Camera
From chi and h
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By camera I mean the electronic detector, the case it is mounted in, and the electronics and software that goes with it. This excludes the lens, which we deal with under optics instead. For a dSLR we would consider only the body to be the camera. A compact digital camera is a camera with a lens thrown in, which cannot be removed. A webcam usually comes with a lens that can be removed. A CCD camera specialised for astrophotography comes without lens.
Types of camera
Compact digital camera
Compacts usually have a zoom lens to allow wide angle and mild tele photography. Travel zoom compacts have significant zoom factors, replacing a dSLR with two zoom lenses. Even so, the zoom factors are not sufficient to photograph typical astronomical objects like the Moon, a star cluster, an interstellar nebula or a galaxy. If you are going to buy a compact, make sure it can take long exposures (30 s or more). Also useful is the "bulb" exposure setting, which allows you to take exposures as long as you like.
In general, you want the option to do things manually. Automatic exposure metering does not work at night; auto-focus does not work on faint or tiny objects. If you may have to stack multiple exposures or to significantly boost the brightness of pictures, make sure the camera can take raw images with more than 8 bit and without JPG compression.
If you are going to buy one of these cameras, perhaps you should consider a dSLR instead. If you have a compact already, use it for astronomical objects that it is suited to. Go for large objects: halos, aurorae, constellations, the Milky Way, etc. Small bright objects are accessible, too. Mount the camera afocally behind a telescope for detailed views of the Moon, or to capture telescopic views of the bright planets.
Digital single lens reflex camera (dSLR)
The dSLR is the big sister of the compact camera. The standard lens makes this camera roughly equivalent to the compact. The dSLR is bigger, because its roots go back to the film SLR of the mid to late 20th century. The old lenses can be used and the detector size is almost similar to the frames on 35 mm film. While these cameras are not so easy to carry around, the larger lenses and larger pixels make for better images. A larger lens has better diffraction-limited resolution; larger pixels collect more photons with less noise. The dSLR is more likely to permit overriding the automatic features when they cannot cope with the strange images you take as an astronomer.
The principal difference to the compact is that the lens can be removed. You can buy a range of photo lenses from fish-eye to super-tele, or you can fix it to the back of a telescope for seriously large image scale.
CCD camera for astronomy
Amateur astronomers can also buy CCD cameras similar to those used by professionals. These are expensive and useless for your holiday snaps. They come without lens, need a computer for power, for control, and to store images. Proper CCD cameras are monochrome. They should be actively cooled to reduce noise. Their digitiser can probably generate 16-bit numbers, compared to the typical 12 bit in raw dSLR images.
These cameras are used by serious amateurs, often in the context of scientific or quasi-scientific work, such as photometry and astrometry. The preferred mode of working in that context is to have a monochrome camera and use filters to select the colour of light that is of interest.
Without filter these cameras have good sensitivity in the extreme red and near infrared. Deep-sky photographers may prefer CCD cameras over compact or dSLR, which have built-in infrared-blocking filters that also block a lot of extreme red, including the Hα line of hydrogen that gives HII regions its characteristic colour.
Webcam
The webcam is the complete opposite of a CCD camera. It is designed for real-time video and generates quite poor 8-bit images. It is limited to very short exposure times. It is also very cheap, with a pair of webcams available for perhaps £20 from your local supermarket. It may appear cheap, but you also need a computer to do anything with it.
A field of astrophotography has developed where the webcam is the weapon of choice. The webcam can take vast numbers of short exposures in very little time, and they pile up immediately on a computer where they can be stacked quite efficiently. This makes the webcam ideal for imaging detail on the bright planets.
Some amateurs have extended the use of webcams to deep sky imaging [1]. They modify the control electronics, such that an exposure can last a lot longer than the typical 1/25 second. It seems to me that the time for such modified webcams has passed: A dSLRs can be had for a few hundred pounds, not much more than the cost of a webcam plus laptop.
Recent compact and dSLR cameras can also be used as a webcam, as they tend to support recording video. The detector will be of a higher quality than in a webcam, and these cameras have their own power source and data storage; no need for a computer during the exposure. The recording quality of the video may also be better than in traditional webcams. However, the user's control over the exposure is more limited for video than for still photography, and this may render the video camera useless for certain objects:
- Autofocus may be mandatory in video modes, and may fail on small or low-contrast objects.
- Automatic exposure may be mandatory, and may fail on small objects.
- Gamma correction may be mandatory, so that the stacked data need to have this reversed for better contrast at medium to high brightness values.
Which camera for the job?
Stefan Seip [7] gives a table of how suitable each of the four camera types is for particular uses:
Camera types and their use. compact dSLR webcam CCD use for + ++ −− −− scenic shots during twilight +− + −− −− star trails +− + −− +− wide field (constellations or Milky Way) + ++ + + Sun, Moon, their eclipses −− − ++ − planets + ++ −− − special events (e.g. meteors, aurora, halo) − + ++ deep-sky, large and bright objects −− +− −− ++ deep-sky, small and faint objects
If you use a video mode of a compact or dSLR, consult the webcam column.
Although the compact is not very good at anything, it can be used in a variety of instances. A dSLR will always be better, can be used for almost anything and is very good overall. The only advantage a compact has over the dSLR is that it is easier to take with you and that it is ready without having to change lenses. The webcam has its niche use where nothing can compete with it. The CCD camera is better than the dSLR only for deep sky imaging.
Attaching the camera to the optics
Having defined the camera as being without lens, we need a way of mechanically bringing together the two. To put a webcam on the telescope the general approach is to make up something cheap and simple. Remove the eyepiece from the telescope and the lens from the webcam. Cut an old film container into a tube and tape this in front of the webcam. The tube will probably fit into the telescope as if it were an eyepiece.
For compacts, the approach is sometimes even more ad hoc. Afocal projection will be used, so the telescope has an eyepiece in it. The camera replaces the human eye and can be held free hand behind the eyepiece. To fix the camera more permanently to the telescope you probably need some Heath Robinson contraption involving brackets around the eyepiece and the camera lens combined with several levers and joints. Digiscoping seems to be the buzzword for this, from mounting a digital compact behind a spotting scope.
A CCD camera will have a T2 thread and you can buy a T2 adapter for your telescope as well. T2 is an M42x0.75 thread, i.e. a thread of 42 mm diameter and with pitch 0.75 mm/rev. Astrophotography suppliers offer all sorts of gadgets for T2 assemblies, including extension tubes for eyepiece projection and T-junctions to divert light into an eyepiece for guiding. You can also get a T2 adapter for your dSLR, making it in this respect equivalent to a CCD camera. Conversely, you can probably get T2 thread for your dSLR lenses so that the CCD camera can be put behind those for a wider field of view than through the telescope.
I have followed a similar route, but I use the M42x1 (aka M42) thread. This has the same diameter as T2 but a steeper pitch of 1 mm/rev. In the 1980s, I had two Praktica SLR bodies with this thread, and most of my lenses still have this thread. For the telescope, I had an adapter ring made that screws onto the regular eyepiece adapter. With a set of M42 extension rings, I can also use eyepiece projection.
Detector
The light-sensitive detector in a digital camera is a rectangular area of semiconductor, subdivided into small - in most cases square - pixels. Quantum physics is at play once again, when a photon of light passes its energy to an electron of the crystal. This liberates the electron from its atom and enables it to move freely through the crystal. However, during the image exposure the electron is limited to its own pixel and cannot move further than that. Not every arriving photon will create a free electron, the chances are somewhere between a half and one, depending on the colour of the photon and the quality of the detector. In "good old" film the odds are a lot worse, 10% of photons or less have a chemical impact on the emulsion.
After exposure, the electrons collected in each pixel can be "read out" of the pixel array. One way or another they become an electric signal in the detector electronics, are amplified, and then converted from analogue current or voltage to digital numbers ready to be transferred and processed like computer data. In essence, this is what your camera delivers as raw format, if it supports raw format.
Bayer matrix
With a monochrome CCD camera, you would have to take three separate exposures through red, green, and blue filters to get a colour image. Single-shot colour cameras record colour by means of a Bayer matrix: Each group of 2 by 2 pixels has tiny coloured filters such that two of the pixels record only green light, one records only blue light, and one only red. The Bayer matrix can become a bit of a nightmare for serious analysis of images: One star might send all its light into a green pixel while another might hit a blue pixel. In this extreme, the colours seen can be wrong. Add to this that the starlight might have real intrinsic colour and your photometry can go haywire. Astrometry will also suffer from a Bayer matrix.
CCD and CMOS
There are two different kinds of detectors in use, CCD and CMOS. CCD stands for charge-coupled device and refers to the way the image is read out from the pixel array. CMOS stands for complementary metal-oxide-semiconductor and refers to how the semiconductor is manufactured. CCDs need a specialised production line, and so CMOS manufacture is much more cost-effective. The major differences between CCD and CMOS are [2,3,4]
- A CCD detector is read out to electronics outwith the pixel array. Each pixel's signal is processed by the same electronics so that the result is more uniform.
- A CMOS detector has readout electronics alongside each pixel. This takes space and results in dead areas between pixels where light would be lost. Since each pixel is processed by distinct electronics, the output is less uniform. Micro-lenses in front of pixels are used to mitigate the dead area issue.
- CCD detectors have higher dynamic range and lower noise.
- CMOS detectors require less off-array space for circuitry. They also require less power so that camera batteries last longer.
- CMOS detectors can be read out faster - essentially all pixels in parallel - and parts of the array can be read out easily.
- The division of CCD for high-quality applications and CMOS for compact, low-cost devices is no longer so clear-cut.
There is still a marginal advantage in a CCD. Nevertheless, this should probably not drive you from one dSLR to another, but make you consider an actual CCD camera for astronomy.
Image resolution and field of view
The size of the pixels limits the resolution of the images. We need to know what angle on the sky corresponds to one pixel in the image plane. We also need to know what angle on the sky the whole detector covers. The following table shows the pixel sizes and detector sizes of my cameras. For comparison, I have included one that I don't have, a low-end CCD camera (SXV-H5, £800).
Pixel sizes and detector sizes. pixel x y resolution camera 14.7 μm 22.70 mm 15.10 mm 1536x1024 Canon EOS 300D, Bayer matrix binned 7.4 μm 22.70 mm 15.10 mm 3072x2048 Canon EOS 300D, Bayer matrix interpolated 11.5 μm 22.20 mm 14.80 mm 1936x1288 Canon EOS 400D, Bayer matrix binned 5.8 μm 22.20 mm 14.80 mm 3872x2576 Canon EOS 400D, Bayer matrix interpolated 2.9 μm 5.84 mm 3.88 mm 2048x1360 Panasonic Lumix DMC-TZ8, 2.5 Mpix output 8.8 μm 5.60 mm 4.20 mm 640x480 Panasonic Lumix DMC-TZ8, VGA output 11.2 μm 3.60 mm 2.70 mm 320x240 Philips ToUcam Pro, Bayer matrix binned 5.6 μm 3.60 mm 2.70 mm 640x480 Philips ToUcam Pro, Bayer matrix interpolated 2.8 μm 3.60 mm 2.90 mm 1280x1024 Logitech C300, Bayer matrix interpolated 5.6 μm 3.60 mm 2.70 mm 640x480 Logitech C300, Bayer matrix binned 7.4 μm 4.90 mm 3.65 mm 660x494 Starlight Xpress SXV-H5
Comparing the compact Lumix to the 400D dSLR, the resolution (number of pixels in the output image) is similar, while the linear sizes are four times smaller. Comparing the Lumix to the ToUcam webcam, the linear sizes are similar, but the Lumix has more pixels after binning and should have far better image quality (before compression to video format).
How these numbers translate to angles on the sky depends on the focal length f (cf. Optics). For the resolution in sky angle per pixel, at the centre of the field, we can assume small angles, so that the pixel size δ corresponds to an angle
- Δα = δ / f
- Δα/' = 3437.75 δ / f
- Δα/" = 206265 δ / f
When we consider the size d of the detector - the field of view - this can be a sizable angle α and we have to keep the tangent function:
- α = 2 atan[d / (2 f)]
- α/° = 114.59 atan[d / (2 f)]
- α/' = 6875.5 atan[d / (2 f)]
The table shows some typical combinations of optics and camera I use:
Pixel-limited resolution and detector field of view (in most cases Bayer matrix binned). f/mm δ/μm x/mm y/mm Δα fov optics and camera 4 2.9 5.8 3.9 2.4' 70° x 50° Panasonic Lumix DMC-TZ8, 2.5M, 1x zoom 18 14.7 22.7 15.1 2.8' 64° x 46° 18 mm, Canon EOS 300D 50 11.5 22.2 14.8 0.8' 25° x 17° 50 mm, Canon EOS 400D 55 14.7 22.7 15.1 0.9' 23° x 16° 55 mm, Canon EOS 300D 135 11.5 22.2 14.8 17.6" 9.4° x 6.3° 135 mm, Canon EOS 400D 49 2.9 5.8 3.9 12.0" 6.8° x 4.6° Panasonic Lumix DMC-TZ8, 2.5M, 12x zoom 400 11.5 22.2 14.8 5.9" 3.2° x 2.1° 400 mm, Canon EOS 400D 800 11.5 22.2 14.8 3.0" 1.6° x 1.1° 400 mm with 2x adapter, Canon EOS 400D 800 11.2 3.6 2.7 2.9" 15' x 12' 400 mm with 2x adapter, Philips ToUcam Pro SIF 2000 14.7 22.7 15.1 1.5" 39' x 26' telescope, Canon EOS 300D 2000 11.5 22.2 14.8 1.2" 38' x 25' telescope, Canon EOS 400D 2000 2.8 3.6 2.9 0.3" 6.2' x 5.0' telescope, Logitech C300 1.3 Mpix 3900 8.8 5.6 4.2 0.5" 4.9' x 3.7' afocal 80x, Panasonic Lumix DMC-TZ8, 12x zoom, VGA 3900 4.4 5.6 3.2 0.2" 4.9' x 2.8' afocal 80x, Panasonic Lumix DMC-TZ8, 12x zoom, 720p 6100 8.8 5.6 4.2 0.3" 3.2' x 2.4' afocal 125x, Panasonic Lumix DMC-TZ8, 12x zoom, VGA 6100 11.2 3.6 2.7 0.4" 2.0' x 1.5' eyepiece proj., Philips ToUcam Pro
Be careful about the resolution listed in this table. The actual resolution of the image is also limited by diffraction (cf. Optics) and by "seeing" (turbulence in the Earth's atmosphere).
Again, the compact camera appears similar to the dSLR, just all linear dimensions scaled down fourfold. As a conventional camera, the Lumix travel zoom is lighter, smaller and with a single 12x zoom lens covers the same range in field of view as the dSLR does with two zoom lenses, 18-55 and 55-200 mm.
Comparing the Lumix to the webcam, the focal can be shorter, thanks to the smaller pixels. Since it has more pixels, the Lumix's field tends to be larger. However, much depends on the practicalities of controlling focus and exposure.
Bias, dark and flat
The camera is not a perfect converter of light into charge and ultimately digital pixel values. The camera itself will contribute to the image. In normal pictures, this is not visible, and even at high contrast these effects are probably masked by noise. If you stack images these effects may become visible, and they will then limit how faint an object you can detect. Professional astronomers will take care of these effects by taking three reference images that record the effects:
- A bias frame is an image read out without prior exposure. A CCD camera will allow you to take a bias frame as well, but with a consumer camera you would have to approximate this by taking a very short exposure and with the lid on the lens to keep the light out. The bias frame will show a pattern of brighter and darker columns or rows.
- A dark frame is an image of finite exposure time taken with no light, only dark current and bias. Professionals will take a long exposure for this, subtract the bias, and scale it down according to the exposure time they used for the real image of the astronomical object. We can do the same, but it is usually easier to record bias and dark current jointly in a single frame with exposure time matching our real image. Often we will call a "dark frame" what in fact is a frame containing bias and dark current for a given exposure. Professional use of the term "dark frame" would mean a frame containing the dark current per second (and no bias).
- A flat field is an image taken of a uniformly white object. Professionals may have screens they point the telescope at (a dome flat) or use the twilight sky. We often have large fields of view where the sky will have a significant brightness variation. A tee shirt flat is a good method, where a white tee shirt is pulled tightly over the front of the lens. A variety of light sources can then be used, best perhaps simply outdoor light on a bright and sunny day. The bias and dark current are subtracted from the flat field. The flat field will not be uniformly bright: It will show the vignette of the lens the dust grains on the detector, and pixel-to-pixel variations of response to light. A flat field is usually normalised to have an average or maximum value of one.
Bias and dark current are subtracted from the real image. The remainder is divided by the flat field.
Dynamic range and saturation
Saturation is, when a pixel is as bright as it can be, more light is simply wasted. You might call this overexposure. Avoiding saturation is a high priority: Once the central area of a bright nebula has become all white in the image there is no way of getting some contrast back into these bright regions.
The dynamic range is related to this. Say your image is exposed just right, the brightest bits are just below saturation, and the dark background is as dark as you can make it. What is the faintest star you can still distinguish from the background? The ratio of the brightest to the faintest object is the dynamic range.
There are potentially several places in the camera where saturation and dynamic range are determined. First there are the pixels themselves: We are counting electrons and there are no half electrons in the universe. In principle, the smallest signal is one electron. In reality, noise introduces random variations of electron count so that the smallest signal is several electrons. Once enough light has entered the pixel to liberate all available electrons, more light will have no effect.
Second, the electronic amplifier has an influence. It adds more noise, making the smallest recognisable signal larger. The amplifier might be overdriven and clip at the highest signals. Third, the conversion from analogue to digital signal introduces a digital signal step as the smallest unit above zero. Depending on how many bits of storage it uses - 8, 12 or 16 bit - saturation occurs at some multiple of that step size - 255, 4095 or 65535.
Chances are the camera is designed well, so that the number of bits it digitises to is an indicator of its actual dynamic range. But be careful. Consider a dSLR with 12-bit digitisation in raw format. True, saturation occurs at about 4095. However, darkness will not actually result in digitisation as zero. We have bias and dark current, which move the black background level up above zero. In addition, the smallest signal is not simply one digitisation step above the bias and dark current. Noise will mean that the smallest meaningful signal will be at least three or five steps above bias and dark current.
Noise and detection limit
Noise is a random contribution to the pixel values in our images. It arises in several places. The electron count itself is inherently noisy. Say, you expect on average to count 100 electrons each second. In one second you may get only 95, while in another second you may get 105. There is a fundamental formula for counting measurements, namely that the standard deviation - the average deviation from the long-term mean - is the square root of the mean. So if we expect 100 per second then on average we expect 10 more or 10 less than 100. Similarly, when the BBC asks 1000 voters who they will vote for then they expect that 30 of them (√1000) are the wrong people and will skew the result. Hence, the poll is only to 3% (30/1000) accurate.
Another common source of noise is thermal noise. In an electronic circuit, the electrons bounce around due to the temperature of the circuit. This introduces random fluctuations into an electronic signal. Some of this happens before the amplifier and is amplified; some of it is added by the amplifier itself, unless it is very cold.
We can detect a faint star only if it is bright enough not to be mistaken for a random peak of noise. Professional astronomers will believe the detection of a faint star or of a faint line in a spectrum only if it has a signal to noise ratio of at least three for a marginal detection, or six or ten for a definite detection.
Processor
With a CCD camera there should be no processing: Raw data is what it makes and raw data is what you get to your computer, straight from the analogue to digital converter, as numbers in a FITS file.
With consumer cameras, data may never be quite as raw as that. Turning raw data from the proprietary format into something that is of practical use often implies processing of the data of some sort. The "raw" data may have been scaled to make best use of the 12-bit storage, which then are not the same as the 12 bit of the analogue to digital converter. The red and blue pixels may have been scaled up to compensate for filter transmission or lower response to photons of those wavelengths.
The Bayer matrix will have been taken care of: In red and blue only a quarter of the pixels have actual data; the other three quarters have to be filled in by interpolation. Even for green half the pixels are missing and need filling in. Colour correction may have been applied to make the colour pictures look more agreeable to the human eye, i.e. photon counts may have been redistributed from one colour to another.
Raw data is not what consumers want, and so the camera will usually process the raw data further, making assumptions about the content of the image and about the sort of picture the consumer wants. The processing will include:
- White-balance adjustment. The human eye wants to see a reasonably white image even when the camera had to work with, e.g., indoor available light.
- Gamma correction and scaling to 8 bit. The raw data contain numbers proportional to the number of photons. However, the human eye is a logarithmic detector. Accordingly, the gamma correction will increase the contrast where the image is dark and reduce contrast where it is bright. In addition, the raw data is probably digitised to 12 bit, which is somewhat unpractical, due to the image storage and display conventions that have developed. The 12-bit data are rescaled to 8 bit to be more acceptable to common image display and image processing software.
- JPG compression. The amateur photographer wants to cram many holiday snaps onto a single memory card. The JPG compression algorithm works well for normal pictures, but is not so good when there is a very sharp edge between light and dark, or when the light areas are just pinpricks in the dark.
We have to be cautious and sceptical about the built-in processing, but not all of it is bad all the time. Some of the objects we want to image are similar to landscapes (halos, rainbows, etc.), and using all the camera's processing saves us time and trouble doing it later. For use at night (noctilucent clouds, aurorae etc.) we should, however, at least use a fixed, standard white balance, such as outdoor/sunlight.
Interface
We have to tell the camera what to do: When to take a picture and how, how to process and store it. A compact or dSLR has this in the camera, as a combination of buttons, software, lights and displays. In addition, software may be available to control a compact or dSLR from a computer. A CCD camera and a webcam rely on software on the computer to provide the interface to the human user.
I use the Linux operating system, where the software included with cameras will probably not work. You may be lucky with MacOS, but your best chances are with Windows.
The Linux kernel does have drivers for a number of webcams, and with the qastrocam application [5] you can control them. As the name suggests, this is optimised for our use. It will normally not take movies, nor stream the video to another Internet user. Rather it will take a certain number of individual frames in a common two-dimensional format. These are then ready for stacking.
Many compacts and dSLRs are supported by the Linux library and application gphoto2 [6]. You can make all the settings and capture the images under control of your Linux computer, then download the images. Support for newer camera models may be patchy, though.
Detector controls
- Easily forgotten, there is the shutter release. A button on the camera is often not appropriate, as pushing it may blur the image. Cable releases can usually be obtained for dSLRs. Failing that you can use the delayed release, where the camera waits about 10 s between pushing the button and taking the image. The manufacturer thinks you use this time to put yourself in the image; we use it to allow the vibrations to die down.
- We can choose the exposure time, also called the "shutter speed". We have to avoid overexposure and blurring, but that aside a longer exposure collects more signal and makes best use of the digitisation.
- We can choose the ISO setting or amplifier gain. Adjust this to make best use of the digitisation - brightest objects of interest are near but below saturation. If possible, longer exposure is better for image quality than higher gain.
You might consider the f ratio another detector control, but it isn't. In compacts and dSLRs the f ratio is set from the same interface, but it is a property of the optics and not the detector.
Processor controls
- Resolution, compression, and gamma correction. CCD cameras can usually bin 2x2 pixel groups into single image pixels for you. If you don't need the resolution, this is probably a good idea. In colour cameras, it is probably best also to bin the Bayer matrix into single pixels, i.e. to ask for half the maximum resolution. However, raw images without JPG compression and without gamma correction are much preferred, if you need to stack or correct for bias and dark current.
- Colour balance. Automatic white balancing will not work for us, as our images do often not contain a mix of colours from which the balance could be calculated. Rather, we should fix the white balance, probably on outdoor/sunlight. Alternatively, determine a custom white balance for your camera and use that instead.
Feedback display
The software for webcam control will show a live, full resolution, display on the computer screen; this is what it is designed for. A compact camera displays a live image of reduced resolution on the back of the camera; this will illustrate the field of view, image brightness, and contrast. A dSLR cannot do this, because one of its major features is the reflex mirror that diverts the light to a visual viewfinder. Compact and dSLR can display the image after it has been taken, and the display can be zoomed to judge not only brightness and contrast, but also focus and resolution. A CCD camera has no preview at all. You will have to guess the exposure time, field of view and focus, take a test image and inspect it. Then try to do better.
References
- QuickCam and Unconventional Imaging Astronomy Group. http://www.qcuiag.org.uk/.
- DALSA Corporation (2009). "CCD vs. CMOS". http://www.dalsa.com/corp/markets/CCD_vs_CMOS.aspx.
- Dave Litwiller (2001). "CCD vs. CMOS: Facts and fiction". Photonics Spectra. January 2001. http://www.dalsa.com/public/corp/Photonics_Spectra_CCDvsCMOS_Litwiller.pdf.
- "What is the difference between CCD and CMOS image sensors in a digital camera?". How stuff works. http://electronics.howstuffworks.com/question362.htm.
- Franck Sicard (2007). Qastrocam. http://3demi.net/astro/qastrocam/doc/. Newer CVS repository at http://sourceforge.net/projects/qastrocam/.
- gPhoto2 digital camera software. http://www.gphoto.org/.
- Stefan Seip (2008). Digital astrophotography: A guide to capturing the cosmos. Rocky Nook. ISBN 978-1933952161.
