# Can a telescope ever increase the apparent luminance of an extended object?

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From what I know about common telescope designs, telescopes don't increase the apparent luminance of extended objects compared to the luminance seen with the naked eye. In this sense extended objects don't appear "brighter" (per unit solid angle of object/image), although the total light flux received from the object (the illuminance) can increase due to the increased magnification (the object looks bigger through the telescope). The best that can be done is keep the luminance the same, which requires eliminating transmission losses. In addition to transmission losses, the light received by the eye per unit solid angle of image is further reduced at high magnifications when the exit pupil is smaller than the viewer's pupil.

Can there be no telescope design (purely optical, e.g. without using electronic eyepieces) that makes extended objects appear "brighter" (in the above sense) than with the naked eye, by somehow overcoming the limitation of the observer's pupil? If so, can this be proven? If not, what would such a design look like?

Can there be no telescope design… that makes extended objects appear "brighter"… than with the naked eye, by somehow overcoming the limitation of the observer's pupil?

I've left out all of the parenthetical qualifications and I'll answer this, let's see if it gets at the heart of the question.

tl;dr: No, because etendue; the same reason that a wall doesn't get brighter when we walk towards it and why we can't go outside with a magnifying glass and concentrate blue sky.

I thought I'd leave the other answer in place for contrast and as a teaching moment, but the down voters had a go at it, so now you can't see it unless you have 10k reputation.

I'd claimed that a pair of 7x50 binoculars would make the object 49 times larger in solid angle but collect $$(50/6)^2$$ or about 69 times more light.

If my fully dark-adapted pupil is 6mm in diameter then the aperture is 8.3 times larger in diameter than my pupil, but the image is only 7 times larger. We square the ratio to get the ratio of surface brightnesses, so it will appear to be

$$left( frac{50/6}{7/1} ight)^2 approx 1.42$$

However the OP pointed out in a comment that this would produce an exit pupil larger than the 6 mm entrance pupil of the eye.

Thanks for the answer. I'm not sure about the conclusion though. With 7x50 binoculars, the exit pupil is 7.14 mm in diameter, meaning not all of the incident light falls on the retina since the eye pupil is smaller. Specifically, the fraction of of light that does enter the eye is $$(6/7.14)^2=0.705$$, which is precisely the reciprocal of the factor 1.42 that you calculated. So in the absence of transmission losses I think the luminance remains the same as that seen with the naked eye.

That was an Aha! moment, nature is smart, or at least I'm not.

I replied:

omg I think that I have I failed to recognize something as fundamental as conservation of etendue. Now it looks like my answer is wrong. :-(…

This failed for the same reason that a wall doesn't get brighter when we walk towards it and why we can't go outside with a magnifying glass and concentrate blue sky on a sheet of paper. In classical mechanics the analogy is conservation of phase space and Liouville's theorem

## Does increased aperture not increase object brightness?

Surface brightness is not a matter of human perception.

https://tonyflanders. ace-brightness/

The surface brightness of an object does not decrease with distance like the total light output, it is an inherent property of the object, not dependent on the observer. When an object is twice as far away, it appears one quarter as bright however, it also appears to be half as big, or one quarter the area, so the surface brightness, or brightness per unit area, remains the same.

http://www.skyandtel. -look-brighter/

In fact, no telescope can ever increase surface brightness beyond your naked-eye view.

http://www.rocketmim. Brightness.html

What the heck is that saying? What -- that the very brightest image I can get with the scope is exactly the same brightness that I see with the naked eye.

Yup.

Well, remember we're talking about surface brightness, meaning the brightness per unit area, or the brightness density.

Of course, there is always some light loss through a telescope so the naked-eye view is actually a bit brighter. This applies to extended objects and not stars, which are point sources.

Dave Mitsky

Well the surface of something could be giving off all sorts of energy that the human eye cannot see. A different animal (or alien) might thus perceive it as brighter (or higher surface brightness) or fainter than we do. We're still talking human perception.

### #52 Jon Isaacs

A point source, when magnified, does not increase in area. The surface brightness of a star does not change with magnification because the area does not change. At magnifications where the Airy disk is visible, then a star behaves as an extended object.

A surface, when magnified does increase in area. The surface brightness decreases because the same light energy is spread over a greater area. If you want to treat an extended object as a summation of point sources, you will have to consider conservation of energy when magnifying the image, the sum total of the energy will have to be conserved.

### #53 Dave Mitsky

A star, other than the Sun, is so distant that amateur telescopes can't resolve the actual stellar disk. (Using interferometry, a number of giant stars can be resolved with professional telescopes.) Therefore, all the light received remains within a geometric point as the image is magnified. This is not the case with an extended object.

### #54 Asbytec

A star, other than the Sun, is so distant that amateur telescopes can't resolve the actual stellar disk. (Using interferometry, a number of giant stars can be resolved with professional telescopes.) Therefore, all the light received remains within a geometric point as the image is magnified. This is not the case with an extended object.

Dave Mitsky

Yes, waves of light collapse on the focal plane and amplify themselves into a diffraction artifact of a given angular diameter. This act of diffraction, as I understand it, destroys any information (resolution) the light waves may be carrying from a distant star. So, it is not an extended image, no matter how small, of a small and distant extended object. Due to diffraction, this is what an image of a single very small distant object looks like against the black of space. An object is small enough to present simply a single PSF as a point source at around 1/4 the Airy diameter and less.

The infinite numbers of point sources on an extended object, no matter how small, behave very differently than a collection of individual Airy discs separated by the Dawes or Abbe limit. It's interesting to contemplate how these points actually behave in a region where Rayleigh and Dawes do not apply, but where contrast and surface brightness do apply at all magnifications. It may be we can model an extended object as an infinite collection of Airy discs, but their combined effect on the extended image is not the same as two point sources isolated in space, or even a single Airy disc.

### #55 Migwan

Each surface area of an extended object contains an infinite number of points. How does one deal with assigning any level of radiance to those infinite points? (I think it's a trick question. )

Since we are talking light then there are not an infinite number of points. Light come in measurable and distinct quanta.

### #56 Migwan

If you really ponder on light for awhile, its merely a means for matter to communicate its presence to other matter across space. Think about it. And that space has its import, consider the following question. Could we even perceive stars if we did not differentiate them from what we perceive as the relatively empty three dimensional space in between?

I do believe our theory is definitely missing something here. Certainly magnification cannot amplify the "surface brightness" of an object beyond what it in reality is. M33 comes to mind. At least in my area, I cannot see it with the naked eye. I can see it with moderate magnification via a small telescope. However, it quickly disappears if I raise the magnification too high. So "magnification" does seems a little fickle and complicated by LP and physiology.

The only possible point emitter is an antimatter matter reaction, which would be virtual at best. Why I think this is important is that the light emitted from any point on a star that spreads at 4pi c^2/2 relative the observer, is added to by the same spreading of the light emitted from other points on that star. That we might want to think of them as point sources, is purely for convenience.

All said and done, I can now clearly see that "magnification" does not amplify "surface brightness. That really makes sense now.

I remain unconvinced with the notion that aperture does not in some manner take up "more" light from a target than that which my naked eye takes up. Basically a telescope transmits more light waves than do my pupils and that equates to more energy utilized.

Great thread guys and thanks for the links. Still working on a couple of those.

### #57 Jon Isaacs

The telescope magnifies the image. That is the other half of the equation, it makes the image larger. So while the image is not brighter or more intense, there is more light entering your eye, those photons cover a larger part of the retina.

### #58 REC

For a good visual on the subject of how scope size works with objects, go the Astronomics site and look at the eyepiece section. They show what a globular cluster looks like in different size scopes and resolution needed to resolve the cluster (M13). If I remember right, a 8" scope was need to really start to resolve the stars in it?

Maybe that will help with light gathering power and resolving objects?

### #59 Asbytec

Since we are talking light then there are not an infinite number of points. Light come in measurable and distinct quanta.

The quanta result in a diffraction artifact with a central disc of a specific size dependant on aperture and wavelength. Extended resolution, to the extent I understand it and in my limited experience with it, is not limited by the diffraction artifact. It's possible to resolve extended features well below Rayleigh and sometimes well below Dawes.

Far as I know, no point on any extended surface has any special properties, relative to any other point, that allow it to produce a discrete Airy disc independently of all other points nor defines their discrete spacing or overlap. It may be, though, if a surface is comprised of a finite number of diffraction discs, then each would retain its brightness as a star does with magnification. The extended object would not follow the inverse square law over a range of magnification.

Ganymede is about the angular diameter of a 6" aperture Airy disc, yet smaller detail can be seen on its tiny surface. It's the angular size of the Airy disc, but it is not an Airy disc itself. It is an extended object comprising more than one point. A lot more, maybe an infinite number of them.

Enough of them so we actually get a high resolution and smooth look at Ganymede consistent with the contrast of its surface albedo. Even small, sub Dawes lunar craters are not pixelated with a finite number points.

Discrete points behave differently from extended images.

Edited by Asbytec, 27 April 2018 - 10:59 AM.

### #60 Asbytec

"I remain unconvinced with the notion that aperture does not in some manner take up "more" light from a target than that which my naked eye takes up. Basically a telescope transmits more light waves than do my pupils and that equates to more energy utilized."

Well, aperture does gather more light and forms a bright image at large exit pupils (lowest useful magnification.) However, the image is also magnified by the ratio of the entrance pupil to the exit pupil. For example, 150mm aperture/7mm exit pupil is about 21x.

At 7mm iris and about 20mm focal length, your eye has a large (fast) f/3 relative aperture operating at 1x. It needs the telescope to provide magnified virtual image for our 7mm f/3 eye to look at.

If your iris could expand to 150mm and magnify 21x, you would never need a 6" telescope to provide a galaxy image of the same brightness per arc second^2. Your naked eye view would be identical to the telescopic image. Our eye cannot expand that large, so we need a 6" aperture and an afocal system with a 7mm exit pupil which, in turn, provides 21x magnification.

In both cases, the image has the same surface brightness. The only difference is our eye can only magnify 1x at 7mm entrance pupil. The telescope provides the same exit pupil (7mm entrance pupil to our eye) except the image is magnified 21x.

Edited by Asbytec, 27 April 2018 - 10:53 AM.

### #61 Starman1

If you really ponder on light for awhile, its merely a means for matter to communicate its presence to other matter across space. Think about it. And that space has its import, consider the following question. Could we even perceive stars if we did not differentiate them from what we perceive as the relatively empty three dimensional space in between?

I do believe our theory is definitely missing something here. Certainly magnification cannot amplify the "surface brightness" of an object beyond what it in reality is. M33 comes to mind. At least in my area, I cannot see it with the naked eye. I can see it with moderate magnification via a small telescope. However, it quickly disappears if I raise the magnification too high. So "magnification" does seems a little fickle and complicated by LP and physiology.

The only possible point emitter is an antimatter matter reaction, which would be virtual at best. Why I think this is important is that the light emitted from any point on a star that spreads at 4pi c^2/2 relative the observer, is added to by the same spreading of the light emitted from other points on that star. That we might want to think of them as point sources, is purely for convenience.

All said and done, I can now clearly see that "magnification" does not amplify "surface brightness. That really makes sense now.

I remain unconvinced with the notion that aperture does not in some manner take up "more" light from a target than that which my naked eye takes up. Basically a telescope transmits more light waves than do my pupils and that equates to more energy utilized.

Great thread guys and thanks for the links. Still working on a couple of those.

jd

"In practice, most larger scopes are not, because of seeing, used at quite as small exit pupils as the small scopes. But they are usually used at higher magnifications. Combine, in practice,

a brighter image (larger exit pupil), more magnification (a larger image), and improved resolution, and is it any wonder we see more in the larger scope?"

Now, assume the small scope is the eye. And match the exit pupil of a low power eyepiece in the larger scope with the pupil of the eye. That keeps the brightness of a target, per unit area, the same, but makes it a lot larger.

My scope equals my pupil diameter at 70x magnification. That makes the object 4900x larger by area than my naked eye, and occupy 4900x as much area of my retina.

Wow. Do you think an object in the sky would be a bit more noticeable and seem brighter, perceptually? Uh, yeah.

The Full Moon at low power (say the 70x in my scope) would also occupy 4900x as much area on the retina as with the naked eye. Impressive. most impressive (geek reference).

Sure, if you count photons, the scope's image is sending a whole lot more "light" into your eye.

But on a surface brightness per unit area level, it's the same as the naked eye. Not intuitive, until you take size into account.

## Rant: busting the myth that the purpose of a telescope is to collect light

Folks, I happened upon this old post from Glenn LeDrew who I think most here on CN respect as a trusted source of knowledge. My apologies to Glenn in advance if I am crossing a faux pas line for providing the link.

As usual, Glenn has a meticulous and succinct way of explanation I'm sure you'll agree on.

### #202 Saravanja

So first of all, those same ones are directly contradicting the very things they have said in the past. I won't bother calling out specifics, but it's kind of. humorous.

Secondly, I am not wrong. I *know* I am not wrong because I have all of the observational evidence I need to confirm that.

If I'm to believe that a telescope is capable of increasing surface brightness, then when I looked at M57 through The Yard Scope at the CSP, I should have seen a blazing bright red ring, not just a larger version of the same gray smoke ring as I see in my 8, 12, or 15" scopes.

If I'm to believe that a telescope is capable of increasing surface brightness, my 15" dob with 3,000x the light gathering power as my eye, would scorch my retina when I look at the Moon.

If I'm to believe that a telescope is capable of increasing surface brightness, again, my 15" dob with 3,000x the light gathering power as my eye, would make it painfully bright to look at the tree tops across the other side of my property.

If I'm to believe that a telescope is capable of increasing surface brightness, my humble 4.5" newt should be showing an already bright object like M42 in rich, vibrant color. Certainly if not a 4.5" then an 8"? Certainly if not an 8", then a 12"? Certainly if not a 12", then a 15?".

If I'm to believe that a telescope is cable of increasing surface brightness, then light pollution through a telescope of any aperture at a 7mm exit pupil should be considerably brighter than it is to the naked eye.

None of that happens though. Clearly there is something going on. It's. it's almost as if the light being collected by the telescope is being diluted somehow by the time it forms an image on my retina. I wonder why?

You're absolutely correct!

Collect 10,000x more light than the human eye and then spread it out over 10,000x larger, and you will have exactly the same intensity of light per unit area as you did before you collected 10,000x more light than the human eye. Energy has been conserved!

Firstly, you are making assumptions about when and how much you should see at what light level. Then when you fail to see Colors you declare that a telescope can't increase surface brightness compared to the human eye. The reasoning is flawed. The human eye is not linear, nor is it continuous in it's iteration.

It's responses at low light are very different compared to intense light.

If you don't get enough intensity, you don't see colors. You declared that you don't see colors based on simple intensify. The gain you have so far used is not enough.

Starman has explained he does see color in much larger telescopes. Many others have seen this effect. You simply are expecting too much from too small a device.

The reason you don't see it is there are not enough photons to see color, even with all that huge capture area.

As to why you don't get blinded by looking at the trees, telescopes not only increase the amount of light captured, they also as you keep stressing, magnify.

You assume that all the light that is being captured by the objective gets to your eye. That is not the case. Magnification cannot divide photos. This should be quite apparent as it would not comply with the conservation of energy.

Magnification shows you a smaller area, but the area it shows you has greater surface brightness than what it would have by the naked eye. The amount more is as I explained determined by the ratio of apertures.

Speaking of which, the ratio of apertures is simply the ratio of areas. Which should intuitively suggest that magnification goes not divide, it shows a smaller area. Thus you are seeing the smaller area at a higher surface brightness but that smaller area also doesn't have the same amount of energy as the larger area. Magnification is selectively showing you smaller areas at higher surface brightness.

Let's do some simple physics.

As everyone knows, a telescope shows you what an object would look like if you were closer. That's the magnification factor. What you are saying is that if you were closer, you would not see the increase in surface brightness. You are incorrect.

Your eyes area remains the same. Moving it closer exposes it to more photons. Moving it further exposes it to less photons precisely because the area is constant. Surface brightness calculations when correctly applied show that the surface brightness of an object is constant based on angular dispersion. They at the same time show that keeping the same area while moving closer increases the surface brightness. The two are different ways of saying the same thing. If you go further, you need a larger area to capture the same energy. If you have the same area but move further, you capture less energy.

The telescope increases the area.

Again, I must stress that magnification cannot change the amount of photons collected. It can only show you a smaller section of that brighter image. Iow, the surface brightness of the telescope cannot be changed by the magnification. But, it makes no sense to think that it must be the same as the surface brightness of the eye to begin with.

Let's try another way of looking at it. A telescope increases the brightness of every point on the image equally because of its larger aperture.

Project that on paper. The image is bright. Every point is brighter than the human eye would have captured with it's smaller area. Now, look at a small section of that image, say 10% of the area. All those points are still brighter than what the eye captured. Notice that they didn't lose photons just by us looking at them. That's impossible.

Next, line up a bunch of lenses and make them form the same sized image of the Sun on paper. The larger lens is brighter.

Now examine a smaller but same sized area of each image. Do they magically have the same surface brightness?

The answer is no. If you used a 7mm lens, you would instantly see that it shows a far dimmer image when they are all the same size. The telescope increases the surface brightness because it concentrates the light into a smaller area.

This should now also make it clear why you don't go blind looking at trees during the day. Each point is brighter, but you are looking at less points. Again, lenses don't divide photons. They show different areas. All of the points that they show however are brighter than if seen by the naked eye.

### #203 B l a k S t a r

Average luminance (surface brightness) nor contrast increase occurs as they are innate. Stars are point sources and it is well established that they do brighten. Stating an average luminance to be made up of individual point sources has no basis.

Area due to magnification can trigger colour reception but the object does not make itself brighter. It is perceived to be but it does not physically occur. That is what I think goes to the heart of the OP. Please read the link above.

add: there seems to be a conflating of average luminance (surface brightness) and total luminance (total brightness).

Im also trying to use the RASC published term

Edited by B l a k S t a r, 23 March 2020 - 09:51 AM.

### #204 Starman1

--look at a galaxy at, say, 150x in a telescope.

--Now, install on the front of the scope a series of aperture masks of, say, 4", 3", 2".

Each mask you install doesn't change the magnification, but the object gets dimmer with each smaller mask.

We could say that what happens is that the f/ratio changes, so the exit pupil gets progressively smaller.

And that is a valid way to look at it.

The thing is, we all know from experience that the 2" aperture will see the galaxy dimmer at ALL exit pupils, not just the one

that happens with the mask.

Reconciling what we see with what we can easily calculate defies experience, and therein lies the issue with the arguments in this thread.

We all know extended deep sky objects appear dimmer in smaller apertures.

Is, therefore, anything wrong with describing that objects get brighter in larger telescopes?

### #205 CrazyPanda

The thing is, we all know from experience that the 2" aperture will see the galaxy dimmer at ALL exit pupils, not just the one

that happens with the mask.

The only thing that governs brightness of extended objects is the exit pupil. A 3mm exit pupil is a 3mm exit pupil, regardless of aperture.

A galaxy in a 3mm exit pupil in a 60mm telescope is going to have the same apparent surface brightness as a 600mm telescope at a 3mm exit pupil.

The difference is the 600mm aperture telescope will produce an image 10x wider in apparent angular size at that same surface brightness.

This goes back to that image I posted before:

The surface brightness of each of those is same. The one on the right is just larger.

Sure, the total flux of the one on the right is greater because it's the same luminance value for each pixel, and there are more pixels, but nobody is going to say the one on the right is brighter, they will say it's bigger.

And yes, as you said, if you back away far enough, the one on the left looks dimmer. But that's because it's dipping below your eye's resolution limit and fewer receptors are being engaged. That has nothing to do with brightness, and it only reinforces my point: its magnification that counts.

Actually, that's the exact opposite for me.

What I can calculate now fully explains why viewing DSOs in huge apertures has never produced what I had always assumed would be the case: a super bright, photo-like view of these objects.

My understanding of how a telescope works is now consistent with what I'm able to observe through them. I was able to come to this understanding thanks to observers like Glenn LeDrew and Mel Bartels. Here's a particularly important article from Mel Bartels that helped me transform my knowledge:

1000x. However in order to fit all of the light from the 10' aperture into the eye's exit pupil we must use at least 33x. 33x will dilute the image brightness by 33^2 =

1000x so we are back where we started. In fact because of mirror coatings not reflecting 100% and the small obstruction caused by a diagonal the image brightness per area will actually be a little less than with the unaided-eye.

This leads to the interesting conclusion that the brightness of the sky glow as seen in the eyepiece is entirely dependent on exit pupil. At a given location on a given night no matter the size of scopes if they are giving the same exit pupil then the sky glow brightness will be very similar.

Here is another interesting quote I found:

There is more to the equation than contrast and surface brightness. Ask yourself why you can't see the Veil naked eye but you can see it in a telescope? The contrast is the same, the brightness is the same. What has changed is the image size, it's larger, it is more easily seen.

This is what telescopes do, they do not increase the brightness or contrast, they simply magnify the image and make it larger. And that is why I can see galaxies in my 22 inch that I can't see in my 12.5 inch. The contrast is the same, the surface brightness is the same. They galaxies are just larger in the larger scope.

Does that sound familiar to you? Like it's EXACTLY the same argument I've been making this whole time?

Take a wild guess whose quote that is.

Edited by CrazyPanda, 23 March 2020 - 11:04 AM.

### #206 Jon Isaacs

There is more to the equation than contrast and surface brightness. Ask yourself why you can't see the Veil naked eye but you can see it in a telescope? The contrast is the same, the brightness is the same. What has changed is the image size, it's larger, it is more easily seen.

This is what telescopes do, they do not increase the brightness or contrast, they simply magnify the image and make it larger. And that is why I can see galaxies in my 22 inch that I can't see in my 12.5 inch. The contrast is the same, the surface brightness is the same. They galaxies are just larger in the larger scope.

Does that sound familiar to you? Like it's EXACTLY the same argument I've been making this whole time?

Take a wild guess whose quote that is.

I suspect I know who the author is.

However whoever it is, you are not making EXACTLY the same argument. Your argument is very different.

- That author, whoever he or she is, was very careful to specify surface brightness, to distinguish between surface brightness and total integrated brightness, something you have failed to do.

- More imporotantly, that author made no claims about the purpose of a telescope. I have to think that were he or she asked, the response would be that the purpose of a telescope is to allow the observer to see objects more clearly and that to do this, it must collect more light than the human eye and make the image on the retina larger.

- I also have to think that this particular author would almost certainly point out if asked, that a telescope does increase the overall integrated brightness of an object and indeed that is why we can see the object more clearly.

- Since you seem to think that this particular author clearly states your case for you, I strongly recommend you ask him/her to comment on whether he/she agrees with you or if he/she sees significant differences and if so, maybe ask this particular author if they would be willing to help you understand any differences.

To me, that sounds like a plan. Are you in?

### #207 CrazyPanda

I address this (several times) in this thread.

And I quote: "That is what telescopes do: they do not increase the brightness or contrast, they simply magnify the image and make it larger."

This is not what this quote says:

"..they simply magnify the image and make it larger. And that is why I can see galaxies in my 22 inch that I can't see in my 12.5 inch."

But in reality, total integrated brightness is defined as a given surface brightness over a given area (similarly, surface brightness is defined as total integrated brightness divided by the square area).

But describing an object as its total integrated brightness is not useful because it lacks information about the view of the object formed by the telescope.

Describing an object by its surface brightness and size, is a much, much more precise way of describing its visibility than just referencing its total integrated brightness.

Edited by CrazyPanda, 23 March 2020 - 11:52 AM.

### #208 Starman1

Look, I can produce 5mm exit pupils easily in both my 4" and 12.5".

Certain galaxies of about 2' in size are visible in the 12.5" and invisible completely in the 4" at the same exit pupil.

OK, the surface brightness per unit area is identical in both scopes.

But, at the large exit pupil, the resolution of the eye sees the galaxy as barely larger than a point in each scope because that size of galaxy

is small enough to be near the limit of the resolution of the eye. To the eye, the galaxy is very very small in both scopes.

And yet, the galaxy is completely invisible to averted vision in the 4" and easily seen in the 12.5".

The surface brightness is the same. The size is greater than a star point.

Why is it easily seen in one and not the other? It can't be magnification because I can see small objects of that size easily in the 4" at that power.

It's how I identify I've found certain objects.

Could it be brightness? I, and many others, think it is. So there is more to being able to see faint targets than merely size.

Total magnitude is also important. It's why both of those figures should always be taken into account when looking for objects.

### #209 Jon Isaacs

Clearly you are more interested in your rant than in clarifying the issues.

"The purpose of a telescope is to allow the observer to see objects more clearly and that to do this, it must collect more light than the human eye and make the image on the retina larger."

Also, please provide a link when you quoted me, I couldn't find it.

"There is more to the equation than contrast and surface brightness. Ask yourself why you can't see the Veil naked eye but you can see it in a telescope? The contrast is the same, the brightness is the same. What has changed is the image size, it's larger, it is more easily seen.

This is what telescopes do, they do not increase the brightness or contrast, they simply magnify the image and make it larger. And that is why I can see galaxies in my 22 inch that I can't see in my 12.5 inch. The contrast is the same, the surface brightness is the same. They galaxies are just larger in the larger scope."

"b. Short (one or two line) quotes of textual material may be posted, as long as the quote is properly attributed to the author and a link to the material is posted if available. Moderators have discretion to determine if posted quotes are too long or improperly attributed."

With your habit of picking and choosing, I want to see the rest of the post.

### #211 Tony Flanders

When you aim a telescope at Andromeda's core, some people might perceive an increase in brightness of the object, but what they are perceiving is the same (or dimmer) core brightness taking up a much, much larger area of their retina.

Here's how I would phrase that, to avoid confusion:

When you aim a telescope at the Andromeda Galaxy, you may perceive its light as being more intense. That perception is a result of the same actual intensity on your retina occupying a larger area on your retina.

Part of the problem here is that there exists no standard term for the intensity of the light on your retina. But calling it "perceived brightness" only adds to the confusion, when in fact it correlates rather poorly with real-life people's real-life perceptions.

### #212 Redbetter

However, what I have said is total brightness is a useless, irrelevant measure. The same way integrated magnitude of an extended object is also a useless, irrelevant measure - the example I gave was M42 vs NAN. Both mag 4, but ask 10,000 people which appears brighter in a telescope, they’re all going to say M42 does. Why? M42 has higher surface brightness (yet another fact I hope we can agree upon). The light from M42 is literally more intense than NAN.

The intensity of light, which can be described as how strongly the light “tickles” a given photoreceptor in your eye (just like an F5 telescope will hit a give camera pixel with 4x more light than F10), is 100% determined by the surface brightness of the point on the object that hits that photoreceptor. Want to activate a cone so you can see color? You need more light intensity, therefore you need higher surface brightness.

TO ME, therefore, I don’t give two rat’s ***es about integrated magnitude or total brightness. It’s a fundamentally flawed measure that does nothing useful to describe the properties of the object you’re viewing. So when I say “brightness”, always mean “surface brightness”. I use the terms interchangeably because to me “total brightness” might well be the same as describing an extended object in terms of leprechauns. Literally a useless measure of extended objects.

So when I say a telescope doesn’t make extended objects brighter, I mean it does not increase their surface brightness, which is the only *practical* measure of brightness to concern to yourself with for this general class of objects.

The above comments about integrated magnitude/total brightness are some of the most poorly thought out things I have ever seen stated on CN, by someone who ought to know better 8 pages into their own rant. It manages to get things completely backwards.

Surface brightness tells me almost nothing about how visible most objects are with a given aperture. The primary determinant of what is visible is the integrated magnitude or total brightness if you prefer. Surface brightness is a secondary consideration, not irrelevant, but secondary. Surface brightness becomes increasingly important for objects on the margin with respect to contrast with the sky conditions (light pollution/transparency, even seeing for very small extended objects.)

The overwhelming number of high surface brightness extended objects can't be seen with the naked eye or with small apertures. (Ditto for stars.) The photons are not arriving rapidly enough through a given aperture to stimulate a signal. Use enough aperture and the count finally reaches a perceptible level. It is the integrated magnitude that matters most, brightness--not surface brightness.

Ironically, low and ultra low surface brightness objects are often best seen with the lowest magnification (turning the magnification thesis on its head.) They become harder to perceive/recognize as magnification is increased.

Surface brightness is secondary as to whether we actually can detect an object, but very important to how we perceive it. Surface brightness matters more to novice observers who are not accustomed to seeing lower contrast targets or features. Typically, as an observer gains experience and recognizes more subtle features, whole classes of objects become visible that went unseen before. It is part of the learning curve.

In this respect the rant reads more as a reflection of an observer's inexperience with low surface brightness targets. Some folks, even those with decades of observing experience, never really take to observing low surface brightness objects. They seem quite happy with the "bright and beautiful." There is nothing wrong with that style of observing, but it is a very limited subset of what is out there to be seen. In some cases the reason to gravitate toward higher surface brightness targets is the light pollution level in the skies they typically observe from.

Surface brightness becomes more relevant/important in brighter sky. This can be a major factor in how things are perceived and in people's experiences. Folks that rarely observe in dark sky have a perception that is skewed far more by their observing environment than the intrinsic properties of the object in the night sky.

### #213 CrazyPanda

Sorry, but no. Not even close.

You think you're going to see an object with a poorly defined edge and maximum MPSAS of 25 from mag 19 MPSAS skies? Never in a million years. The contrast is far too low. Doesn't matter what that object's integrated magnitude is. Doesn't matter what size telescope you have.

Edited by CrazyPanda, 23 March 2020 - 05:16 PM.

### #214 Starman1

Sorry, but no. Not even close.

You think you're going to see an object with a poorly defined edge and maximum MPSAS of 25 from mag 19 MPSAS skies? Never in a million years. The contrast is far too low. Doesn't matter what that object's integrated magnitude is. Doesn't matter what size telescope you have.

No one is arguing you can see a galaxy with a peak of mag.25mpsas in mag.19 skies.

It would take an observatory-sized scope to image it because most of the galaxy would be well below m.25.

That galaxy would have an exceedingly low total integrated magnitude unless it were huge, and all the really big galaxies have high integrated magnitude figures.

So that's just a straw man argument.

But, if there were a galaxy like that, and it was huge, it's possible you could see it, despite its low surface brightness.

You can see M33 in a 30mm finder scope (I've done it), and that's because its total integrated magnitude is bright (5.7), even though its surface brightness average is 14.1mpsam (23mpsas).

In such a case, total magnitude counts.

### #215 Redbetter

Sorry, but no. Not even close.

You think you're going to see an object with a poorly defined edge and maximum MPSAS of 25 from mag 19 MPSAS skies? Never in a million years. The contrast is far too low. Doesn't matter what that object's integrated magnitude is. Doesn't matter what size telescope you have.

Oh, come on. Completely moving the goal posts to bright skies. Why not daylight? Sheesh. Ok, let's do daylight for kicks: Venus has a surface brightness that is about 2+ MPSAS better than the Moon, but it is very hard to find naked eye in the daytime sky while the Moon is easy to see this way. Of course the Moon is many many times brighter (integrated magnitude) than Venus and still has enough surface brightness to be seen well in daylight. Mercury is about 1 MPSAS brighter than the Moon, good luck catching Mercury naked eye with the Sun above the horizon.

I stick with evaluating faint astronomical targets in dark skies, as it provides a needed anchor for discussion. Seeking out cases there the background itself washes out the object is specious at best. Contrast is only one aspect to what is seen or detectable, it is not everything. And I do see 25 MPSAS stuff in dark sky, some naked eye, but others through scopes. Some observed through scopes include dwarf galaxies that are large enough to be visible naked eye. except that they aren't visible that way because they aren't bright enough (integrated magnitude) for that.

As Jon said earlier, this thread is not likely to prove helpful for those seeking to understand these sort of things. Instead this rant mostly confuses things and draws the wrong conclusions. That forces many of us to speak up, as much as we would prefer to let this thing die. Worst rant ever.

### #216 charlesgeiger

If you are viewing M13 or M42 with a 50 mm f/4 with a 5 mm exit pupil and right along side you are viewing M13 and M42 with a 500 mm f/4 telescope with a 5 mm exit pupil are you saying that you see the same number of stars with the same brightness in M13 and brightness/nebulosity in M42 as in the 50 mm telescope? Or are you saying that with a 50 mm scope at f/4 with say 75 power (assume a 2X barlow and 5.2 mm eyepiece) that you would see the same brightness in M13 and brightness/nebulosity in M42 as you would in a 500 mm f/4 at 75 power? (assume use of a 26 mm eyepiece in the 500 mm scope). I am not including differences in eyepiece design but for the sake of the argument saying that one would be getting a similar AFOV in both scopes.

I can only say from practical experience that the 500 mm would not only give much brighter images but would also give so much more contrast and resolution. As previously said, one would get so many more photons using the larger telescope. Photographically I understand that at the same f stop one would use the same exposure but I believe that leaves out the visual experience regarding telescopes relative to the observer's skills and visual acuity.

I am not arguing but trying to understand.

### #217 Tony Flanders

The primary determinant of what is visible is the integrated magnitude or total brightness if you prefer. Surface brightness is a secondary consideration, not irrelevant, but secondary.

I certainly agree with that, but it is to some extent a selection effect, due to the fact that the catalogs from which amateur astronomers draw their targets consist primarily of objects whose surface brightness is high enough not to pose a major problem for visual observers.

There's pretty compelling evidence that most galaxies have surface brightnesses too faint to detect easily through photography, let alone through visual observing. However, most of these have not been detected yet -- especially once you get beyond the confines of the Local Group. So you won't find them in (say) the PGC, much less the IC, much less the NGC, which was complied based on visual observations.

### #218 B l a k S t a r

Well it looks like this gem of a thread remains unresolved and contains several off-axis viewpoints eluding concensus. It has been quite illuminating at times, shining a light on how all of this telescope viewing and eyesight works together in fact, and perception. Perhaps higher magnification will shed a better light on it.

I've learned much and investigated more in a quest to understand this. What a wonderful topic to be presented in say S&T or somewhere. Perhaps all of the parameters at play elude most and yet I can't help but wonder, surely there is a single unifying treatise out there that is unambiguous and scientifically sound.

### #219 tommm

Well it looks like this gem of a thread remains unresolved and contains several off-axis viewpoints eluding concensus. It has been quite illuminating at times, shining a light on how all of this telescope viewing and eyesight works together in fact, and perception. Perhaps higher magnification will shed a better light on it.

Maybe larger aperture to shed more light on it.

### #220 Roger Corbett

BlakStar, given the confusion sown, the repetition, and that any edification has long been lost, I’d vote for the following.

Let the “debate” conclude!

### #221 CrazyPanda

Oh, come on. Completely moving the goal posts to bright skies. Why not daylight? Sheesh. Ok, let's do daylight for kicks: Venus has a surface brightness that is about 2+ MPSAS better than the Moon, but it is very hard to find naked eye in the daytime sky while the Moon is easy to see this way. Of course the Moon is many many times brighter (integrated magnitude) than Venus and still has enough surface brightness to be seen well in daylight. Mercury is about 1 MPSAS brighter than the Moon, good luck catching Mercury naked eye with the Sun above the horizon.

I stick with evaluating faint astronomical targets in dark skies, as it provides a needed anchor for discussion. Seeking out cases there the background itself washes out the object is specious at best. Contrast is only one aspect to what is seen or detectable, it is not everything. And I do see 25 MPSAS stuff in dark sky, some naked eye, but others through scopes. Some observed through scopes include dwarf galaxies that are large enough to be visible naked eye. except that they aren't visible that way because they aren't bright enough (integrated magnitude) for that.

As Jon said earlier, this thread is not likely to prove helpful for those seeking to understand these sort of things. Instead this rant mostly confuses things and draws the wrong conclusions. That forces many of us to speak up, as much as we would prefer to let this thing die. Worst rant ever.

Moving the goal post? Bright skies is all relative. Contrast is relative. Contrast and size is what matters.

I can barely see M101 as a smudge in my 60mm, and a bigger smudge in my 15" from my 21.2 skies. I cannot see spiral structure. The 15" just makes it a bigger smudge. It's not brighter, it's not higher contrast. It's just a bigger smudge.

Someone with 20.0 skies likely won't see it at all.

Funny you should say that, because S&T published just such an article in the December 2017 issue called "Understanding Surface Brightness"

And once again, another great Glenn LeDrew summary from the topic I raised about that very article:

Edited by CrazyPanda, 24 March 2020 - 11:28 AM.

### #222 B l a k S t a r

Moving the goal post? Bright skies is all relative. Contrast is relative. Contrast and size is what matters.

I can barely see M101 as a smudge in my 60mm, and a bigger smudge in my 15" from my 21.2 skies. I cannot see spiral structure. The 15" just makes it a bigger smudge. It's not brighter, it's not higher contrast. It's just a bigger smudge.

Someone with 20.0 skies likely won't see it at all.

Funny you should say that, because S&T published just such an article in the December 2017 issue called "Understanding Surface Brightness"

And once again, another great Glenn LeDrew summary from the topic I raised about that very article:

https://www.cloudyni. -tel/?p=8274019

Frustrating that I can't seem to get anywhere at S&T for a back issue. Paper editions listed number 20 or so and don't go back to December 2017. I'd rather have a PDF edition but that does not seem to be offered either. Not sure if that relates to the old/new ownership.

edit 2: nix that, I tried once and was informed I only have access to digital issues from my subscription date onward.

Edited by B l a k S t a r, 24 March 2020 - 04:13 PM.

### #223 Redbetter

I certainly agree with that, but it is to some extent a selection effect, due to the fact that the catalogs from which amateur astronomers draw their targets consist primarily of objects whose surface brightness is high enough not to pose a major problem for visual observers.

There's pretty compelling evidence that most galaxies have surface brightnesses too faint to detect easily through photography, let alone through visual observing. However, most of these have not been detected yet -- especially once you get beyond the confines of the Local Group. So you won't find them in (say) the PGC, much less the IC, much less the NGC, which was complied based on visual observations.

However, when one considers telescopic limiting magnitude the selections move even further from the ultra low surface brightness galaxy subset. So in terms of selectivity, overall brightness (magnitude) is even more important than surface brightness. The problem with most of the ultra low surface brightness galaxies is that they are also very dim, even in the Local Group, so the vast majority are too high of a magnitude to be seen other than a few closer ones. These very low/ultra low surface brightness galaxies are often only a few hundred thousand suns luminosity, some are as low as a few thousand or even a few hundred suns.

Consider the sampling and populations for the moment: there are perhaps a few hundred or so galaxies with surface brightness that are 25+ MPSAS and are within the integrated magnitude range of a large aperture scope. By comparison there are hundreds of thousands of other galaxies within range of the 20". No doubt there are many times more very low surface brightness/low luminosity galaxies unseen out to the same distances as those few hundred thousand, but nearly every one of the low surface brightness galaxies is well beyond the magnitude range of the instrument as well.

The selectivity issue is one of distance and absolute magnitude rather than surface brightness. It is only the close local group galaxies that are very low/ultra low surface brightness that are out of visual range of large aperture because of surface brightness rather than total magnitude.

It is the combination of very low surface brightness and high magnitudes that makes them so difficult or impossible to see. That is the problem with this ill-conceived rant thread, it pretends that only one thing matters, which is simply false.

There is no doubt that it becomes very difficult to see 25 MPSAS and above even in pristine sky. The toughest/lowest surface brightness object that I believe I have seen is the Ursa Minor dwarf galaxy with a few hundred thousand suns luminosity. The surface brightness is given as

26 MPSAS, essentially an exceedingly slight over brightening of the background sky that was difficult to define. However, I have observed it several times in excellent conditions and the impression of overall shape and orientation has remained the same each time. I have tested at different times of the night, as well as repeating years apart, and the impression has been independent of which way the tube/focuser was oriented so it wasn't an issue with stray light/reflections in the scope, although I can't rule out background star patterns or IFN masquerading as the galaxy. It has a few 17th mag stars, but I haven't tried to ID them in the field--to do that I would have to increase the magnification several fold to get closer to the stellar limiting magnitude of the scope. Doing that actually makes the extent of the galaxy impossible to see.

## Can a telescope ever increase the apparent luminance of an extended object? - Astronomy

To illustrate the dramatic effect of combining a larger apparent field (yielding greater deep sky details) with smaller exit pupils (yielding fainter stars with darker sky background) we propose the "Majesty Factor". We define it simply as the cube of the ratio of any two different apparent field eyepieces having the same field stop diameters (same true field). Examples:

After showing the first Ethos 13mm at a number of events since April 2007, we can safely conclude it brings the observing experience to a new level. This is based solely on user's reactions to views of familiar objects, not on any prejudgments, publicity or hype on our part. While we were quite confident of success, we wanted, and still want, to explore all the ramifications of what a sharp 100° field really represents.

Right after NEAF in April, Rodger Gordon, the acknowledged "eyepiece junkie" of all time, wrote me "Definitely the finest wide-angle eyepiece I've ever seen. If God is an astronomer, this is the wide-angle eyepiece he'd choose. You can quote me." Thanks, Rodger. I waited until now to avoid "priming the pump", so to speak before quoting your unbridled enthusiasm publicly.

For some time, I've been pondering just why the response has been so overwhelmingly positive. And if I really understand why, is it possible to quantify? My views of the Double Cluster at Stellafane pointed the way.

The 1991 article I wrote for Sky and Telescope on magnification provides the key. A major conclusion for low power states: "The best view occurs with the highest power that comfortably includes the target object. Higher powers darken the background sky, reveal fainter stars and show more detail. The resulting smaller exit pupil also minimizes the effects of eyesight defects."

Considering the potential of Ethos, let me posit a more general conclusion.

For deep sky viewing of star fields, open and globular clusters, nebulae and galaxies, choose the highest power that frames the subject, so long as the sky background does not reach black, and the atmosphere does not degrade the resolution. The smaller exit pupils permit a darker sky background which achieves greater contrast against the fixed brightness of stars, while the greater magnification reveals more structural details on extended objects. Using eyepieces with larger apparent fields maximizes the viewing experience.

The result is an increase in what I would call the Majesty Factor, the nexus of contrast, power and field.

It's clear that the largest possible apparent field for a given true field yields the most magnification for greater resolution, with a darker sky background for more contrast as a result of the smaller exit pupil. I believe this combination of contrast, power and field causes the typical "wow" reaction &mdash the Majesty Factor. I think Tom Trusock said it most succinctly in his Starfest (Canada) report: "The same true field at higher magnification means that you'll see blacker skies and more detail." Dennis di Cicco in his 5-star review of Ethos in his October 2007 Sky & Telescope review noted something similar: "Observing with the 12-inch scope, I typically bounce between a wide-field eyepiece for star-hopping and a high-power one for detailed views. But the Ethos gave me both. The field was large enough to star-hop, and the magnification was high enough to bring out faint stars and resolve details in galaxies and star clusters." (He coincidently also illustrated field sizes using the Double Cluster.)

• apparent field: perceived span of sky seen through eyepiece (without telescope). Not used in true field (see) calculation.
• exit pupil: image of objective formed by eyepiece. Location where full apparent field is seen.
• f/#: a ratio that describes the relation between the aperture and focal length of the telescope -- important for photography
• field stop: ring inside the eyepiece barrel that limits true and apparent field size
• focal length: effective distance from entrance of an optical system to focal point
• magnification: relative change in angular size of object
• true field: span of sky seen through telescope/eyepiece combination

Let's try to quantify the so-called Majesty Factor. While we cannot quantify the majesty of a great symphony, work of art or edifice, I think a meaningful Majesty Factor is quantifiable for those great deep sky views. Here's how:

Let's consider a range of possible eyepieces with apparent fields of 50°, 60°, 68°, 82° and 100°. Now let's pick an object, (like the Double Cluster) and let's say it's properly framed in the field of a 50° Plössl with a 26-mm focal length in an f/4 telescope so the exit pupil = 6.5-mm. Let's arbitrarily assign a factor of 1 to the power (magnification) of this telescope and a factor of 1 to represent the contrast for the 6.5-mm exit pupil. Therefore, for the given true field:

Now let's replace the Plössl with a 100° (apparent field) Ethos with a 13-mm focal length. This yields the same true field of view at twice the power with twice the apparent field and half the exit pupil. The 3.2-mm exit pupil is only ¼ the area of 6.5-mm, so the sky background darkens by a factor of 4 (contrast factor). The magnification power factor yields twice the detail or resolution. Therefore:

Working out the math for all the apparent fields listed above, we have:

Apparent Field (°) Power Factor Contrast Factor Majesty Factor
Plössl 50 1.00 x 1.00 = 1.00
Panoptic 68 1.36 x 1.85 = 2.52
Delos 72 1.44 x 2.07 = 3.00
Nagler 82 1.64 x 2.69 = 4.41
Ethos 100 2.00 x 4.00 = 8.00

A simple rule of thumb is that for any two eyepieces having the same true field of view, the Majesty Factor equals the cube of their apparent field ratios. Example is (100°/70°) 3 =2.92.

## Rant: busting the myth that the purpose of a telescope is to collect light

If one just does a simple thought experiment, I believe you will see that a larger mirror, as well as narrowing the field will also brighten the light throughput. Say one was to create a mirror system that was so large as to extend radius to Jupiter. Say it had an f/4 focal ratio. It's airy disk would be extremely small and it could magnify the actual nearest stars disks. Those disks become extended objects. As well as being able to directly see the actual disk of the star, you would be able to see so many undetected stars. The airy disk being so small would bring such light intensity that very faint stars would be visible. Yes, the larger the scope the more light gathering capability and the smaller distant object will be resolved. When you look at this same portion of the sky with a 10" scope you would not be able to see any of the 25+magnitude stars available to the large scope. So the superior light gathering, resolution and added contrast would just kill the 10". Again, some of the stars would possibly be resolved into disks (actually resolved) and the light intensity would be much greater to provide the many fainter objects to be viewable. The light intensity would be much greater than the human eye as the human eye would have no way to detect them. So both light gathering and magnification would trounce the smaller scope on a great scale. I believe that the largest scopes would show that even at f/4 for both scopes the big one's light gathering/intensity would be vastly greater along with contrast, resolution and energy production in the airy disk.

### #177 Asbytec

If one just does a simple thought experiment.

In a similar way, I "think experiment" it like this. Take a 10" scope and look at a faint galaxy at 100x. Compress all of the galaxy's light into a star like point. Increase the aperture to 20", the star like point get's brighter. We all know this. Now, still at 100x, expand that brighter point back into the extended faint galaxy. What do you have? Expand magnification to the same 2.5mm exit pupil as the 10" aperture, what do you have?

### #178 CrazyPanda

In a similar way, I "think experiment" it like this. Take a 10" scope and look at a faint galaxy at 100x. Compress all of the galaxy's light into a star like point. Increase the aperture to 20", the star like point get's brighter. We all know this. Now, still at 100x, expand that brighter point back into the extended faint galaxy. What do you have? Expand magnification to the same 2.5mm exit pupil as the 10" aperture, what do you have?

Here are my thought experiments:

1. If telescopes are capable of increasing the brightness of non-point objects beyond what the human eye can see, at what aperture will the North America Nebula become so bright it causes you to enter 100% photopic vision and present itself as a firey red nebula?

2. If telescopes are capable of increasing the brightness of non-point objects beyond what the human eye can see, at what aperture will viewing daytime objects become so bright they are painful to look at?

3. If telescopes are capable of increasing the brightness of non-point objects beyond what the human eye can see, at what aperture will viewing the moon become physically dangerous?

The answer to all three is "none", and every single observer in this thread should know this instinctively just from sheer observing experience.

Edited by CrazyPanda, 21 March 2020 - 07:30 AM.

### #179 Asbytec

The answer to all three is "none", and every single observer in this thread should know this instinctively just from sheer observing experience.

I'm glad you supplied the answer, because I didn't know the answer to any of those questions. I doubt it would burn your eyes if you were right up on it because it's so huge. So, it seems you're right, no afocal system would do that. The aperture would be so large and the exit pupil equally enormous (maintaining high image surface brightness) our own small iris (small effective aperture) would cut off almost all of the gains in brightness. I think it's related to the total flux of photons, but I have to leave that to others more knowledgeable than me.

### #180 Cotts

Here are my thought experiments:

1. If telescopes are capable of increasing the brightness of non-point objects beyond what the human eye can see, at what aperture will the North America Nebula become so bright it causes you to enter 100% photopic vision and present itself as a firey red nebula? An aperture exists where extended objects such as the N.Am. Neb. will be made bright enough to see colour in them. Probably something north of 60 or 100 inches. There are some deep sky objects that begin to show colour in 32" scopes - I have seen it myself. This happens because the telescope makes the object bright enough to trigger our colour receptors.

2. If telescopes are capable of increasing the brightness of non-point objects beyond what the human eye can see, at what aperture will viewing daytime objects become so bright they are painful to look at? Again, it would be a very large aperture. Even larger than in your #1 because the eye's iris can stop out a great deal of the light.

3. If telescopes are capable of increasing the brightness of non-point objects beyond what the human eye can see, at what aperture will viewing the moon become physically dangerous? Again. Very large telescope. Your iris.

The answer to all three is "none", and every single observer in this thread should know this instinctively just from sheer observing experience.

Your entire argument fails because you continue to make a distinction between point source objects and extended objects.

You have admitted here that telescopes make point sources brighter. No question. You are correct. And you continue to contend that telescopes do not make extended objects brighter. In your three scenarios, above, you are careful to insert the term 'non-point objects'.

But you fail to admit that the same physics applies for point sources and extended objects which are nothing more than the integrated overlapping of myriad point sources of light.

I asked you this before but you ducked it. If a field of view has both stars and an extended object in it how does the telescope "know" * to make the stars brighter but not the extended object? What about stars that are 'in front of' the extended object? What does the scope do then?

179 posts and, yet, here we are.

* Three old men were sitting on the veranda of the rest home when Ernie said, "What is the greatest invention of all time?" They thought for a while in silence when Fred said, "Fire - the harnessing of fire. Our entire civilization began there. " Then Bob argued, "No, I would say it is the wheel. It allowed agriculture, cities and our modern way of life." Fred and Bob then stared at Ernie. Finally Ernie spoke up, saying, "No you are both wrong. The greatest invention of all time is the Thermos bottle." The other two men howled with laughter. "The Thermos bottle? How in the world did you come up with that answer?" And Ernie smiled and said, "The Thermos bottle keeps hot liquids hot and cold liquids cold." "Yeah, so what?" said the other two. Ernie answered, "How does it know?"

Your entire argument fails because you continue to make a distinction between point source objects and extended objects.

You have admitted here that telescopes make point sources brighter. No question. You are correct. And you continue to contend that telescopes do not make extended objects brighter. In your three scenarios, above, you are careful to insert the term 'non-point objects'.

But you fail to admit that the same physics applies for point sources and extended objects which are nothing more than the integrated overlapping of myriad point sources of light.

I asked you this before but you ducked it. If a field of view has both stars and an extended object in it how does the telescope "know" * to make the stars brighter but not the extended object? What about stars that are 'in front of' the extended object? What does the scope do then?

179 posts and, yet, here we are.

Dave

* Three old men were sitting on the veranda of the rest home when Ernie said, "What is the greatest invention of all time?" They thought for a while in silence when Fred said, "Fire - the harnessing of fire. Our entire civilization began there. " Then Bob argued, "No, I would say it is the wheel. It allowed agriculture, cities and our modern way of life." Fred and Bob then stared at Ernie. Finally Ernie spoke up, saying, "No you are both wrong. The greatest invention of all time is the Thermos bottle." The other two men howled with laughter. "The Thermos bottle? How in the world did you come up with that answer?" And Ernie smiled and said, "The Thermos bottle keeps hot liquids hot and cold liquids cold." "Yeah, so what?" said the other two. Ernie answered, "How does it know?"

The misconceptions stated in the OP have now become one of the subjects in my "Telescopes, Eyepieces and Optics" class I teach at the observatory when I cover total brightness and surface brightness as they relate to point-like and extended objects.

### #182 Starman1

Here are my thought experiments:

1. If telescopes are capable of increasing the brightness of non-point objects beyond what the human eye can see, at what aperture will the North America Nebula become so bright it causes you to enter 100% photopic vision and present itself as a firey red nebula?

2. If telescopes are capable of increasing the brightness of non-point objects beyond what the human eye can see, at what aperture will viewing daytime objects become so bright they are painful to look at?

3. If telescopes are capable of increasing the brightness of non-point objects beyond what the human eye can see, at what aperture will viewing the moon become physically dangerous?

The answer to all three is "none", and every single observer in this thread should know this instinctively just from sheer observing experience.

1) I don't know about the North America Nebula, but M42 in my scope is bright enough to damage my night vision and my vision becomes mesopic, allowing me to see lots of color tints.

Naked eye, it's quite faint. but visible.

2) Can't say--who looks at a daylight image with a 30" scope using a 7mm exit pupil?

3) I can stare at the full moon all the time without feeling my pupils constrict. It's bright, but not that bright. Through a 12.5" scope, though, it is so bright it is painful, even with pupils constricted, and my eye starts tearing up. When I look away, I have so dazzled my eye I cannot see for many seconds. That never occurs without a telescope. It is the equivalent of staring directly at a 60W lightbulb from a few inches away.

The Moon is *obviously* significantly brighter in the telescope. As well as significantly larger.

So my observing experience says the answer is not "none".

### #183 B l a k S t a r

you observe an extended object with max 7mm pupil with 30% of the FOV background sky. Then pan to fill the FOV completely with the extended object and no background sky. Average luminance is unchanged, but total luminance is increased by 30% with more photons (cowbell). So it becomes brighter.

but, if you observe an extended object of higher average luminance than the first example it will look brighter, and again as the FOV of the target saturates.

Are these extended objects made brighter than naked eye? I'm feeling the answer is no, it is contrast and resolution that allow increased visibility allowable by the luminance of the extended object.

Lastly, I thought it well established in this thread and in fact that point source stars and extended objects do indeed behave or represent differently in telescopic views by their nature.

Ok lastly again, glad to see this thread take another breath!

### #184 CrazyPanda

1) I don't know about the North America Nebula, but M42 in my scope is bright enough to damage my night vision and my vision becomes mesopic, allowing me to see lots of color tints.
Naked eye, it's quite faint. but visible.
2) Can't say--who looks at a daylight image with a 30" scope using a 7mm exit pupil?
3) I can stare at the full moon all the time without feeling my pupils constrict. It's bright, but not that bright. Through a 12.5" scope, though, it is so bright it is painful, even with pupils constricted, and my eye starts tearing up. When I look away, I have so dazzled my eye I cannot see for many seconds. That never occurs without a telescope. It is the equivalent of staring directly at a 60W lightbulb from a few inches away.
The Moon is *obviously* significantly brighter in the telescope. As well as significantly larger.

So my observing experience says the answer is not "none".

At the next star party look at M42 at the same magnification (or hell, any magnification) in a 30” as you do in your 12.5” and let me know if you feel M42 shows color 5.6x more intensely.

Look at some daytime object in the 30” at a 7mm exit pupil and tell me if it looks exceedingly brighter than the naked eye view. He’ll, just point it at the blue sky and see if it makes the blue sky several thousand times brighter than the naked eye.

And no, the moon is not brighter. It is larger, at the same surface brightness. It has exactly the same intensity as the naked eye sees it. It’s “painfully bright” because it’s hitting a larger area on your retina, and your retina is dark adapted. Look at the moon in daylight in a 30” telescope when your retina is not dark adapted. Will it be painful then? No. The answer will be no. Why? Because your eyes are not dark adapted, not because the moon is somehow several thousand times brighter

Edited by CrazyPanda, 21 March 2020 - 12:52 PM.

### #185 CrazyPanda

And you continue to contend that telescopes do not make extended objects brighter.

I’m going to say this once more as clearly as I can. A telescope does not make *surface brightness* brighter. That is a fact I hope we can agree upon.

It does increase *total brightness*. This is logical since surface brightness is the same, but there is a bigger area. There is more light in total. That is another fact I hope we can agree upon. I have never ever said anything to the contrary.

However, what I have said is total brightness is a useless, irrelevant measure. The same way integrated magnitude of an extended object is also a useless, irrelevant measure - the example I gave was M42 vs NAN. Both mag 4, but ask 10,000 people which appears brighter in a telescope, they’re all going to say M42 does. Why? M42 has higher surface brightness (yet another fact I hope we can agree upon). The light from M42 is literally more intense than NAN.

The intensity of light, which can be described as how strongly the light “tickles” a given photoreceptor in your eye (just like an F5 telescope will hit a give camera pixel with 4x more light than F10), is 100% determined by the surface brightness of the point on the object that hits that photoreceptor. Want to activate a cone so you can see color? You need more light intensity, therefore you need higher surface brightness.

TO ME, therefore, I don’t give two rat’s ***es about integrated magnitude or total brightness. It’s a fundamentally flawed measure that does nothing useful to describe the properties of the object you’re viewing. So when I say “brightness”, always mean “surface brightness”. I use the terms interchangeably because to me “total brightness” might well be the same as describing an extended object in terms of leprechauns. Literally a useless measure of extended objects.

So when I say a telescope doesn’t make extended objects brighter, I mean it does not increase their surface brightness, which is the only *practical* measure of brightness to concern to yourself with for this general class of objects.

There are no misconceptions, only misinterpretations of what I have been saying

Edited by CrazyPanda, 21 March 2020 - 01:46 PM.

### #186 Starman1

At the next star party look at M42 at the same magnification (or hell, any magnification) in a 30” as you do in your 12.5” and let me know if you feel M42 shows color 5.6x more intensely.

Look at some daytime object in the 30” at a 7mm exit pupil and tell me if it looks exceedingly brighter than the naked eye view. He’ll, just point it at the blue sky and see if it makes the blue sky several thousand times brighter than the naked eye.

And no, the moon is not brighter. It is larger, at the same surface brightness. It has exactly the same intensity as the naked eye sees it. It’s “painfully bright” because it’s hitting a larger area on your retina, and your retina is dark adapted. Look at the moon in daylight in a 30” telescope when your retina is not dark adapted. Will it be painful then? No. The answer will be no. Why? Because your eyes are not dark adapted, not because the moon is somehow several thousand times brighter

Already done it, comparing M42 in an 8", 12.5", 20" and 32" on the same night at the same hour.

In the 32", the red colors were intense and the palette seen resembled a photograph. Everyone commented on it.

In the 20", the colors were there, but very subdued.

In the 12.5", they became slight tints to grey, as in bluish-grey, reddish-grey, etc.

In the 8", no color except a slight greenish tint to the central region..

Average exit pupil was 2-3mm in each scope, but not exactly the same.

I've looked at the moon in daylight and it is still painfully bright in the scope.

My pupils contract to tiny points. My eye is less dazzled than at night, of course, because the retina has lost a lot of sensitivity.

But it still makes my pupils contract painfully to block out the light.

The magnitude of the full moon is -12.5 and you can stare at it all day long without discomfort.

I can look away from the Moon, and I haven't lost much night vision. And I don't feel my pupils constrict when looking at it.

The magnitude of the sun is -26.7 and that is far too bright to look at it for more than a brief second.

I'd say the full moon in the scope falls in between those two.

### #187 CrazyPanda

Already done it, comparing M42 in an 8", 12.5", 20" and 32" on the same night at the same hour.
In the 32", the red colors were intense and the palette seen resembled a photograph. Everyone commented on it.
In the 20", the colors were there, but very subdued.
In the 12.5", they became slight tints to grey, as in bluish-grey, reddish-grey, etc.
In the 8", no color except a slight greenish tint to the central region..
Average exit pupil was 2-3mm in each scope, but not exactly the same.

I've looked at the moon in daylight and it is still painfully bright in the scope.
My pupils contract to tiny points. My eye is less dazzled than at night, of course, because the retina has lost a lot of sensitivity.
But it still makes my pupils contract painfully to block out the light.
Try it.

The magnitude of the full moon is -12.5 and you can stare at it all day long without discomfort.
I can look away from the Moon, and I haven't lost much night vision. And I don't feel my pupils constrict when looking at it.
The magnitude of the sun is -26.7 and that is far too bright to look at it for more than a brief second.
I'd say the full moon in the scope falls in between those two.

Funny. M42 looks the same brightness to me in my 8, 12, and 15 scopes. Same general shade of gray in all three at the same exit pupil. Only difference is size. I would have thought that at 3,000x the light gathering power of my eye, M42 would be blindingly bright red/pink in the 15”. That’s not the case though.

I don’t doubt the 30” at a given exit pupil is going to make the bright core of the nebula fill a greater percentage of the FOV, but the surface brightness of any given point in that core has not changed.

Edited by CrazyPanda, 21 March 2020 - 01:48 PM.

### #188 Redbetter

Look at some daytime object in the 30” at a 7mm exit pupil and tell me if it looks exceedingly brighter than the naked eye view. He’ll, just point it at the blue sky and see if it makes the blue sky several thousand times brighter than the naked eye.

And no, the moon is not brighter. It is larger, at the same surface brightness. It has exactly the same intensity as the naked eye sees it. It’s “painfully bright” because it’s hitting a larger area on your retina, and your retina is dark adapted. Look at the moon in daylight in a 30” telescope when your retina is not dark adapted. Will it be painful then? No. The answer will be no. Why? Because your eyes are not dark adapted, not because the moon is somehow several thousand times brighter

Once again, that is 100% factually incorrect. If you understood the difference between brightness and surface brightness you would not be conflating the two this way. Surface brightness is the brightness divided by the apparent surface area. When viewed through a larger aperture the extended object is both larger and brighter, even if the pupil and therefore apparent surface brightness are the same.

This whole rant is an example of misapplying the real lessons to be learned from surface brightness (both intrinsic and apparent.) It is fair to point out that a telescope's function is more than just gathering light, but instead this has been taken to the opposite extreme of declaring that its purpose is solely to magnify and that the brightness is the same--which it is NOT. Replacing one mistake for another doesn't result in the correct answer.

We can't really have productive discussions about the changes in apparent surface brightness, because brightness is being dismissed as being the same or irrelevant.

### #189 Saravanja

Arrg. Long post got lost hitting back button.

The formulas found online that show telescopes are incapable of increasing surface brightness compared to the human eye are simply wrong. They fail the basic test for conservation of energy.

If we have 1 photon per square cm and a 100cm squared objective, it captured 100 photons.

If the human eye is dilated to 1cm squared, it captured 1 photon.

Now focus that large lens till it makes a 1cm squared area. It now has 100 photons inside that 1cm squared area.

Now, place the eye such that it recieves that 1cm squared cone. It now has 100 photons entering the eye.

The conservation of energy is preserved. The surface brightness is increased when compared to the naked eye.

Where most go wrong is the next step. They assume if the image gets magnified, it reduces surface brightness by 1/magnification.

The exit pupil is just an area. A very specific area. If the ratio of apertures squared is 1:10, most assume that all light is reduced to 10%. That is not correct. It just means that only 10% of the objectives exit cone is used. Lenses pass through light, they do not divide it by some magic.

What happens is that 10% of the area around the center of the fov is passed through at whatever transmission efficiency the optical system has. It does not mean the photon count is reduced to 10%.

That 10% area, just happens to also be what we see in the eyepiece.

The net result is that the telescope increases the surface brightness by the ratio of apertures squared when compared to the naked eye. Because otherwise, the conservation of energy would not be preserved.

So all images brightness (points of light and extended objects) are increased by the same amount. Regardless of magnification. When we magnify, we spread that brighter image over a larger area, however, it cannot reduce the increased surface brightness. Making it larger will however reduce the apparent brightness. Think of it this way, magnification does not divide the image brightness, it just examines a smaller section of a brighter image. It loses brightness as we increase magnification because it looks at smaller areas. Less area means less light. Again, conservation of energy in practice.

Having a larger objective does not increase the surface brightness of objects, but, it does increase the surface brightness compared to the naked eye, because of the conservation of energy.

Magnification does not reduce surface brightness, it spreads the light over a larger area, making it dimmer. Thus preserving the conservation of energy.

The equations showing that no telescope can increase the surface brightness greater than that of the naked eye are simply wrong. They DO NOT preserve the conservation of energy. It is a trivial mistake, but one that seems to keep persisting.

If the math doesn't preserve the conservation of energy, it is wrong. No ifs, no buts.

Oh, yes, moonlight can blind you. To find out how big an objective you need, follow the conservation of energy. Don't forget to take the exit pupil(area) into account.

So yes, you will see colors in dso's once you get enough photons. It's all about those photons.

## Rant: busting the myth that the purpose of a telescope is to collect light

Well it looks like this gem of a thread remains unresolved and contains several off-axis viewpoints eluding concensus. It has been quite illuminating at times, shining a light on how all of this telescope viewing and eyesight works together in fact, and perception. Perhaps higher magnification will shed a better light on it.

I've learned much and investigated more in a quest to understand this. What a wonderful topic to be presented in say S&T or somewhere. Perhaps all of the parameters at play elude most and yet I can't help but wonder, surely there is a single unifying treatise out there that is unambiguous and scientifically sound.

Let the debate resume!

The "debate" was over many pages ago. Only one person, the OP, has been maintaining that surface brightness was the only aspect that mattered, and that total magnitude (brightness) was irrelevant. It is a faulty basis, no better than the one the OP claims to be debunking, and in many ways it is actually worse. Replacing one error for another is no way to understand or explain a subject.

Extremes rarely adequately define something, unless that something itself occupies an extreme of the range and provides the definition in that manner. What is seen visually with a scope does not meet such a criteria. Instead, there is an interplay of different aspects: light gathering, magnification, resolution, the scotopic/mesopic/photopic range one is operating in, etc. Learning how to use each is how one can get the most out of observation with any aperture from naked eye to the largest scope available.

There have been good discussions about surface brightness and the limitations of scopes and aperture in other threads, but this isn't one of them. This is a distraction from a real discussion. To me that it is unfortunate because it has not really been possible to deal with mistakes made regarding surface brightness by others, because I have had to waste my time in the thread addressing the silliness of claiming total magnitude (brightness) is irrelevant or equivalent. And unfortunately, addressing that nonsense has been more pressing than other things I would otherwise gladly take issue with.

I enjoyed and learned from discussions with Glenn LeDrew and agree with his analysis. But this isn't an example of that, rather it is a perversion of it.

Keep in mind that I have used various scopes from 60mm to 20" to explore the limits of scotopic contrast detection in dark skies, finding my own limit somewhere past 28 MPSAS in all of them, where my ability to detect the field stop fully around the eyepiece becomes spotty. (I have been able to confirm this limit with threshold studies from the WWII time frame that achieved similar results with the better young observers.) I have noticed and examined shadows cast by Jupiter and Sirius under dark skies. when I wasn't looking for or expecting them. I have seen Barnard's Loop naked eye, and-when conditions allow, I routinely track the zodiacal band from the zodiacal light through the gegenshein across the sky when conditions support it. There are others more skilled or with more innate acuity/talent, but I appreciate surface brightness and contrast more than most.

Yet surface brightness remains only a part of what defines the visibility of an object. At/very near the limit of conditions, surface brightness is the most important factor. Away from the limits it is not that important, and is secondary. If surface brightness is not at a threshold, then total brightness (magnitude) is the primary consideration--which is the vast majority of the time. Most of the descriptions in this thread by the OP are nowhere near the limits of detection.

## Exit Pupil, Brightness, and Stars

I think this is a situation where there theory is interesting but one has to look at the underlying assumptions. One assumption is that at all but the smallest exit pupils, stars behave as points because the eye cannot resolve the Airy disk. An interesting assumption but as you and others have pointed out, stars are not necessarily seen as points at moderate to large exit pupils, as magnification is increased, they actually get smaller..

I think the workings of the eye are most important but the most complex. Basic optics can provide qualitative guidelines and limits but the eye's response makes quantitative numbers suspect.

What does experience tell me/us.. M103 in a 3 or 4 inch telescope it a beautiful small cluster with tiny stars that can pop in and out with averted vision. In a 25 inch, they are nominally about 40 times brighter, 4 magnitudes, than the 4 inch and this beautifully subtle cluster is now in your face bright and not in the least bit subtle.

### #27 Tony Flanders

Doubling the diameter of a scope will always increase the limiting magnitude by a mere 1.5 .

### #28 Starman1

Just for another visual effect on scope size, go the the host's site, Astronomics. I think it's in one of their learning pages? They show what M13 would look like through different size scopes. A 6" almost stars to resolve some stars on the cluster, but more likely it starts with an 8" scope.

I think you mean the ones in the middle of the page here:

Those charts are a little misleading because they are heavily influenced by light pollution and magnification.

At high powers, M13 in a good 12.5" looks more like the image for a 20" for example.

Here are some specs for M13:

Total Integrated Magnitude: 5.8 (naked eye)

Magnitude of brightest stars: 11.9 (visible in a 2" scope)

Magnitude of Horizontal Branch (where the bulk of the stars reside): 15.0 (reachable with 8" in dark skies)

Size at magnitude 22.0 isophote (typical 3-4" scope): 10'

Size at magnitude 25.0 isophote (typical 15-20" scope): 20'

A 6" will show stars across the cluster, but not resolve the multitude of really faint ones. The view will be "grainy" with quite a number of resolved stars.

An 8" will be similar though the number of resolved stars will increase tremendously and the graininess of the background will only be confined to the core.

A 12" will resolve stars to the very center if seeing and optical quality allows.

Edited by Starman1, 29 August 2014 - 11:47 AM.

### #29 Steve OK

Doubling the diameter of a scope will always increase the limiting magnitude by a mere 1.5 .

I don't know why I ever bother posting.

### #30 areyoukiddingme

I suspect the difference is attributable to the nature of human perception of brightness.

Weber-Fechner law (the general area is psychophysics, it's a field that German physicists put together over 100 years ago).

### #31 Asbytec

Jon, sure, the eye is a crazy thing. At higher magnifications, stars get smaller? Not sure I follow, but maybe due to their radiance, seeing and such, they look brighter and larger. That would make sense. I rarely ever view bright stars at low power or even use low power much at all, so I'm just not familiar with how they look in terms of relative size.

I would guess the reason they do is because we're packing the same energy into an increasingly smaller disc (and ring) structure. As I remember, Sidgwick was not specific at which point stars begin to act as extended sources as viewed by the eye. I'd think the diffraction pattern is always an extended object regardless of magnification.

It's intuitive in you cluster examples stars would be much brighter in larger apertures and dimmer with averted vision in smaller apertures. I'm not sure what you're illustrating other than a larger surface area collecting more light.

I think star images are never point sources so they behave like extended objects. Even if the eye cannot see their apparent angular dimension, maybe we can infer it because they are brighter at lower magnifications.

### #32 Jon Isaacs

Just ask yourself this question:

Why is it we see brighter stars as larger? There must be many reasons but it is something we all accept as fact without questioning it. One look at Rigel and its companion tells us it's definitely the case.

### #33 Mentor

Doubling the diameter of a scope will always increase the limiting magnitude by a mere 1.5 .

I don't know why I ever bother posting.

LOL this place does seem to be stocked full of argumentative contrarians, doesn't it?

### #34 Starman1

Norms:

Just ask yourself this question:

Why is it we see brighter stars as larger? There must be many reasons but it is something we all accept as fact without questioning it. One look at Rigel and its companion tells us it's definitely the case.

Jon

Jon, this is because the brain identifies brighter as larger. Look at the new crescent moon in a telescope--the crescent and the earth-lit part are the same diameter. Look at it with the 1 power naked eye, where the sunlit part of the Moon is brighter--the sunlit part appears larger than the earthlit part. This is an optical illusion. Vega and 52 Cygni are the same size to the eye, just not to the brain.

### #35 Starman1

Jon, sure, the eye is a crazy thing. At higher magnifications, stars get smaller? Not sure I follow, but maybe due to their radiance, seeing and such, they look brighter and larger. That would make sense. I rarely ever view bright stars at low power or even use low power much at all, so I'm just not familiar with how they look in terms of relative size.

I would guess the reason they do is because we're packing the same energy into an increasingly smaller disc (and ring) structure. As I remember, Sidgwick was not specific at which point stars begin to act as extended sources as viewed by the eye. I'd think the diffraction pattern is always an extended object regardless of magnification.

It's intuitive in you cluster examples stars would be much brighter in larger apertures and dimmer with averted vision in smaller apertures. I'm not sure what you're illustrating other than a larger surface area collecting more light.

I think star images are never point sources so they behave like extended objects. Even if the eye cannot see their apparent angular dimension, maybe we can infer it because they are brighter at lower magnifications.

Nope. Double stars of exceedingly close separations become more separable with magnification. Resolution continues to improve as magnification is added because the separation grows, but the apparent size of the star does not. Once the stars (i.e. the Airy Discs) achieve a "size" to the eye, no further improvement in resolution occurs because the "spurious discs" get larger with magnification and the separation doesn't appear to get larger. There is not universal agreement on this, but it seems that about 1x/mm of aperture is the point where this starts to occur. From this point up, no improvement in resolution occurs, but you may see more in the way of details on extended surfaces because the details grow larger and the eye/brain combination notices larger things more easily than smaller.

So, stars ARE essentially point objects up to the size where the spurious discs become visible.

### #36 GlennLeDrew

From one of the argumentative contrarians.

The bulk of most globular clusters' reside on the main sequence, not the horizontal branch. Now, of those stars detectible in amateur scopes, then yes, there are often more HB stars than giants.

Why would a larger aperture detect to a fainter isophote? If that isophotal diameter is above the size detection threshold in the smaller scope, it will be detected. A 20' diameter isophote in a 3" aperture at a 6mm exit pupil (13X) subtends 260', or 4.3 degrees. This is sufficient to detect to lowest contrast. Of course, the very dominant brighter core region of most globulars could well 'swamp' the much dimmer outer halo, not to mention the presence of the scattered outer giants. I think you were being specific to M13's case, but I thought I'd toss this more general info into it for clarification.

### #37 Asbytec

Norms:

Just ask yourself this question:

Why is it we see brighter stars as larger? There must be many reasons but it is something we all accept as fact without questioning it. One look at Rigel and its companion tells us it's definitely the case.

Jon

Jon, this is because the brain identifies brighter as larger. Look at the new crescent moon in a telescope--the crescent and the earth-lit part are the same diameter. Look at it with the 1 power naked eye, where the sunlit part of the Moon is brighter--the sunlit part appears larger than the earthlit part. This is an optical illusion. Vega and 52 Cygni are the same size to the eye, just not to the brain.

Unless I am misunderstanding Jon, I think this is what he is saying. 52 Cygni might appear to be the same size when both Airy discs are visible, or maybe in that small range of magnification when the first ring is just visible. Surely Vega spews out a lot of light at low power appearing larger than a less radiant star, even though it really is not larger at all in the strictest sense.

Still, Sirius appears larger than dimmer stars when the Airy disc is visible, to me. probably due to the same illusion (and maybe because the central disc is actually smaller for dimmer stars not peaking above the visible threshold as much.) At low power, Sirius is an overwhelming blaze of light with an apparent diameter (even if the disc itself is not resolved by the eye.) And it would certainly appear "larger" than a much fainter star simply due to its radiance.

Again, I still think the apparent angular size of the Airy pattern (magnification) and the amount of energy in it (aperture) are related by unit area of the image and surface area of the aperture. This happens when the star image is large and the pattern clearly seen at small exit pupils and when the the star image is very small at large exit pupils (smaller than the iris.)

When the exit pupil is large enough to loose acuity, the image is still behaving like an extended object (because it really is an extended image of a point source diffracted on the focal plan) becoming brighter with smaller apparent diameter at lower magnifications. So, at low magnifications, stars should "look" brighter by squeezing the same energy into a smaller area.

The effect of higher magnification making dimmer stars more visible has to do with. uh oh, getting confused real time, darker background? Magnification darkens the background because the background is an extended object. The contrast between a dim star and the dark sky is unchanged, but the star's image is larger and less bright because of it?

By the way, I agree with comments about magnifying low contrast detail a bit more than 1mm exit pupil, but not much more. say larger than 0.5mm and less than 1mm works for me on low contrast planetary detail. Maybe a small bright, higher contrast planetary nebula can use a smaller exit pupil to good effect.

#### Attached Thumbnails

Edited by Asbytec, 29 August 2014 - 11:58 PM.

### #38 Jon Isaacs

Norms:

Just ask yourself this question:

Why is it we see brighter stars as larger? There must be many reasons but it is something we all accept as fact without questioning it. One look at Rigel and its companion tells us it's definitely the case.

Jon

Jon, this is because the brain identifies brighter as larger. Look at the new crescent moon in a telescope--the crescent and the earth-lit part are the same diameter. Look at it with the 1 power naked eye, where the sunlit part of the Moon is brighter--the sunlit part appears larger than the earthlit part. This is an optical illusion. Vega and 52 Cygni are the same size to the eye, just not to the brain.

I disagree. The physical size of Rigel A and Rigel B are clearly different and its not just brain, one is a tiny pin point, one is quite large and may be quite aberrated. This same effect is seen in photographs, brighter stars are larger.

Regarding closely separated double stars: At the Dawes limit, the central disks of the two stars are overlapping and the observer is looking for a small minima, supposedly only a 5% drop in brightness, this appears as a thin, dark line. In order to see that minima and make the split magnifications sufficient to view the Airy disks as extended objects are necessary. This can require magnifications on the order of 80X/inch.

### #39 Asbytec

But are smaller stars (images a less magnification) brighter? Hence larger?

### #40 Starman1

The FWHM width of the Airy disc for an exceptionally bright star should be the same as the FWHM for a faint star.

The spurious disc in the center of the Airy disc should vary in size, however.

The resolution of the eye sees both as points until magnification is high enough the spurious disc has become large enough to NOT be seen as a point.

I seem to recall a size of 1', but I think it varies a bit, just as visual acuity varies.

Hence, at low powers, the eye sees a bright star as larger, assuming good seeing, because of the brain's optical illusion, NOT because the star is

actually larger in appearance. On paper, the spurious disc is larger, but we simply cannot see it or resolve it at low power.

The "Old Moon in the arms of the New" illusion is a powerful reminder of how the brain works.

Once magnification is high enough to see the spurious discs with size to them, they act like extended objects--further magnification makes them fainter

and whatever contrast they have with the sky will not improve. We see fainter stars at powers higher than that because their spurious discs are smaller

and we cannot resolve a size in them until they get large enough with magnification. I.e. the discs stay points while the background sky, an extended object,

What I am unclear on is whether the actual size of the Airy disc, which includes the spurious disc and half of the dark ring in between the spurious disc and the first ring,

is exactly the same size for stars of all magnitudes in a particular scope. I've read that the size of the Air disc is determined by the aperture and f/ratio,

not the magnitude of the star. If that is the case, and I believe it is, fainter stars appear smaller (once we can resolve the spurious disc) because less of the central

peak rises above the threshold of visibility.

### #41 Starman1

But are smaller stars (images a less magnification) brighter? Hence larger?

In photographs, bright stars will bleed over to surrounding pixels due to turbulence, scintillation, and pixel saturation. The better the seeing, the smaller the bright stars appear in photographs.

And though the spurious discs of stars do have a size to them, our eye sees them as points well up into the magnification range. They will not appear brighter at low power than mid power because the eye sees both as a point. Once we see the spurious discs with a size, then they would obey the brightness per unit area rule of extended objects as the magnification is further increased.

### #42 Asbytec

Don, as I understand it, and please correct me if I am wrong.

Both points are correct, IMV. As you say, that small difference in spurious disc makes little difference to the Star's apparent diameter at modest magnifications depending on acuity, as you say, for middling to bright stars, anyway. I am not sure this is what Jon is (and I am) getting at.

I suspect the key here is star images are always point sources on the focal plane, whether the eye can see them or not. At higher powers the eye can witness the enlarged surface area and apparent dimming just as it would any extended object. At low powers. the image is still an extended object the eye cannot resolve. But we can witness it's behavior as it gets brighter, apparently, by squeezing the same energy into an unresolved, smaller extended object.

A larger and half larger aperture will exhibit this same behavior thus the ratio of brightness between them remains unchanged. One the star begins to brighten, it will radiate a more intense light that the eye observes as a larger "point" source. Some of this may be an illusion like the lit part of the moon. I know Sirius gives the impression it is larger when we can actually observe it's Airy disc. When we cannot, it's a brilliant ball of light that does appear larger. That may be due to it's radiance or an optical illusion, or both.

Not withstanding the above, this sounds reasonable. As higher magnification should actually dim the star per unit area magnified. But, the eye response is to detect this faint speck against the same contrast ratio against the background. I think that's correct. ironing out the differences between what the scope puts up and what we can observe.

Yes, the aperture determines the actual angular diameter of the Airy disc and the focal ratio determines the apparent angular diameter. If less of the central peak rises above the visible threshold (along with the rings, too, disappearing at some middling magnitude), then the star is less radiant and appears dimmer and smaller than FWHM. The aperture does not gather enough light to make it brighter.

### #43 Starman1

"I suspect the key here is star images are always point sources on the focal plane, whether the eye can see them or not. At higher powers the eye can witness the enlarged surface area and apparent dimming just as it would any extended object. At low powers. the image is still an extended object the eye cannot resolve. But we can witness it's behavior as it gets brighter, apparently, by squeezing the same energy into an unresolved, smaller extended object."

I don't think so. To the eye, the same amount of light is in that point until the spurious disc gets large enough to see.

Ergo, the star's apparent brightness will be the same from the lowest magnification up to that point. Whether the star's spurious disc is 1/4 as big or 4X as big,

if the eye sees it as a point, the brightness per unit area will be the same.

I see the stars as the same brightness from lowest power up to the point where the spurious disc becomes visible and it starts acting like an extended object.

And then, though the unit area brightness may decrease, it's apparent brightness may not because the contrast with the background sky will remain constant

as magnification increases.

### #44 Pinbout

vlad has a cool graphic, fig 18 showing "Illustration of the resolution concept based on the foveal cone size." half way down the page

Edited by Pinbout, 30 August 2014 - 03:11 PM.

### #45 Jon Isaacs

But are smaller stars (images a less magnification) brighter? Hence larger?

In photographs, bright stars will bleed over to surrounding pixels due to turbulence, scintillation, and pixel saturation. The better the seeing, the smaller the bright stars appear in photographs.

And though the spurious discs of stars do have a size to them, our eye sees them as points well up into the magnification range. They will not appear brighter at low power than mid power because the eye sees both as a point. Once we see the spurious discs with a size, then they would obey the brightness per unit area rule of extended objects as the magnification is further increased.

I do not believe the eye or at least my eye sees the of brighter stars as single points at large exit pupils. If they were single points, they would have no dimension. Dim stars definitely appear smaller than bright ones. This is so fundamental to our experience that we accept it without questioning it, without understanding it.

I suggest that the eye experiences many of the same issues as a camera. There is also the issue of the resolving power of the eye's lens as function of focal ratio.

Danny's link to Vlad's pages is quite enlightening and will take me some time to digest. But at some point, a discussion like this needs to look at what we actually see, what we experience, because there are so many factors involved that in the final analysis, it's the experimental evidence that's most relevant..

To my mind, Rigel is an good starting place.. I can resolve Rigel at under 100x in my 12.5 inch, that's a 3mm+ exit pupil. The primary is much brighter and much larger than the secondary which appears as a pin point. The primary is clearly larger than the Airy disk. Just why is that. I think it's an interesting question with no simple answer..

A previously referred to a set of observations I made viewing Gamma Arietis, an 8 arc-second double. I used 17x in an 80mm. I used masks to reduce the size of the exit pupil and therefore increase the size of the Airy disk. What happened was that with the smaller exit pupils, the stars actually got smaller at until I reached an exit pupil of around 2mm. This is more than just dimming and seeing a smaller part of the Airy disk because one is not seeing the Airy disk, or at least I wasn't at exit pupils in the 2mm-5mm range.

### #46 drollere

OK. now the question. Since the star point should appear as bright regardless of magnification (again given the background black assumption), should not a particular star point viewed in an 8" telescope, appear visually 4x brighter than it does in a 4" telescope, regardless of magnification?

you don't have enough degrees of freedom there, bill. if you double the aperture you can't hold magnification constant without changing the focal ratio and/or the exit pupil. if the exit pupil is just D/M, and you double D but hold M constant . you have to make the image fainter by reducing the exit pupil artificially or by increasing the focal ratio.

aperture delivers illuminance, the amount of light *on* a surface: larger aperture, greater illuminance, period. focal ratio increases the concentration of light because the image grows correspondingly smaller with shorter objective magnification. images are therefore brighter in a larger aperture or a shorter focal ratio. so jim is correct visually, and we have "fast" photographic telescopes of small ƒ ratio. all that is pure physics.

your question mixes physics with psychophysics. "brightness", a *perceptual* quantity, is light emitted from a surface that creates a luminance contrast with the surrounding visual area greater than

10x luminance. ("lightness", or the light or dark of surfaces, is the perception of luminance contrasts less than 10x.) brightness cannot be equated directly with illuminance or luminance, because surround contrast plays a significant role in the perception.

the luminance paradox is that stars viewed with the naked eye appear fainter at larger celestial distances because they have a constant visual size -- the diffraction disk of the eye pupil. this is the "surface area" we see as the light source. magnification of the telescopic diffraction disk must exceed the angular size of the eye's diffraction disk before a brightness decrease would be perceptible. but magnification also decreases the sky brightness, so the contrast ratio remains the same and the apparent brightness remains the same. it's only when we magnify to the point that the sky brightness nears the luminance threshold, and no longer changes with magnification, that we can significantly reduce the stellar brightness by further magnification.

to compare stellar brightness in different apertures requires us to hold exit pupil and magnification constant, which is impossible without changing the ƒ ratio. because it's impossible, we compare images at different illuminances at different angular areas through different exit pupils at different sky background contrasts, which creates a novel perceptual situation each time.

## An old-looking, dusty galaxy in a young universe

This spectacular view from the NASA/ESA Hubble Space Telescope shows the rich galaxy cluster Abell 1689. The huge concentration of mass bends light coming from more distant objects and can increase their total apparent brightness and make them visible. One such object, A1689-zD1, is located in the box — although it is still so faint that it is barely seen in this picture (click for a enlarged view. New observations with ALMA and ESO’s VLT have revealed that this object is a dusty galaxy seen when the universe was just 700 million years old. Image credit: NASA ESA L. Bradley (Johns Hopkins University) R. Bouwens (University of California, Santa Cruz) H. Ford (Johns Hopkins University) and G. Illingworth (University of California, Santa Cruz) A team of astronomers, led by Darach Watson, from the University of Copenhagen used the Very Large Telescope‘s X-shooter instrument along with the Atacama Large Millimetre/submillimetre Array (ALMA) to observe one of the youngest and most remote galaxies ever found. They were surprised to discover a far more evolved system than expected. It had a fraction of dust similar to a very mature galaxy, such as the Milky Way. Such dust is vital to life, because it helps form planets, complex molecules and normal stars.

The target of their observations is called A1689-zD1. It is observable only by virtue of its brightness being amplified more than nine times by a gravitational lens in the form of the spectacular galaxy cluster, Abell 1689, which lies between the young galaxy and the Earth. Without the gravitational boost, the glow from this very faint galaxy would have been too weak to detect.

We are seeing A1689-zD1 when the universe was only about 700 million years old &mdash five percent of its present age. It is a relatively modest system &mdash much less massive and luminous than many other objects that have been studied before at this stage in the early universe and hence a more typical example of a galaxy at that time.

A1689-zD1 is being observed as it was during the period of reionisation, when the earliest stars brought with them a cosmic dawn, illuminating for the first time an immense and transparent universe and ending the extended stagnation of the Dark Ages. Expected to look like a newly formed system, the galaxy surprised the observers with its rich chemical complexity and abundance of interstellar dust.

“After confirming the galaxy’s distance using the VLT,” said Darach Watson, “we realised it had previously been observed with ALMA. We didn’t expect to find much, but I can tell you we were all quite excited when we realised that not only had ALMA observed it, but that there was a clear detection. One of the main goals of the ALMA observatory was to find galaxies in the early universe from their cold gas and dust emissions &mdash and here we had it!”

This galaxy was a cosmic infant &mdash but it proved to be precocious. At this age it would be expected to display a lack of heavier chemical elements &mdash anything heavier than hydrogen and helium, defined in astronomy as metals. These are produced in the bellies of stars and scattered far and wide once the stars explode or otherwise perish. This process needs to be repeated for many stellar generations to produce a significant abundance of the heavier elements such as carbon, oxygen and nitrogen.

This view includes infrared light images from the WFC3 instrument on the NASA/ESA Hubble Space Telescope as well as visible light views. It shows a close up look at part of the rich galaxy cluster Abell 1689 showing galaxy A1689-zD1 as the elongated reddish object in the box. Image credit: ESO/J. Richard Surprisingly, the galaxy A1689-zD1 seemed to be emitting a lot of radiation in the far infrared, indicating that it had already produced many of its stars and significant quantities of metals, and revealed that it not only contained dust, but had a dust-to-gas ratio that was similar to that of much more mature galaxies.

“Although the exact origin of galactic dust remains obscure,” explains Darach Watson, “our findings indicate that its production occurs very rapidly, within only 500 million years of the beginning of star formation in the universe &mdash a very short cosmological time frame, given that most stars live for billions of years.”

The findings suggest A1689-zD1 to have been consistently forming stars at a moderate rate since 560 million years after the Big Bang, or else to have passed through its period of extreme starburst very rapidly before entering a declining state of star formation.

Prior to this result, there had been concerns among astronomers that such distant galaxies would not be detectable in this way, but A1689-zD1 was detected using only brief observations with ALMA.

Kirsten Knudsen (Chalmers University of Technology, Sweden), co-author of the paper, added, “This amazingly dusty galaxy seems to have been in a rush to make its first generations of stars. In the future, ALMA will be able to help us to find more galaxies like this, and learn just what makes them so keen to grow up.”

## How to Combine Images from Multiple Nights

### Use the tabs to group your image sets

Once you’ve got your picture files (lights) and all of your support files loaded into the main group, it’s time to load up your files from night 2. Click on the small Group 1 tab at the bottom left of the screen, and repeat the process for opening files from imaging night 2.

Remember, you can stack different variations of exposures together in Deep Sky Stacker. This means a range of ISO sensitivity and exposure length.

Some imaging sessions may include all 3 supports files to complement the light frames, some may not. This is fine. After all of the image files have been loaded into their respective categories, it is time to register and stack the frames into a single file. Finally, make sure to click “‘check all“, to make sure that all of the frames you have loaded are selected.

Before we click Register and Stack images, let’s take a look at the current default settings.

Accessing the Register and Stacking settings is accessible by clicking “Settings…” under the options tab.

The default settings for registering is set to a 10% star detection threshold. In my experience, the default value of 10% has worked very well for stacking images captured using my 12MP Canon EOS Rebel DSLR. If you decrease the star detection threshold, DSS will detect fainter stars. The number of stars in a given light frame is displayed in the lower half of the screen.

With a light frame selected, look for the #Starscategory.

The following checkboxes should be checked before moving hitting “OK”, and letting DSS begin its process.

• Register already registered pictures
• Automatic detection of hot pixels
• Stack after registering

The DeepSkyStacker website states that the automatic detection of hot pixels only works if using Super-pixel, Bayer Drizzle, bilinear and AHD interpolation modes. However, I leave this box checked regardless and hot-pixels and stacking errors have never been an issue.

## Increased aperture Vs Perceived Brightness

Couple of days ago I had my XT6i out instead of Z8 to take a quick peak at nebulae (Lagoon & Swan) in Sagittarius.

A few days before that I had seen the same nebulae through Z8. To my surprise the nebulae did not look that different in XT6i. The seeing conditions were similar. The magnification was 80X. I was using DGM NPB Nebula filter in both cases. I had my expectations low as 6" gathers lot less light than 8". However my eyes did not see the difference that way. I was pleased with what I was seeing through a 6" scope.

The question is, does the increase/decrease in aperture creates proportionate difference at the eyepiece or your eyes adapt and the difference is not as significant as the math behind it.

### #2 cpper

You said the seeing conditions were similar, how about the transparency ?

What eyepiece were you using with the Z8 and what with the XT6i ?

Edited by cpper, 18 August 2014 - 12:11 PM.

### #3 Fuzzyguy

I recently purchased an 80 ED refractor and I was pleased with the brightness. I was also expecting things to be less bright in the 3" vs my 8" SCT, but that was not the case. Of coarse I was looking at large objects as opposed to small fainter DSO's, but the North America and Veil nebulas were very evident in the little scope. Exit pupil was large though (

4mm) with the 24mm EP and that may have had as much to do with it as anything else.

### #4 GOLGO13

A 6 and 8 inch are fairly close in aperture. However, I'm guessing if you did the comparison at the exact same time you'd see the difference. However, the jump from 6 to 10 would be much more noticeable.

I had a somewhat similar experience comparing my C6 to a Meade 8 inch. I felt my image was a bit sharper in the C6. but the C8 was just a bit brighter, but not by much. I suspect my C6 was working better for some reason. could have been collimation, cooldown, conditions, etc.

Same as when I compared my 4 inch refractor to my XT6 I had (donated it recently to a friend who is lucky he just moved to dark skies). The 4 inch refractor was sharper but the XT6 was just a bit brighter. Not by a large amount but certainly there. If mags are the same then it's simple exit pupil (depending on the object) or light gathering if stars. The XT6 was very good though. and a whole heck of a lot cheaper.

I find it's a few steps higher where you really see the difference. Going from 6 to 10 inch. 10 to 14. etc.

### #5 GlennLeDrew

You've just made an important discovery. Image surface brightness scales as the area of the exit pupil. A 2" aperture or a 20" aperture working at the same exit pupil present images of sky and other extended objects at the same surface brightness. The 10X larger aperture merely provides a 10X larger image. (And stars 100 times fainter, of course, because the light collecting area is 100X greater.)

Furthermore, no telescope can deliver surface brightness higher than can be seen by the unaided eye. Indeed, telescopic surface brightness is always a little less due to non-perfect transmission of light.

If the exit pupil remains fixed, extended objects in the night sky *appear* to have higher surface brightness as aperture increases only because of the increase in area occupied on the retina. This makes for higher total, or integrated brightness, and more importantly more resolvable detail (information.)

Lastly, even the ideal telescope can never improve contrast real telescooes always degrade contrast. The best we can ever hope for is the preservation of contrast. And so boosting magnification does not 'darken the sky and thereby improve contrast.' Sky and extended object are dimmed equally as magnification increases, and contrast remains constant. The increase in image scale delivers more information, which is *perceived* as a gain in contrast.

Edited by GlennLeDrew, 18 August 2014 - 01:07 PM.

### #6 Tony Flanders

Even the ideal telescope can never improve contrast real telescooes always degrade contrast. The best we can ever hope for is the preservation of contrast. And so boosting magnification does not 'darken the sky and thereby improve contrast.' Sky and extended object are dimmed equally as magnification increases, and contrast remains constant. The increase in image scale delivers more information, which is *perceived* as a gain in contrast.

I guess I would have to quarrel with the use of the word "contrast" here. It is true that if you define contrast as the ratio of brightness between your target and the background, then telescopes cannot improve contrast.

But who says that's the proper definition of "contrast?" To my mind, what really matters is perceived contrast, not this artificial definition which cannot in practice be measured without fancy instruments. And telescopes most definitely can and do increase perceived contrast.

### #7 Tony Flanders

A few days before that I had seen the same nebulae through Z8. To my surprise the nebulae did not look that different in XT6i.

I have a theory about this. I find that perceptions of objects don't increase continuously as you increase aperture and/or magnification instead, they progress by quantum jumps.

So, for instance, Saturn's rings look pretty much the same at 20X, 40X, 60X, and so on . until all of a suddent you see Cassini's Division, at which point they look totally different.

Likewise, M51 looks pretty much the same in apertures of 3 inches, 4 inches, 6 inches, and so on . until all of a suddent you see the spiral arms, and then it looks totally different.

### #8 GOLGO13

interesting concept Tony. Maybe looking at M13 at 200x would really show that difference between a 6 inch newt and an 8 inch newt.

That's usually when I see the big differences between aperture.

### #9 GlennLeDrew

First we need to understand how the instrument works in order that we can differentiate between it and and the operration of the observer's visual system. Too often performance is ascribed to the instrument where it's really the interplay between specific conditions of the object under observation and its image as processed in the visual cortex which is responsible.

Perceived contrast changes depend sensitively on object size, brightness and contrast. Under some conditions the changes are rather profound, under others hardly notable. But underneath all this variability imposed by the organic visual system the instrument remains a constant, predictable factor as dictated by the laws of optics.

### #10 Abhat

interesting concept Tony. Maybe looking at M13 at 200x would really show that difference between a 6 inch newt and an 8 inch newt.

That's usually when I see the big differences between aperture.

That's a good point. I think higher magnifications is where the increased aperture comes in handy. For certain DSO and Planetary Details that magnification can make a big difference in seeing critical details. I have always been able to see M13's propeller in Z8 especially at higher magnifications but in XT6i I do struggle to see it.

### #11 Starman1

If we're talking about a profound difference in the visual appearance of an object in a scope, I go by the "1 Magnitude Rule".

I see a profoundly different view of an object if I change the aperture enough to gain a full magnitude.

That's 6" to 10" to 16" to 25"

Or, 5" to 8" to 12.5" to 20" to 32"

I find it interesting that these progressions also conform to a lot of common sizes for telescopes.

Sure, of course any increase in aperture will make some difference.

But if you want a profound difference, make it a magnitude gain.

### #12 Pinbout

The increase in image scale delivers more information, which is *perceived* as a gain in contrast.

the contrast slope becomes slower, wider. the ration between white and black is still whatever, but the slope between them is lengthened.

the MTF isnt the tool to express this. you need a different chart like gama or average gradient to express the longer slower slope that holds the increase in information.

and when you go from a large telescope to a smaller telescope you speed up, or compress the contrast slope so it looses information.

### #13 Jon Isaacs

Even the ideal telescope can never improve contrast real telescooes always degrade contrast. The best we can ever hope for is the preservation of contrast. And so boosting magnification does not 'darken the sky and thereby improve contrast.' Sky and extended object are dimmed equally as magnification increases, and contrast remains constant. The increase in image scale delivers more information, which is *perceived* as a gain in contrast.

I guess I would have to quarrel with the use of the word "contrast" here. It is true that if you define contrast as the ratio of brightness between your target and the background, then telescopes cannot improve contrast.

But who says that's the proper definition of "contrast?" To my mind, what really matters is perceived contrast, not this artificial definition which cannot in practice be measured without fancy instruments. And telescopes most definitely can and do increase perceived contrast.

Contrast does have a specific meaning and not an artificial definition, the ratio of two brightnesses. If you want to use some phrase such as "perceived contrast", that is reasonable but you really can't have it both ways because "perceived contrast" is still contrast, the ratio of two brightnesses.. But difference is, rather than being the ratio of two brightnesses, it's the observers interpretation of what that must look like. But in reality, when you increase magnification, the reason it is more easily perceived is not because the actual contrast has changed but rather because of the larger image size on the retina covering more rods.

To my mind what really matters in understanding what you are seeing and why you are seeing it. It's worth understanding that increasing the magnification may make an object more easily seen because the image covers more of the retina. With understanding can come reinterpretation of ones perceptions so they more accurately reflect what one is actually seeing.

Personally it took me many years to understand the concept of contrast because it was misused so often, there's just a lot of confusion. Getting the basics straight from the beginning, that's the way to keep simple concepts simple.

### #14 havasman

With understanding can come reinterpretation of ones perceptions

That pretty much covers a lot of ground very elegantly.

Edited by havasman, 19 August 2014 - 02:10 AM.

### #15 GlennLeDrew

In what respect does the 'contrast slope' vary with aperture? Do you mean spatially, as related to image scale on the retina? If so, how would a sharp-edged disk of uniform surface brightness be considered? At exit pupils larger than about 1mm, all views are of a sharp transition from darker sky to brighter disk, and so there is no 'slope' in the brightness transition. Yet the perceived contrast can vary as its size is manipulated via magnification/aperture change.

### #16 Illinois

I compare to my 6 inch and 10 inch. Nebula and galaxies are noticeable that 10 inch is better. I sold my 10 inch and bought 16 inch!

### #17 cpper

I read some time ago this article : http://starizona.com. ing_theory.aspx. Very nice and informative, I learned a lot. An on topic paragraph:

"A more complex theory for the best deep-sky magnification is based on the human eye's response to contrast. The contrast between the sky background and a faint celestial object is critical for observation. An object whose surface brightness is such that the contrast between it and the sky is below the eye's detection threshold will be invisible. Observers have noted that low-contrast objects are often easier to detect at higher magnifications. It is often assumed that this is due to higher power increasing contrast, but this is not true. The relative contrast between the object and the sky background is unchanged by magnification (each is affected equally). However, the eye is more sensitive to low-contrast objects when they appear larger."

### #18 Pinbout

If so, how would a sharp-edged disk of uniform surface brightness be considered?

that's a demonstration of visual acuity not contrast sensitivity.

Edited by Pinbout, 19 August 2014 - 01:24 PM.

### #19 Chuck Hards

You don't know Swift from Astrola

Couple of days ago I had my XT6i out instead of Z8 to take a quick peak at nebulae (Lagoon & Swan) in Sagittarius.

A few days before that I had seen the same nebulae through Z8. To my surprise the nebulae did not look that different in XT6i. The seeing conditions were similar. The magnification was 80X. I was using DGM NPB Nebula filter in both cases. I had my expectations low as 6" gathers lot less light than 8". However my eyes did not see the difference that way. I was pleased with what I was seeing through a 6" scope.

The question is, does the increase/decrease in aperture creates proportionate difference at the eyepiece or your eyes adapt and the difference is not as significant as the math behind it.

Coincidentally, I was looking at both the Lagoon and Swan nebulae on Saturday night with a 70" Dob. You may be surprised to hear that the images were not that much brighter than with our club's 32" Cassegrain, with a similar focal-length eyepiece. The 70" has over 100" longer effective focal length than the 32", so the image was larger as the light was spread out over a larger area, but was still bright due to the increased aperture. There was more detail visible in the 70", however. A Lumicon broadband filter was used to combat some light pollution and the increased contrast made the image more pleasing to the eye, but I don't know that more detail was seen with the filter.

### #20 Jon Isaacs

I compare to my 6 inch and 10 inch. Nebula and galaxies are noticeable that 10 inch is better. I sold my 10 inch and bought 16 inch!

Small nebulae and galaxies show more detail in a larger scope for the reasons discussed above, the combination of brightness and magnification. A larger scope, within the limits of the entrance pupil of the eye, (eye's pupil at at least as large as the exit pupil), the image can be brighter at the same magnification, the image can be larger at the same image brightness or some combination of the two. But for large nebulae, they may be more easily seen in a small scope.

Surface brightness: Light per unit area, intensity.. .Stars and extended objects are given visual magnitudes, this is the total amount of light from the object or star. For a star, this is a useful number because a star is a point. For an extended object like an planet, galaxy or nebulae, isn't so useful since the light does not come from a single point, it comes from an area of the sky, the larger the size of the object, the more the light is spread out and the less intense the light is coming from that object. The Andromeda galaxy has a visual magnitude of 3.4 but that light is spread out over an area of about 3 degrees x 1 degrees, clearly that is much more difficult to see than a magnitude 3.4 star. The units of surface brightness are magnitudes per square arc-minute or magnitudes per square arc-second. Andromeda has a surface brightness of about 13.5 magnitudes per square arc-minutes. The surface brightness is a much better measure of how easily seen an object will be.

Exit pupil: The beam of light leaving the eyepiece, what you observe. The exit pupil can be calculated a couple of ways, I think the most intuitive is the aperture divided by the magnification. The greater the magnification, the smaller that beam will be. The brightness of an extended object depends only on the size of the exit pupil, the smaller the exit pupil, the dimmer both the object and the night sky will be. This makes intuitive sense because the smaller the exit pupil represents less light entering your eye. At the higher magnification, the object is larger, the same amount of light is spread out over a larger area, it must be dimmer.. Since the amount of light is related to the area of the exit pupil, the relative brightness of two images is the ratio of the square of the exit pupils. A 200 mm scope at 200x produces a 1mm exit pupil, a 200mm scope at 100x produces a 2mm exit pupil, the image of an extended object like a galaxy is 4 times brighter..

If the exit pupil is larger than the observers dilated eye, the observers entrance pupil, then that added light does no good, the light does not enter the eye. The size of your dilated eye is an individual thing, generally as one grows older, the eye becomes less flexible and is unable to open as far as it does when one is young. Typically a maximum entrance pupil, the observer's dilate eye, is assumed to be 7mm but it varies between about 5mm and 8mm.. You can measure it..

Contrast: Contrast is the ratio of two brightnesses. In deep sky observing, one mostly thinks of contrast as the brightness of the object compared to the brightness of the background sky but it can also be used to compare different parts of an object. A good example of this are the cloud bands on Jupiter, they are bright but they are relatively low contrast, their brightnesses are quite similar.

Since the brightness of an extended object is directly related to the size of the exit pupil, increasing or decreasing the magnification does not change the contrast since both the object and the background sky are changed in proportion. A telescope cannot increase the contrast of an extended object..

The contrast of a star against the background sky is a different issue. A star is a point source of light, when you magnify it (up to about 25X/inch) it does not dim, it's still just a point. The background sky does become dimmer, you are spreading the light out over a larger area.. With the brightness of the star unchanged but the background sky dimmer, the ratio of the two brightnesses is increased and the contrast of the star against the background sky is increased. This is useful to know, increasing the magnification does allow one to see stars more easily, to see faint stars that could not otherwise be seen.

The surface brightness of the object is proportional to the exit pupil, this is independent of the aperture. Since the largest possible exit pupil is the size of your dilated eye, the surface brightness of an image can be not brighter than it is naked eye.. Telescopes magnify objects but they do not make the brighter.

Looking at an extended object, this certainly can be counter-intuitive. Saturday night I was enjoying the Veil nebula in my my 4 inch scope and my 25 inch, the exit pupils were both about 6mm.. the views were quite different, with the big scope, I was looking at a small portion of the Veil, a patch about 0.7 degrees in diameter, object was large and details unseen in the smaller scope were quite apparent. In the small scope, the entire nebulae was seen and nicely framed. It was much smaller.. At first, it seems like the image in the large scope is much more intense but looking back and forth between the two, it is possible to realize that they are equally bright, one is just larger.

In some situations, that larger image scale can actually decrease the contrast, make an object more difficult to see, the object can overflow the field of of view and one has no background sky for comparison..

There is lots to know.. I believe that if one understands basic concepts and how they affect the image, then one can make wiser choices in both purchasing telescopes and eyepieces as well as what to use in a particular situation. Knowing that increasing the magnification does increase the contrast of a star against an extended object suggests that if one wants to see a super nova in a galaxy or the central star in a planetary nebulae, increasing the magnification is a very powerful tool..

Some stuff to consider, I hope this helps those just starting out in this wonderful hobby understand a bit more about their equipment and what they are seeing.