What kind of things I could “see” with an amateur radio telescope?

What kind of things I could “see” with an amateur radio telescope?

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There are apparently not many reasonably priced radio telescopes available for the amateur users. I only could find a SPIDER 230C 2.3 meter diameter compact radio telescope, which costs ~10 k€. Reading the page, I don't get a good overview what kind of things I could "see" with a radio telescope that has a 2.3 meter antenna.

Is the instrument already a (semi-)professional one?

Rather than looking for ready-made systems, take a look at projects. Right now, plenty of amateurs are using software defined radio coupled to various antennas for astronomy. Start here:

And while it has nothing to do with imaging, there's plenty of radio astronomy that amateurs can do using simple (albeit sometimes large) antennas:

From my simplistic analysis, it's not good for much.

For comparison, the first radio telescope was 9 meters.

One of the favorite parts of the spectrum for radio telescopes is the water hole - 21 cm.

From my quick mental arithmetic, this dish would be able to resolve sources of 21 cm signals of they were about 5 degrees apart.

Why Amateur Astronomers Are Important, and How to Become One

For centuries, astronomers have looked to the stars in an effort to better understand the nature of the universe and our place in it. There's also a long history of amateurs contributing to astronomical discoveries.

By one estimate, there may be as many as 300 sextillion stars in the universe. Want to see that written out?

Even with some 7 billion people on Earth, that leaves more than 40 trillion stars per person. In other words, there are far more stars in the universe than professional astronomers could possibly study. Then there are planets, and nebulae, and quasars, and . you get the idea. There's plenty of work to go around.

And you don't need a big radio telescope to make meaningful contributions. In fact, since the size and type of the telescope determine what and where it is capable of seeing, there are important observations that must be made by smaller telescopes. For example, the Werner Schmidt Observatory, run by the Cape Cod Astronomical Society, monitors near-Earth asteroids.

Interested in getting involved in astronomy? Here are a few tips from Dr. Michael West, director of astronomy for the Maria Mitchell Association, and Ed Swiniarsky, secretary of the Cape Cod Astronomical Foundation.

SARA Western Regional Conference in Prescott, Arizona

The Society of Amateur Radio Astronomers (SARA) announces a regional conference will be held at Embry-Riddle Aeronautical University in Prescott, Arizona March 11 to 13, 2016. SARA member Ray Fobes is a Staff Radio Astronomer at the University and is coordinating the event. He can be reached by e-mail at westernconference_at_radio-astronomy_dot_org.

Papers on radio astronomy hardware, software, education, research strategies, philosophy, observing efforts and methods are welcome. Deadlines for submitting a letter of intent, including a proposed title and informal abstract or outline can be found at Call for Papers

    Registration for the 2016 Western Conference is just $60.00. This includes breakfast and lunch Saturday and Sunday. Payment can be made by going to and direct payment to Please include in the comments payment is for the Western Conference. Credit card payment can be made thru

You can mail a check payable to SARA Treasurer at 904 Towering Oak Court, Purcellville, VA 20132 . Please include an e-mail address so a confirmation can be sent upon receipt of payment.

You can receive a reduced hotel rate at the Marriot SpringHill Suites in downtown Prescott, Arizona. When you call, ask for the "Embry-Riddle Friends" rate.

SpringHill Suites by Marriott
200 East Sheldon

SpringHill Suites is located in downtown Prescott, approximately 2 blocks from Historic Downtown Prescott. Studio Suites include a kitchenette with refrigerator, microwave, sink, sitting area and work area. Guest room rate includes complimentary breakfast and free wireless Internet, indoor swimming pool, business center and fitness center all within walking distance to Downtown Prescott Courthouse Shops, Restaurants and Museums.

We would expect that you could leave at early afternoon on Sunday.

Give yourself at least two hours to get to the airport (about 100 miles) but Sunday should be an easy day driving thru Phoenix.

Ray Fobes offers the following information about things to see and do in the Prescott, Arizona area.

As to National parks and scenic beauty - we have lots of it. Of course its of the desert type. Weather in April should be nice, cool and sunny. Watch it rain!!

If its your first time to Arizona I would not miss the Grand Canyon, Sedona, Flagstaff and the Indian areas. Painted Desert, Monument Valley, Zion and Bryce Canyon are also close.

Prescott is the principal town in Yavapai County, the size of New Jersey with only a little over 200,000 population. Prescott is only about 50,000. This link will give you some background on the town:

Embry-Riddle Aeronautical University is about 5 miles north of town. There are several modern motels, a couple of historic hotels downtown and some B&B's that are nice but vintage style.

Meteor Crater is a 35 miles East of Flagstaff. Formally called Barringer Crater it is now a tourist destination (read trap). You can't go into the crater anymore.

A car is necessary to do much sightseeing. If you fly in, fly into Phoenix and either rent a car or take one of the shuttle buses that go directly to Prescott area. About $30 each way I think. Its a two hour drive up I-17 and Route 69 to get to Prescott. You ascend from 1500 feet to 5500 feet elevation so its an interesting trip. We never get tired of the trip unless its weekend traffic.

Some of the astronomical observatories in our area.

Flagstaff Area:
Lowell Observatory
Anderson Mesa
Discovery Channel, 4-meter under construction
US Navel Observatory

Southern Arizona:
Arizona Radio Telescope on Kitt Peak
Kitt Peak - many telescopes
Fred Whipple Observatory, Mt Hopkins
VERITAS gamma-ray telescope, Mt Hopkins
Steward Observatory, UA Tucson
Steward Mirror Facility, UA Tucson

Mt Graham:
Heinrich Hertz Submillimeter Telescope
Vatican Advanced Technology Telescope
Large Binocular Telescope

Radio Astronomy

The intermediate frequency, ( IF ), amplifier is a radio frequency amplifier which processes the output of the mixer. In addition to amplifying the signal, the IF amp usually has some form of bandpass filtering so that only a selected range of frequencies is allowed through. These filters are often of the SAW, crystal lattice, or ceramic varieties. Common, IF frequencies are 70, 45, 21.4, and 10.7 MHz, however there is no restriction to these frequencies. One difference between most radio telescope IF amps and those found in communications receivers is that communications receivers usually employ some form of automatic gain control, (AGC). AGC circuits need to be disabled in communications receivers modified for radio astronomy use as they tend to mask the subtle changes in signal strength we are trying to detect.

The amount of gain needed for the IF amplifier is determined by the signal level exiting from the mixer, the amount lost in the IF amp filter(s) and the appropriate level required for the square law detector which follows. It is sometimes difficult to determine all of these values ahead of time and so a variable attenuator is sometimes introduced at the IF amp output prior to the detector

Radio Telescope Square Law Detectors

If you become a dedicated amateur radio astronomer, you will no doubt hear much talk about "square law detectors". It is interesting that one of the simplest electronic configurations in the radio telescope show draw so much attention, but all of this attention is due to its' very important role. The radio frequency energy exiting from the earlier portions of the receiver alternates in polarity around some central voltage. If we could just hook up a DC (direct current) meter to this signal it would read zero volts because the positive and negative swings of the voltage would cancel each other out. In order to measure the intensity of the signal we thus must throw half of it away! We need a door which only allows passage of the signal in one direction and this is the semiconductor diode. Even the symbol we use for the diode suggests this quality, an arrowhead pointing towards a line.

There are several types of diodes. The types most commonly used by amateur radio telescope makers are the germanium diode and the schottky diode. If we pass just the right range of current through these diodes, the voltage we measure coming out of them will be the square of the input, and thus will be proportional to the power which is fed to them from the receiver. It is the power received by the radio telescope antenna that we want to measure and we owe to detector the ability to measure it. If this is a bit confusing, fear not. You will no doubt have time to sort it out in one of those discussions I refer to above.

DC Processors for Radio Telescopes

Once the radio frequency energy has been converted to a DC signal by the detector, we need to transform it in other ways that make it easier to record. Even though we have tried very hard to not introduce much additional noise from our receiver into the signal, there will usually be much more of this unwanted noise at this point than the actual noise we are trying to measure. In other words the output of the detector will consist of a lot of receiver noise added to a small amount of noise from the cosmic source. Lets say our recording system could measure from 0 to 5 volts. If we were to amplify our DC signal to fill most of this range, only a small change in the recorded output would be due to the source. What we need to do is remove most of the noise contributed by the receiver before we amplify the DC signal by a large amount. This function is provided by an offset circuit which simply subtracts a steady DC voltage from the signal voltage. This is easily accomplished with a operational amplifier connected as a voltage adder.

Even though we refer to the detected signal as a DC (direct current) signal, it still varies rapidly in intensity because it retains much of its noise character. The smoothing out of these rapid fluctuations is accomplished by an integrator. The integrator function is accomplished by using a capacitor as a holding tank for the incoming signal. Imagine that the signal is water coming through a hose and that the water pressure fluctuates in this hose. If we empty the hose in a large water tank and take the outflow of water from a small tap in the bottom of the tank, it is easy to imagine that the fluctuations in water pressure will be largely absent from our outflow tap. The integrator performs an additional service in that by averaging the signal over time it greatly increases the sensitivity of the measurement.

Lastly out DC processor amplifies the detected signal to a level where it matches the range of our recording device. The amplification function as well as the other functions of the DC processor is usually accomplished by use of integrated circuits called operational amplifiers. It is very important to use high quality "op amps" and other components in the DC processor.

Radio Telescope Recording Devices

To be of any value, the output of a radio telescope must be recorded. For the simple radio telescope described here, what we want is a record of how strong the signal is over time. If we are using drift scan observations, we can relate the time a particular value of signal strength was recorded to where in the sky our antenna was pointed. The result is often called a strip chart. Below is a great example of a strip chart from the web pages of the radio observatory at the University of Indianapolis in Indiana. This is a chart of the radio source, Taurus A, taken as the rotation of the Earth moved the beam of their antenna across this region in the constellation Taurus

Drift scans are nice, but what if you want to make a map of the skys radio emissions ? Well, all you have to do is plot drift scans of the sky at a series of elevations separated by somewhat less than the angular beamwidth of your antenna. If you live in the northern hemisphere, you could begin by pointing at an elevation close to the north celestial pole near Polaris, and running your telescope for 24 hours. If you had a beamwidth of say 10 degrees, you would then lower the elevation by about five to seven degrees and making a strip chart for that elevation. You would continue the process until the beam was point a bit above your horizon and then combine the data files to make a 2 dimensional map of the sky. In reality, there is quite a bit more to it than this. Calibrations half to be maintained and other factors like the interference and radio noise from the ground considered. Still, you get the idea.

Step 3: FM Tuner

The FM Tuner will be the core component of your lab. Sounds expensive, doesn't it? Well, it doesn't have to be. The picture above is a used Digital FM Tuner that I bought off eBay for $8.00 total (including shipping). The thing I like about this tuner, is that it's not complicated and doesn't have too many controls. The memory presets come in real handy as well. When I find stations that work well, I program them into the memory presets for easy access later.

On the back of the tuner, make certain that you are able to plug in an external antenna, and that you have some sort of audio outputs to plug into your computer. All FM tuners have audio outputs of some sort. Become familiar with the type of output jacks that you have because if you are going to hook a computer up to your tuner, you will need the appropriate cable to make the hookup.

What would a &ldquocrystal mixer&rdquo have been in an 1960's radio telescope at 960 MHz?

The 1963 paper is Accurate Measurement of the Declinations of Radio Sources (click the little PDF icon) and the two excerpts containing the phrase "crystal mixer" are quoted below along with the block diagram.

When I hear "crystal" I think of either quartz crystals for oscillators, or metal crystals (e.g. galena) with oxide surfaces used to make diodes using cat's whisker contacts, but for diode detection in the mixer certainly both vacuum tube and solid state semiconductor diodes existed in the 1960's, though I don't know about the performance at 960 MHz.

Question: What would a "crystal mixer" have been in an 1960's radio telescope at 960 MHz?

Block diagram of an astronomical interferometer made from two radio telescopes separated by 200, 400 or 1600 feet and operated at 960 MHz. The mixers in question are shown just below each dish:

"Crystal mixer" appears twice in the paper on page 2:

The arrangement of the components of the receiving equipment is shown by the block diagram of Figure 1. The receivers were of the superheterodyne type, and the crystal mixers were connected by short lengths of cable to the antenna feeds without any pre- amplification at the signal frequency. The local oscillator frequency was 960 Mc/s, and the center frequency of the IF amplifiers was 10 Mc/s, with a band width of about 4 Mc/s. No attempt was made to reject the image response of the superheterodyne. Note that the IF amplifiers were split into two sections. The IF preamplifier was located at the prime focus of the paraboloid along with the mixer and amplified the signal sufficiently to allow it to be fed through a long connecting cable to the remainder of the receiver, which was located in the laboratory building.

The local oscillator power for each half of the receiver was supplied by a separate klystron oscillator, which was phase-locked by a closed-loop servo system to reference signals of a common, central origin. The high-frequency refer- ence signal power required by the system for a satisfactory lock was about six orders of magnitude weaker than the available local oscillator power required by the crystal mixers of the superheterodyne receivers. The problem of getting phased local oscillator power to the two antennas when they were being used at large separations was thus greatly simplified.

Please note that starting from version 9 the software makes usage of latest Skyfield version (1.3). You can install Skyfield using pip install skyfield

It is no longer necessary to install using pip install skyfield==0.6.1

I also fixed a bug about proper management of negative (southern) longitude values.

Many thanks to Dave Typinski for the additions/corrections/phase plane probability map I added the various edits to paragraphs "Which kind of signals could I receive ?", "A bit of scientific background", "About CML-Io Phase Plane Probabilities".

I updated the software used in this project to use a different FFT library and different file format for improved performance and reduced storage space.

New software is available on github here

This new version makes use of a modified version of rtl-power-fftw by Klemen Blokar and Andrej Lajovic. The new sources are available here

Old version of the software is available here

All the software has been built and tested both on Raspberry PI 2 and 3.

It runs well also on a Debian Jessie virtual machine.

rtl_power_fftw can be compiled also on Windows but I did not try this yet.

Amateur radio-astronomy with your Raspberry PI

Amateur radio-astronomers built several devices in order to receive, listen and/or display signals from outer space sources, many use custom made radio receivers, others use HAM radio receivers. Recently, various experiments have been done using the cheap rtl-sdr dongles (radio receivers originally meant to listen radio and watch TV on your laptop).

With this project I want to try receiving radio emissions from the Sun or Jupiter and its satellite Io. I want to build an automated radio station, cloud connected as an IoT device that any amateur could easily replicate, joining the project and forming a collaborative, open science effort to easily share the spectrograms of the received emissions.

Which kind of signals could I receive ?

It is very easy to receive Radio Frequency storms from the Sun, but also Jupiter produces strong RF emissions. You can receive radio emissions from these sources on different frequencies. The upper part of the HF band ( between 10 and 30 MHz ) is often the target of scientific monitoring for both Sun and Jupiter. Many other frequencies can be monitored up to 1.7 GHz with these radio dongles but I will focus on the HF band with this project.

A bit of scientific background

Jupiter decametric radio emission, first discovered by Burke and Franklin in 1955, can be received between 15 and 30 MHz with a modest antenna array. The Earth's atmosphere attenuates the emission progressively more the further one looks below 15 MHz. Jupiter’s magnetic field strength limits its decametric emission to a maximum of slightly under 40 MHz, but Jovian emission also gets weaker the higher one goes in frequency. Most Jovian emission above 30 MHz is weak, requiring large antenna arrays to see it.

You can read the whole story of the discovery of Jupiter’s radio storms on the wonderful Radio Jove NASA website . On the same website you will find the description of a very good dedicated radio receiver on 20.1 MHz .

The tuning frequency is not very critical since the radio storms can be received across most of the HF spectrum. The emissions are produced by the interaction of charged particles with the Jovian magnetic field. The Jovian magnetic field is about 20,000 times stronger that Earth’s. As with Earth’s aurorae, the Jovian magnetic field is responsible for aurorae formation in Jupiter’s upper atmosphere (like pictured below by Hubble). The Jupiter emission observed in the HF band does not come from the aurorae, but from a region around 20,000 km above the visible cloud layer.

The relative geometry between Jupiter, its satellite Io, and Earth has been observed to have a modulating effect on the emission, making it more or less likely to be observed at different times. There's a probability distribution chart that you will find in various scientific papers that has been built based on years of data logging since 1957. Data is still being collected and the probability parameters updated.

It is being used by some programs to forecast and display the best listening periods. Here is an example:

The plot above, called a CML-Io Phase Plane Probability Map (or just “phase plane” for short), is an adaptation from the probability map used by Radio Jupiter Pro .

Central Meridian Longitude tells us what part of Jupiter is facing Earth, while Io Phase tells us where Io is in its orbit relative to superior geocentric conjunction (the point in its orbit furthest away from Earth, directly behind Jupiter).

The zone boundaries (with the exception of Io-D) are marked according to the University of Florida Radio Observatory (UFRO) definitions see . UFRO does not define an Io-D zone, so the Io-D zone is marked according to the definition provided by Carr, et al. in “Physics of the Jovian Magnetosphere” (1983).

About CML-Io Phase Plane Probabilities

The Io-CML phase plane image above attempts to depict the relative probability of receiving Jovian emissions at 20.1 MHz. This is done by first making an average of probability data generated from observations made at 18, 20 and 22 MHz at the University of Florida Radio Observatory (UFRO) from 1957 to 1994. The resulting average is then scaled so that the peak probability, in the Io-B source region, becomes 100% relative probability. The probability of observing Jovian emissions is affected by many variables. Some of these are the observing frequency, transparency of the earth's ionosphere, duration of the observing session, antenna gain, receiver sensitivity, galactic background noise level, man-made noise level, position of Jupiter relative to the Sun, and the jovicentric declination of Earth. While this image is a useful guide for the Jove observer, it cannot be used to predict events with absolute certainty.

Experience with small antenna arrays and swept frequency spectrographs shows that 100% relative probability is perhaps 50% absolute probability. That means even for the high-probability Io-B zone, one has only a 50% chance of observing Jovian emission when Jupiter, Io, and Earth are in such a configuration.

Thanks to Drs. Chuck Higgins, Francisco Reyes, and James Thieman, for their assistance in making this UFRO data available, and to Dave Typinski for generating the phase plane graphic image above.

How to Convert a Satellite Dish Into a Radio Telescope

If you find yourself with an old 30 meter satellite communication antenna, what should you do with it? One option is to convert it into a radio telescope, which is exactly what astronomers at the Auckland University of Technology in New Zealand have done with an old dish lying around in the northernmost reaches of the country.

So what exactly do you have to do to convert a communications antenna into a radio telescope? Today, Lewis Woodburn at the Auckland University of Technology and a few pals, answer this question by detailing the process they have gone through to make the conversion.

The old satellite communications dish in question was built in 1984 for the New Zealand Post Office and transferred to Telecom New Zealand in 1987. By 2010, the dish had become obsolete and the company stopped maintenance with the intention of demolishing it. That’s when the Auckland University of Technology stepped in.

What they inherited was a far cry from a state-of-the-art radio telescope. The dish is located near a remote township in the very north of New Zealand’s North Island. Being only five kilometers from the sea, salt corrosion was significant issue, particularly given the lack of recent maintenance.

So the team’s first task was to clean the dish service and replace rusty bolts and equipment. In particular, the motors that move the dish had become rusted and in any case were old and inefficient.

What’s more, the dish’s pointing mechanism allowed the dish to travel through only ±170° compared with the ±270° required for radio astronomy. So the power cables and metal chain that did all this steering also had to be replaced with the longer ones to allow for this extra movement. The dish also required new emergency stop circuits to prevent the dish moving beyond its mechanical limits.

Next, the team looked at the dish’s control system. Originally, the antenna had a pair of large induction motors for slewing and a set of small DC servomotors with extra gearing for tracking the small daily motions of geostationary satellites. The team replaced both sets of motors with a single set of DC servomotors with optical shaft encoders that work both for slewing and tracking.

One of the challenges they came up against was designing a control system without a detailed knowledge of the antenna’s mechanical characteristics, such as its stiffness, inertia, wind loads, and so on. “However, recommissioning tests showed the system to be stable with a servo accuracy of better than one millidegree under light wind conditions,” say Woodburn and co. And they say that there is sufficient margin to improve the performance during gusty weather, if needed.

The team also used a laser scanner to map the shape of the reflector surface. Any serious warping could have a significant influence on the instrument’s accuracy. The shape is generally satisfactory. However, “the result of data processing revealed a noticeable gravitational deformation of the antenna,” say Woodburn and co. They say this is the result of the vertical elevation required to do the mapping which places the dish at an angle of just six degrees.

Knowing the exact shape should allow astronomers to allow for any gravitational deformation. However, this requires them to work out how the deformation changes with the dish’s elevation, something that the team is currently working on.

Finally, they fitted the dish with the instruments necessary to detect radio waves from space. The dish has a waveguide that sends the signal into the building underneath telescope. In this area sits a new receiver designed to match one at the radio telescope at Jodrell Bank in the U.K., along with various other bits and pieces such as a recording system and an upgraded network for transmitting data and communicating with other radio telescopes when this dish operates as part of an array.

That’s a handy new piece of kit that should have significant impact on the kind of radio astronomy that can be done in New Zealand. The team envisage that the dish will work both as a standalone instrument and also with other dishes as part of a radio interferometer, although some upgrading is still required. “This 30m antenna adds significantly to New Zealand’s capability in radio astronomy with a large surface area and is a highly sensitive instrument capable of significant single dish work,” say Woodburn and co.

Incidentally, the New Zealand dish is by no means the only satellite communication antenna converted for radio astronomy. Several similar sized dishes have been converted in Australia, Japan and Africa.

Amazing what you can do with a lump of old metal scheduled for demolition!

How is noise removed in radio telescopes?

A rather simple example: if you are measuring signal from a compact source (e.g. a distant galaxy), you can have two identical receivers, one getting signal from the source, and another from "empty space" nearby (say,

0.1 degree away). Terrestrial and Solar system noise sources affect both receivers equally, since receivers are identical, and they use the same antenna at the same time. Subtract the signals, and you can get rid of those types of noise.

They reduce noise by various clever methods.

A rather simple example: if you are measuring signal from a compact source (e.g. a distant galaxy), you can have two identical receivers, one getting signal from the source, and another from "empty space" nearby (say,

0.1 degree away). Terrestrial and Solar system noise sources affect both receivers equally, since receivers are identical, and they use the same antenna at the same time. Subtract the signals, and you can get rid of those types of noise.

since you touched on the subject

There are 2 main sources of noise in modern astrophotography ( digital imaging)
Both of them can be quite successfully dealt with.

1) manmade light pollution …. use dark site and or filters …. guys are doing deep space imaging even during full moon from suburbia
stacking multiple (long = 5 minute exposures) images also substantially reduces random noise

2) noise generated by the sensors themselves

cooling the sensor as in the example below. Doing dark frames and subtracting them from the lights gets rid of hot pixels


There are 2 main sources of noise in modern astrophotography ( digital imaging)
Both of them can be quite successfully dealt with.

1) manmade light pollution …. use dark site and or filters …. guys are doing deep space imaging even during full moon from suburbia
stacking multiple (long = 5 minute exposures) images also substantially reduces random noise

2) noise generated by the sensors themselves

well that's why you do the things I commented on

( analog imaging on film and no, you are pretty screwed …. digital imaging leaves you wide open for
noise removal)

taking multiple images and stacking those images substantially reduces random noise

Adding "Dark Frames" to the stack removes ALL the hot pixels

and some creative post processing on that final stacked image results in a very noise free image

this is an example of a very noise free amateur astro image …..

that's 81 minutes of accumulated exposure time
Credit Franky T Astro Cop‎ an Australian astronomy mate from up north


Neither of those remove noise, they just remove non-uniform 'signal'. By signal I mean the actual electrons generated by incoming photons and the bias added by the sensor. The noise is the random variation in this electron count that causes a 'grainy' image.

It increases the SNR, but the amount of noise as I defined above actually increases. Luckily the signal increases linearly while the noise typically increases as the square root of the signal, which is why the SNR rises.

Neither of those remove noise, they just remove non-uniform 'signal'. By signal I mean the actual electrons generated by incoming photons and the bias added by the sensor. The noise is the random variation in this electron count that causes a 'grainy' image.

It increases the SNR, but the amount of noise as I defined above actually increases. Luckily the signal increases linearly while the noise typically increases as the square root of the signal, which is why the SNR rises.

It removes the unwanted signal from these pixels, but the noise is not removed.

your idea of noise in an image and my idea seem to be very different
I really cant figure where you are going ?

noise is noise is noise, regardless of how it is generated and the processes mentioned do lots to remove those various types of noise.

I don't know if Drakkith thinks that I think that all … 100% … of noise can be removed. Of course not and I am not stating or advocating that

But the noise can be substantially reduced by various methods that I have stated, including the use of bias frames that help remove the bias noise signal generated by the sensor

The sensors coming out these days have extremely low bias noise that many guys are not even bothering with bias frames to lower electron noise

this is the comments for one of my cameras

Cameras of 5 - 20 years ago suffered from a lot of self generated noise. These days it is hardly an issue
even just in the last 5 years, the technology has advanced well

Mine comes from my book on astronomical image processing and is more technical than what I've usually seen describe noise.
Imagine taking two images of the same starfield. These two images have identical exposure times, identical filtering, were taken with the same camera, the same software, at the same location, etc. If you were to inspect every pixel on both images you find that these pixels are not identical. The number of electrons counted from each pixel on one image is slightly different than the number of electrons from corresponding pixels on the other image (I say electrons and not photons because that's what's physically being counted by the detector). If you were to take a third image, you would find that, again, all of the pixels have slightly different values. This variance is noise and while you cannot predict the exact value a pixel will have between successive images, it follows a certain statistical pattern, namely that the noise varies as the square root of the signal.

That signal could be the actual photons being captured by the sensor (either from your target or from stray or unwanted light), the electrons generated in the sensor by dark current, or the electrons generated by the onboard electronics. All of these things serve as 'signals' and all contribute their own noise to the resulting image. Basically anything that generates electrons in the detector is a signal.

Subtracting dark frames, flat frames, or bias frames does not subtract the noise added to the image from dark current, from bias, or from the flat, dark, and bias frames taken to do the subtraction. The reason that these are taken and subtracted is to remove the signal from each of these sources, in addition to fixing hot/cold pixels. That leaves you with, ideally, the signal from your target, the signal from the background and ambient light, and the noise from all of the sources.

Taking multiple exposures and adding/averaging them together does the same thing that taking a longer exposure would do. It increases the signal to noise ratio. If we examine the same pixel from images taken of the same object at 5, 10, and 20 second exposures we would find that the pixel value increases approximately linearly, with the 10 second image having twice the signal as the 5 second image and the 20 second image having 4 times the signal. However, the noise does not increase linearly. The noise in the 10 second image is only ##sqrt<2>## times the noise in the 5 second image, and the noise in the 20 second image is ##sqrt<2>## times as much as in the 10 second. So increasing the exposure time from 5 seconds to 20 has increased the signal by 4x but the noise by only ##sqrt<2>*sqrt<2>##, or 2x. Hence the SNR has increased by a factor of 2 also. Stacking images is almost identical except for the fact that the readout noise of the sensor is comparatively higher than it would be if you just increased the exposure time.

Narrowband filtering 'removes noise' by blocking all of that pesky background light that you don't want which would only add lots of noise to the image. After all, you'd be able to subtract some quantity from all the pixel values of the image digitally so that this background light wasn't visible except that the noise can potentially be larger than the signal of your target! That's why shooting in heavy light pollution is so bad. The target's signal is swamped by the inherent noise of the ambient light.

For example, when shooting from inside a city, in a 30 second exposure I might see pixel counts of more than 40,000 electrons per pixel all across my sensor. The noise inherent with this background light is roughly ##sqrt<40,000>##, or ±200 e. Technically I should mention this is a mean, since it's a random variation about some central value. The fluctuation in the measured value of a particular pixel after many different exposures would have a high chance to be within 200 e of that 40,000, a slightly lower chance to be a little more than 200 e above or below, an even lower chance to be a bit further beyond that range, etc. When I compare this to the expected signal of my target, which may only be a hundred electrons per pixel over that 30 seconds or even less, you can see that the variation per pixel because of the noise can much larger than the signal from my target. This is what it means for a signal to be buried in the noise.

I hope I've made myself a bit clearer now.

I agree with Drakkith. The term noise is as commonly, widely and horribly misused in astrophotography to a similar extent of the the common practice of using the kg to measure weight is in everyday usage.

All relevant signals when it comes digital sensors in astronomy are modeled as stochastic processes with a Poisson distribution. This means that the signal is equal to the number of photons/electrons detected by the sensor (this relates to the Nobel Price Einstein did receive on the photoelectric effect) and the noise is thus equal the square root of the signal (in statistics terms the noise is the variance of the signal).

The main three signals are:
1. Bias/offset/pedestal signal - a small constant signal added by the sensor circuits to every exposure to make sure the final number can't become negative.
2. Dark current signal - electrons leaking into the "pixels" over time which is accelerated greatly by higher sensor temperatures. This is why serious astrocameras are both chilled and temperature regulated (to keep the variation down).
3. Light signal - all the actual photons your sensor detects (or in the case of Vigenetting, photons it fails to detect but can be measured/modeled). This includes:
+ The actual object you want to observe
+ Cosmic rays
+ Vigenetting, dust bunnies, etc.
+ Zodiacal light
+ Air glow/Aurora
+ Atmospheric dispersion of light sources
+ Reflections/dispersion due to (nearby or not) light sources reaching the sensor even though they shouldn't due to the properties of the optics
+ Light pollution
+ Aircraft and satellite trails
+ other stuff I forgot to mention

In serious astrocameras the digital output has a "unity" gain, this means that the ADU (Analog-Digital Units), more commonly known as the pixel value, is equal to the the number of electrons detected from all sources. Only the electrons from 3 are actually due to photons and you'd ideally want to avoid having to bother with both the spurious electrons from case 1/2 and many of the photons (you are after all only interested in the photons from the objects you want to observe) from case 3.

There are limits in what we can do to avoid detecting the spurious electrons from 1 and 2 but in most relevant cases 1 only matters if your exposures are too short (and the signal but not the noise can be removed using bias frames) and 2 can be limited by cooling the sensor and if the temperature is steady you can easily remove the signal (but again not the noise) by using dark frame subtraction.

For case 3 some of the signals can be reduced (and thus their inherent noise) by placing the telescope in the right spot (say high and dark), taking the image at the right time (no moon overhead, no aurora), using filters (very narrow-band filters works even from some of the most light polluted areas (I've seen amazing narrow-band images from amateurs in Rome and Athens)), etc. If the signal you want to avoid ever hits the sensor the best you might be able to do is to measure or model it (flat frames, background subtraction, etc.) but then you are again left with removing just the signal and not the noise.

The techniques used to stack sub-exposures (average, median, Sigma-Kappa, etc.) then also have a impact on how the noise and spurious signals are controlled.

Ep. 7: Getting Started in Amateur Astronomy

Got your eye on that $40 telescope at Walmart? Wait, hear us out first! Fraser and Pamela discuss strategies for getting into amateur astronomy – one of the most worthwhile hobbies out there. We discuss what gear to get, where to look, and how to meet up with other astronomy enthusiasts.


Check out Episode 33: Choosing and Using a Telescope for more specific suggestions and links
Basic Optics

Other Resources for Amateurs

    – Buyer’s Guide, reviews of equipment and lots of free observing tools including interactive star charts for all latitudes. – an excellent book for amateurs anywhere, including starcharts good for Northern Hemisphere locations.

Transcript: Getting Started in Amateur Astronomy

Fraser Cain: Last week we discussed the beginning of everything: the big bang. Pretty heavy stuff, so we thought we’d give you guys a couple of weeks to absorb that, and talk about our favourite hobby: amateur astronomy (obviously) and how to get into it.

So before you rush off to Wal-Mart and by that $40 telescope, hear us out.

First, Pamela, I wanted to know how you got into amateur astronomy.

Dr. Pamela Gay: I don’t think I necessarily was going to be given a choice. My dad was an electrical engineer who was deeply passionate about astronomy and physics, but just for various career reasons went into electrical engineering instead. As far back as I can remember I was getting sci-fi influences, and getting drug out of bed to see pictures coming back from the Voyager space probes.

One of my earliest memories is going out to the backyard around age five or so, and looking through a little tiny refracting telescope that much more strongly resembled a pirate’s spyglass than any piece of astronomical instrumentation. He showed me the Moon through it, and I lied: I said I could see Russian cosmonauts (and how I knew those things existed at age five, I don’t know – I was an uber-geek at five). I lied and said I could see Russian cosmonauts walking their dogs on the Moon, and my dad let me get away with the lie!

I remembered what happened because he let me lie and I didn’t get punished. It’s strange the things you remember, and by being allowed to get away with that lie, I somehow ended up becoming a professional astronomer.

Fraser: How did you know that you were going to become a professional astronomer? How did you make that decision?

Pamela: I sort of wasn’t smart enough to know when to stop taking classes? I actually started out college at James Madison College at Michigan State University, which is an international relations program. The program was nice and all, but I was taking astronomy classes (and I’d been taking astronomy classes for forever). No one – well, very few people – actually believe, “I can be a professional astronomer” and I wasn’t one of those people that believed I could do it, so I figured I’d get a degree in international relations and do science policy or something like that.

I was hacking it in the astronomy classes, and going a little bit crazy with all of the young Republicans in my international relations programs (no offence to Republicans out there, but at 9am I can’t deal with them in classes). The astronomy just sort of sucked me in and I got my undergraduate degree and said, “Okay, I’m going to graduate school,” got my masters degree, got my PhD, and just kept going and landed as a professional. I’m just doing it because I love it: it’s not a job, it’s a hobby I get paid to do sometimes.

Fraser: I actually had a really similar upbringing with you, actually. Both my parents were quite into science fiction – we watched Star Trek whenever the repeats were on. My dad woke me up in 1981 to watch the launch of the space shuttle, but he didn’t do a lot of observing. My dad would by a copy of Sky and Telescope every month, so I always had the pictures to look through. I think, for me, what really affected me was I got a copy of Our Universe which was this time-life book that came out in the mid-80s and was really thick. It had wonderful pictures and information about space, and I read it over and over again.

I think I was about 14 when I bought my first telescope. We’d set it up in the backyard and observe every night. It was a 4″ Newtonian telescope – not very good, it didn’t have a good mount or anything. When my parents’ friends would come by, I would set up the telescope and show everybody, “here’s Saturn!” and, “here’s the Moon!” It was great – I think I always loved to share it, which was funny.

Later on, I actually organized a star party on the island I grew up on, Hornby Island (west coast of British Columbia), and organized this star party with a friend of mine who was also into astronomy. We’d have all of our parents’ friends over and they were all – they weren’t into it, no one else had a telescope, but they really enjoyed it.

Then I went into the business world, but I also maintained my interest in astronomy on the side, and eventually worked on Universe Today and kept going with it.

Pamela: And now you’re here!

Fraser: So let’s say that a listener wants to get involved in astronomy – is obviously listening to this podcast, but maybe hasn’t done any observing with a telescope but isn’t really sure what to get and is eyeing that Wal-Mart telescope with $40 in hand. Where would you suggest people get started?

Pamela: I’d walk next door to the photography store and get a pair of binoculars. The thing about telescopes is there is technique to using them. A lot of the Wal-Mart telescopes, you’re going to get frustrated with trying to figure out how to find something in the sky and stop using it before you actually do anything cool.

If you go out and instead get a pair of binoculars, first of all you can convince your spouse it’s a good investment. You can also use them to look at your kids when they’re in marching band or soccer or whatever sports they play. You can watch parades, go to the zoo and see animals. Binoculars are one of these investments you can use for your hobby, but you can also use them for all sorts of other different things.

They’re easy to use. With a telescope there’s fussing and fussing and fussing – and eventually you find an object. With binoculars, you look at the object with your eyes, pull the binoculars in front of your eyes and look at the object through the binoculars with your eyes. The learning curve to get from having the binoculars in a box to using the binoculars to look at the Moon is almost none.

Fraser: Yeah, using binoculars is so fast. You can look up in the sky, see some blurry spot or something, pull the binoculars, turn and see things that have a much better view. If you’re not sure if you’re looking at the right object, you just look again and make sure you’ve got it lined up.

Yeah, I totally agree that binoculars are one of the first, best ways to go.

Pamela: They’re entirely satisfying because you can see objects that are fainter than you could ever see with your eyes, but you can get from object to object quickly. You’re not lugging around a heavy object. Telescopes can get heavy and cumbersome to carry around with you. So you’re getting a whole new face to the sky with this $70 piece of glass and plastic, and you’re having fun and they’re easy to share. You just hand them to your kid and help your kid find an object. Hand them to your friend, your spouse, whoever is nearby going, “what are you doing in the dark?” and you can get them hooked as well.

Fraser: I think the best thing with binoculars kind of goes hand-in-hand with one of the most important things, to learn your constellations and be able to go out with a star map in hand and actually start learning where all the stars are. Everything in astronomy is based on those constellations, so if you know where Hercules is, you can find the globular cluster. If you know where Pegasus is, you can find Andromeda. So I think the next step is to learn your constellations.

Pamela: Here I think you and I have a slight disagreement. I love planispheres, and there’s a really good one available for free.

Fraser: What’s a planisphere?

Pamela: It’s a round piece of paper within two pieces of paper. It’s a dial that’s just… take three pieces of paper, cut a hole in the top piece and allow the middle piece to rotate freely. The top piece you cut a hole that represents your sky. The back piece has all the constellations on it, and you rotate it until the constellations that are visible on the planisphere match the constellations visible in the sky.

Fraser: They have like a month and time so you can turn it so you could match the sky tonight.

Pamela: Exactly. These are also called star wheels, star dials… because you dial the wheels to the time and date you’re at, and it moves your map to adjust to what our current changing sky actually looks like.

Fraser: Yeah, you can get them at museums, online…

Pamela: Edmund Scientific

Fraser: Yeah, and they’re only a couple of dollars. They’re not expensive at all.

Pamela: They’re easy to use, and every good journey begins with a map, and this is one of my favourite maps.

Fraser: Yeah, my favourite is there’s a book called Nightwatch which is what taught me my constellations. It’s this spiral bound book, about 96 pages or so, and inside it’s got a really nice view of all the constellations for each season. You can lay it out flat and use a red flashlight so you don’t wreck your night vision, and you can learn your constellations. The great thing about it is it also has in all the constellations, it’s got closer-in views that show where all the interesting objects are: where galaxies and nebulas and stuff like that are. So as you get better and learn your constellations you can switch over and start looking for some of these objects. Nightwatch is by Terrance Dickinson. I would say it should be on every amateur astronomer’s bookshelf.

Pamela: I have it on my shelf, so it should be on every pro’s shelf as well.

Fraser: The other thing I recommend is before getting a telescope, find your local astronomy chapter: do a search on Google for “astronomy society” and then your city (Chicago, New York, Vancouver, whatever) and you’ll find your local astronomy club. There’ll be some contact people, and they’ll do an observing night several nights a month. What you can do is organize to go out there and look through people’s telescopes and you’ll find amateurs are more than happy to let you take a look.

Pamela: Amateurs are some of the friendliest human beings out there. These are people that have a hobby they’re passionate about that they just want to share with somebody. They want to take you out and help you find the galaxies, help you find the planets, and get you as hooked as they are so they have another person to talk about the stars with.

Finding a mentor can be the best thing you can do if you really want to get started. It’s one thing to learn from a book, it’s another thing to learn from someone who’s already gone through the learning process and knows all the tricks that you have to watch out for.

It’s also good because you can play with their equipment before you have to invest in any of your own. Just like it’s good to test-drive a car, it’s good to test-drive a telescope. What is one person’s favourite telescope might drive another person crazy.

So go out, find someone that you enjoy learning from, a club you enjoy being part of, and get involved with human beings and build a small astronomical community. Here in the United States, we have the astronomical league which keeps a list of major astronomy clubs around the United States. In Canada there’s the Royal Astronomical Society of Canada. Both are good organizations that can help you get started, and help you find the local folks to get you started face to face.

Fraser: So let’s say then that people don’t want to wait and they’ve done the binoculars and they know they want a telescope. What’s the big recommendation?

Pamela: My favourite first telescope is a Dobsonian telescope. This is basically a light bucket. It’s a telescope mounted on what, for lack of a better term, I will call a lazy-susan. It’s easy to point, it’s easy to carry, it’s hard to break and they’re cheap. You can get a really good one for under $200.

Go out, you can get a 6″ mirror which will collect huge amounts of light compared to your binoculars or your eyes, and start looking at fainter galaxies. Start challenging yourself to find little tiny double stars that have very different colours. See what your eyes are capable of, and just go that next level of faintness into the sky.

Fraser: Yeah, the Dobsonians are actually pretty easy to move around. Now, it won’t have computer control, which is good because it’s good to learn the constellations and where objects are. You can swing them around, turn them, move them up and down, and look through the spotting scope to find out what you’re looking at.

So I totally agree that for a couple hundred dollars you can get a pretty powerful telescope. They’re not super portable, but they are definitely a really good view of the night sky. Then you can start looking at some of the more darker, deeper sky objects – galaxies, nebulae, stuff like that.

Pamela: You can get them that have encoders in them that can say, “you just pointed at Aldeberan, you pointed at Betelgeuse before that, I know how you moved the telescope to get between these two objects, now let me tell you how to find something cool you wouldn’t be able to find otherwise,” and they use little arrows to move you around the sky.

So it’s actually possible to get a Dobsonian that will help you find things and be your resident tour guide. It won’t move the telescope for you, but it will tell you how to move the telescope.

Fraser: So let’s say that the Dobsonian is good, but I think people do want some of those features – some of that star-finding, some better optics. What would you recommend about that?

Pamela: Well, there it starts to get into a question of how much money you’re willing to spend, and what are your eventual goals? If you’re someone who wants to always be looking at the sky, the Takahashis are amazingly expensive you’ll spend as much on them as you will on a car, but you’ll have sensational optics. Dobsonians are good, but they are hard to transport but you can get them in huge varieties. I’ve met some that you have to stand on 12′ ladders to look through, but you could see quasars through the Dobsonian.

Fraser: I looked at one this summer and it was 21 inches, I think. It was a 16 foot ladder you stood at the top of, and then you could look in. You could see Andromeda and see the spiral arms of it. It was just unbelievable. It was fairly portable – wasn’t too bad. It was cloth and the tops and the bottoms could come apart and they could move it around. So actually it wasn’t too bad, but still –pretty crazy.

Pamela: Once you start getting into wanting a mechanized telescope, it all depends on how much money you’re willing to spend. Celestron, Meade and Orion all put out perfectly reasonable telescopes in the “it costs as much as my high school student’s car” variety. These will get you started: you can attach cameras to them, you can get what are called CCDs – Charged Coupled Devices, that are basically the same technology that’s in a digital camera, but specially cooled and built just for astronomy use. You can get these and mount them on. You want to look for a telescope that is a Cassegrain focus. This means the light comes out, basically, the butt of the telescope. So the camera is mounted on the bottom part of the telescope, the light comes in the top, does some reflecting in the middle, and everything balances out nicely.

Fraser: A lot of the pictures people see on the internet (I’ll highlight them on Universe Today quite a bit, once a week we do a new astrophoto), those are actually not so hard to get into anymore.

Pamela: No. I actually have a graduate student that I’m working with right now that’s using a Canon EOS Rebel to do digital photography of the sky. You can go out and by any SLR camera on ebay and do perfectly good astrophotography using good quality film, and then scan your negatives in and get amazing photos.

So there’s lots of different directions you can go in. SBIG, Santa Barbara Instruments Group, makes amazing digital devices for measuring the sky, CCDs. They come in all different varieties, all different price ranges, depending on exactly what you want to do you can actually buy them nowadays that take colour pictures (that wasn’t true just five years ago).

Fraser: Something other people are doing as well is using video cameras. They’re stacking up pictures on the video camera. They take five minutes of an object and then stack them up using a computer, and the images they get are just amazing.

Pamela: Meade makes something called the Deep Sky Imager that is fairly cheap and does fairly good quality. You hook it up and it goes. It’s a webcam designed for your telescope. You can stack all of these pictures – there’s a group at Dexter Southfield School in Massachusetts that uses high quality security cameras, actually, that they’ve reprogrammed and re-done and hooked up to their telescope. They’ll take a few thousand images, go through using software and find the best hundred out of those few thousand images and stack them together. You can get images of the space shuttle that allow you to see the paint job. It’s really cool, the type of stuff you can do, and they’re just using a 20-something inch telescope that anyone with a big enough backyard could buy for a few thousand dollars.

Fraser: So just to recap, start with your eyes. Learn your constellations, get a starwheel and a reasonably good set of binoculars. Head outside, learn your constellations, enjoy the time with other people. If you feel like that’s it for you, find a local astronomy club, check out other people’s telescopes to see what you can see, and then you can start the long road to spending your whole mortgage on your telescope.

But you don’t have to, which is the point.

Pamela: No. I personally love my binoculars. So there’s two things to think about: you can spend as much or as little as you want and still do good science. There are people out there who are visually, without binocular or anything else, observing some of the brightest variable stars and doing real science. At the same time, there are amateur astronomers who have built backyard facilities that would make any professional astronomer drool. There’s this fascinating figure I learned last year: a person will often spend as much on their hobby as they spend on their car. So if you go to a star party, find the person with the telescope you love and go look at their car (then do some mental calculations). You don’t have to do that. A good pair of binoculars will take you a long way through the sky.

Fraser: Wonderful. So this episode of Astronomy Cast is going to have some homework. You’ve got to let us know how it went. Have you been wanting to get into astronomy and you listened to the show and it’s kind of getting you inspired? Maybe you’ve been thinking about taking the next step. Let us know how it goes. Go out, spend a night with your binoculars, learn some constellations. It’s especially fun with family and friends to make a night of it. Drive outside of the city nights and spend the night looking at the sky.

Let us know how it goes, drop us an email: we’d love to hear if you’ve caught the bug yet.

Pamela: We can’t wait to see your results!

This transcript is not an exact match to the audio file. It has been edited for clarity.