So far in our equipment introduction in our introduction to Astrophotography we’ve discussed the selection of a telescope for astrophotography and the other obvious topic is the selection of a suitable camera. Here we face a problem similar to that with ‘scope selection: there are many kinds of cameras than can be used for astrophotography in some way, some highly specialized for this purpose, others for general use but still capable of astrophotography.
Ideally you’d like a camera to
- Be easy to learn and use; and
- Be inexpensive to own; and
- Produce excellent results
Like the old joke, “pick any two”. You won’t find a camera that does everything well, is easy to use, and produces superb results – especially if you try to do both lunar/planetary and deep-sky photography. “Pretty good” results are achievable with simple cameras, and excellent results are available relatively easily if you restrict yourself to certain types of targets.
So, like we did with telescopes, let’s develop a structured way to look at cameras, to understand their pros and cons and intended uses, and to plan a sensible approach to this aspect of astrophotography.
We’ll need to define some specialized vocabulary that is specific to certain camera types later, but there are some commonly-used terms that are worth discussing right away.
In our introductory discussion we concluded we were discussing only digital astrophotography, not film. Like the other digital cameras in your life, cameras for astrophotography capture the image on an array of small light-sensitive dots, called pixels. And, like the other digital cameras in your life, the number of pixels is one important factor.
You may be surprised to see the relatively small number of pixels on some astrophotography cameras. Marketing hype has trained us to look at “megapixels” as an indicator of overall quality in consumer cameras – probably to excess, as people frequently think a low-quality 20-megapixel camera is a better choice than a high-quality 10-megapixel camera, when that is often not the case.
Many older dedicated astrophotography cameras have under one megapixel, or between one and two megapixels. (Many others have large megapixel counts too, but these can be very expensive.) The original camera on the Hubble Telescope was 640,000 pixels – less than one megapixel (it has been upgraded to multiple megapixels now), and one of the most successful amateur CCD cameras in the world – the SBIG ST-7 – was under half a megapixel, yet has produced tens of thousands of stunning images.
There are a couple of reasons for this.
- Most important, the raw number of pixels is not as important, for an astrophotography camera, as other attributes, such as the size of the pixels, that determine how large a field of the sky you can image, and how fine a level of detail you can record.
- Second, since high-end astrophotography requires an extremely stable and accurate mount anyway, it is quite practical to use multiple exposures of small side-by-side sections of the sky to make up a mosaic image that is indistinguishable from an image taken with a larger chip.
So, the message is, by all means make note of the number of pixels on a camera you are considering, but don’t obsess over it and definitely don’t make it the primary factor driving your purchase decision. Instead, learn about the other specifications that are important to astrophotography.
So if “megapixels” isn’t what we talk about, what is?
Image Scale is the overall term for one set of factors that are important. Technically, image scale is how small a piece of sky corresponds to one pixel on your camera and is measured in “arc-seconds per pixel”. Combined with the number of pixels in the camera (height and width in pixels, separately), it tells you how big a rectangle of the sky is in your image.
More practically, image scale answers questions like “how large or small a target will fit in the frame of my chip?“, “how many pixels are covering my target?” and, most important, “what kind of resolution am I attempting to record“. These questions determine what you can image and how high a resolution image you can produce.
Image scale depends on your camera’s imaging chip and on your telescope. With a given chip, a telescope of different focal lengths produces a different image scale. Google will find you many online calculators where you plug in information about your telescope and camera and get information about your image scale, or you can calculate it quite simply using this formula:
scale = (205 * pixel size) / (focal length) [pixel size in microns and focal length in millimeters]
For example, my QSI583 camera has a pixel size of 5.4 microns, and my AT8RC telescope has a focal length of 1625mm, so the image scale with that combination would be
(205 * 5.4) / 1625 = 0.68 arc-seconds per pixel
In this example, each pixel on the camera is recording a bit of sky that is 0.68 arc-seconds across (a very small bit of sky).
If you are doing astrophotography with a consumer digital camera or a DSLR this may seem a bit academic since you don’t have any choice about the specifications of the chip, but you can still affect your image scale by your choice of telescope. And even for regular digital cameras, you will still find it worth knowing the chip dimensions and the pixel size. Eventually you will have a camera on a telescope, successfully take an image, and then say “Why is my target so small in this image? How do I make it bigger?” or “Why doesn’t my target fit in the frame? How do I make it smaller?” Then you’re talking about image scale.
More important, image scale is a measure of the resolution you are trying to record – smaller numbers indicate higher resolution. However, the resolution that is available to you is limited by atmospheric conditions – turbulence, heat shimmering, dust, moisture, etc. In most normal conditions, 2 to 4 arc-seconds per pixel is the maximum resolution you can record through our atmosphere. You might be able to get resolution as fine as 1 arc-second per pixel on rare nights of ideal conditions, or in the desert or on top of a mountain.
Having a camera/scope combination that is recording much more resolution than is available through the atmosphere is a waste of time and disk space, so you would normally try to arrange to have an image scale in the 2-4 arc-seconds per pixel range. With the example above, I could use 2×2 binning to get 1.3 arc-seconds per pixel, or 3×3 binning to get about 2 arc-seconds per pixel. My imaging would be faster, my files smaller, and I would see no loss of resolution since there isn’t any more detail getting through the air to be recorded.
Only two reasons to do astrophotography immediately come to mind. One, of course, is to create a permanent record of a view that you can look back on later. The other is that most cameras are more sensitive to light than your eye – especially when you add the effects of long or multiple exposures – so you can capture details that you can’t see with your eye and an eyepiece.
Again, if you are using a camera you already own, this is not something you can control. However, if you are selecting a camera for astrophotography purposes, especially a specialized CCD camera, you will see among the specifications an indication of how sensitive the chip is to light.
The unit of measurement is interesting and serves to remind us what an incredible thing we are doing when we photograph a distant target. There are only so many photons arriving at your telescope from that distant galaxy – and they took millions of years to get here, so there is no opportunity to “turn up the brightness”. The sensitivity of a CCD chip is defined as the percentage of available photons that get detected by the chip and turned into a useful signal. This metric is named Quantum Efficiency (QE) and is measured as a percentage, or a decimal fraction less than 1.0.
My SXV-H9 camera, for example, had a QE of 65% – 65 of 100 arriving photons were accurately captured. For comparison, the QE of the human eye has been measured at about 5% – so my CCD camera is about 13 times more sensitive than my eye and it can take long exposures to gather even more light, something my eye cannot do. The “shutter speed” of the human eye is not a clearly defined value, but various studies suggest numbers such as 1/50 to 1/30 of a second. So, a 1-second exposure with my CCD camera is exposed to 30 times more photons, and is 13 times more sensitive; that’s why my camera captures far more detail (about 13×30 = 390 times more) than I can see with my unaided eye.
It’s not only how sensitive to light your camera is that matters, but also to which colour of light it is sensitive. Your eye is sensitive to the visible spectrum: Red through Violet (which is why it’s called the visible spectrum). Your camera will also be sensitive to those colours, but the sensitivity may vary across the range.
Ideally, you would like the camera to be as sensitive as possible to the lowest frequencies in its range – the Reds – because many interesting astronomical objects, being composed largely of Hydrogen, emit a lot of red light (the colour that Hydrogen emits when it is in its most common excited state). Astronomical CCD cameras are specifically designed for this.
Consumer digital cameras, on the other hand, are actually designed against this: they usually have an internal filter to reduce Infrared light (just outside the visible spectrum, right next to Red) because it can distort the colours in everyday snapshots. Unfortunately, the Infrared filter can’t help but reduce the camera’s sensitivity to Red light somewhat. This is why you will hear about some amateur astronomers modifying DSLR cameras by removing the Infrared filter – to increase the camera’s sensitivity to Red light. This is not critical and is by no means an indication that your household camera won’t work for astrophotography without modification. You should not do this to a camera that you intend to continue using for daytime photography, and, as a beginner, you probably will never notice the difference.
If you’re old enough to remember working with film, you may remember the problem of grain: the more sensitive a film was to light, the more it produced a blotchy, mottled effect in your pictures. Digital cameras have a similar problem, noise.
Noise is simply a measure of the degree to which the chip in your camera reports photons that weren’t there. It has a variety of causes including electronic design, recording heat from the electronics as light (which it is), and errors in the circuits that read the information recorded by the CCD (“readout noise”).
If you have a digital camera that can take long exposures, try this experiment. Take a long exposure (30 seconds to several minutes – whatever your camera will allow) of complete blackness – leave the lens cap on and have the camera in a dark room. If your camera has a “noise reduction for long exposures” feature, turn it off for this experiment. (You may need to do something to tell the camera not to worry about the fact that its auto-focus routine can’t find anything to focus on. Typically there is some way to set focusing to “manual”, which is a way of saying “trust me” to the camera.) You would expect the result to be a uniform, completely-black image.
It won’t be.
You will see the frame filled with a speckled appearance of random pixels, possibly with some standing out brightly, and possibly with other patterns of light. On DSLRs it is especially common to find one corner of the frame showing a light reddish cast. This is “amplifier noise”: heat from the camera electronics leaking onto the chip and being detected as infrared light.
All else being the same, cameras with lower noise are preferred. Noise tends to increase with temperature, so dedicated Astronomy CCD cameras usually have some kind of cooling circuitry to reduce noise.
If the noise in your camera is consistent – always the same at a given temperature – it can be dramatically reduced with post-processing techniques such as dark frame subtraction: taking a dark frame and “subtracting” it from your actual image. Some cameras can do this automatically for long exposures.
Dedicated astronomical CCD cameras are often sold this way, as monochrome cameras. With such cameras, colour images are produced by taking multiple exposures through different coloured filters (usually Red, Green, and Blue, or RGB) and then combining the images in image-processing software. A filter wheel, fitted between the camera and the telescope, makes changing filters easier, and may even be computer controlled for complete automation of the process.
Your household digital camera, of course, produces colour images. It does this by having an array of tiny coloured filters in alternating colours (RGB) in front of the imaging chip, so adjacent pixels are seeing the image through various coloured filters. The camera then combines the images from the filtered pixels into a colour result. The array of coloured filters is called a Bayer mask or Bayer filter and the process of combining the result into a colour image is called debayering.
Dedicated astronomical CCD cameras are available that do this, producing coloured images in a single step. These are called one-shot colour or OSC cameras. While OSC cameras are far more convenient, and produce reliable colour images more easily, they have limitations, and many high-end astronomy cameras are monochrome, giving the user the option of using a variety of filtering techniques. For example, narrowband imaging uses 3 filters, that pass only the specific wavelengths of light associated with various elements in their excited state, in place of the usual RGB filters. This allows detailed imaging of gaseous objects such as nebulae.
With some basic terminology in hand, we can start looking at the kinds of cameras used for astrophotography. We’re going to review them in increasing order of probability that you already have one around the house – which also happens to be the order of increasing difficulty of use.
“Point and Shoot” Cameras
Chances are you have a simple digital camera already. If it is for casual family use, it is probably one of the compact “point and shoot” models with a non-removable built-in lens, built-in flash, rear viewing display, and automatic focus. These little cameras are extremely easy to use and revolutionized personal photography.
(This article was written in the early 2000s. Now (2017), chances are you do not have a simple point-and-shoot camera. They are all but extinct, replaced by the decent cameras built-in to everyone’s cellphones. This comments also apply to cellphone cameras, except there may be fewer available controls with your cellphone camera unless you are using add-on control apps.)
These cameras can be used for astrophotography, but are quite limited in what they can achieve. Because the lens can’t be removed, there is only one way to connect them to the telescope: just set the camera’s focus to “infinity”, hold it up to the eyepiece, and line everything up (called afocal astrophotography). You can get mechanical jigs to help hold the camera aligned in this position (see below).
If you already have a camera such as this, it is worth experimenting with afocal astrophotography, and possibly investing in one of the mechanical mounting jigs. You can use this setup to photograph the moon, the brightest planets, and some of the larger, brighter deep sky objects such as M42.
However, if you don’t have one yet, it is not a good investment to purchase one of these for astrophotography purposes because they have several limitations:
- Alignment of the camera lens to the telescope eyepiece is difficult and error-prone. Mechanical mounting jigs can help but require that the camera have a tripod mounting hole, and can be difficult to use on cameras where the lens moves in and out automatically.
- Focusing is difficult on any targets except the moon.
- Some of these cameras lack some of the needed settings (like locking the focus on “infinity”).
- Long-exposures are not possible on most cameras of this type and, where they are possible, they will be noisy.
Digital SLR Cameras
The next step up the ladder of camera sophistication is the DSLR (Digital Single-Lens Reflex). The name Single-Lens Reflex refers to the fact that the viewfinder looks out through the lens rather than through a separate portal, so you are guaranteed that what you are looking at in the viewfinder is exactly what the camera will see. Most DSLRs have removable, interchangeable lenses. Those that do are good cameras for astrophotography for several reasons:
- The removable lens can be replaced with an adapter for direct connection to a telescope.
- They usually have facilities for long-exposures.
- They usually have a manual focus for afocal use.
- They often have variable sensitivity for use in dim light.
- Some have built-in noise reduction, by dark subtraction, for use with long exposures.
These cameras can be used in afocal mode, exactly like the point-and-shoot cameras described above. They can also be used in eyepiece projection mode where the camera, without lens, is placed over the eyepiece of a telescope, or in prime focus mode, where the camera, without lens, replaces the eyepiece of the telescope. (These techniques are all described later in this series.)
Most people think of “prime focus” when they think of connecting a camera to a telescope without additional lenses or eyepieces. This is an excellent method for long exposures of large deep space objects, because the large chip in the DSLR gives a wide field of view. However, small targets such as planets may appear disappointingly small in a DSLR image taken with prime focus. Eyepiece projection and Barlow lenses are ways to increase the magnification for cases such as these.
Focusing a DSLR connected to a telescope can be a challenge since the dim image is usually very hard to see clearly through the viewfinder. Precise focusing will require taking a series of test exposures, with careful focus adjustment between them.
All-Manual Mode Only!
After seeing a few posts from confused beginners in various online forums, I thought it worth making a point clearly here.
If you connect your DSLR to your telescope for astrophotography, you must set it to all-manual mode.
- Set it to manual focus so the autofocus system doesn’t try to engage (and focus with the telescope’s focuser); and
- Set it to manual exposure (not Auto, Program, Aperture Priority, Shutter Priority, or any of the little preset icons like “portrait”, “sports”, or “landscape”) and set your exposure time manually. You won’t need to worry about setting aperture because you’ve removed the camera’s lens.
I’ve seen some online posts from beginners who left their cameras in “full auto” mode and then just got error messages on the camera’s displays when trying to do exposures. Sorry: it’s harder than that; you need to go “full manual” on everything.
Issues with Long Exposures
If you plan to do long exposures (30 seconds to many minutes) with a point-and-shoot camera or a DSLR, you will discover this attracts several problems.
- First, you need to work out how to even ask your camera to take a long exposure. Most point-and-shoot cameras have no ability, or limited ability, to set the exposure time at all. While most DSLRs have the ability to set the exposure time, there may be an upper limit (such as 30 seconds) to how long the camera will expose an image for you.
However, most DSLRs have a “B” setting (for Bulb – an archaic reference to when photographers would open the camera shutter in a darkened room and then manually trigger a flash) in which the shutter is held open as long as the shutter button is pressed. This setting can be used with a manual or timed remote control cable to take exposures longer than the camera will do by itself. Also, more advanced DSLRs may have the ability to be connected to a computer and to take “open/close” instructions from it.
Some DSLRs also have the ability to lock the mirror up separately from the act of opening the shutter. It is a good idea to use this technique if it is available on your camera, starting the exposure after the vibration from the mirror snapping upward has subsided.
- Next, you may need to improve on your camera’s power supply. Most digital cameras use power to hold the shutter open and, in the case of DSLRs, to hold the mirror up, so taking very long exposures is an unusual draw on the battery’s power reserve. If you plan to take many long exposures, you will want spare batteries or, better, an external power adapter for your camera.
- Finally, as mentioned above, noise increases with the length of exposure. Your camera might have built-in noise reduction which is convenient, but doubles the length of every exposure by following the exposure with a dark frame of identical duration. Or you can take dark frames manually and subtract them from your collected images later, in post-processing.
Whatever the practical limit of your ability to take long exposures, you can always approach the effect of longer exposures by taking multiple images and “adding” them together on your computer. This technique, called stacking, is described in more detail later in this series.
Mirrorless Cameras (“Compact System Cameras”)
Newer than DSLRs, Mirrorless cameras also allow you to focus and compose directly through the lens. However, rather than achieving this by diverting light with a flip-up mirror, they use a different electronic sensor chip that can produce a live image during composition. Good mirrorless cameras are smaller and lighter than DSLRs and have their own system of interchangeable lenses. The comments given for DSLRs above apply almost exactly to Mirrorless cameras.
Webcams and Small Video Cameras
You may be surprised to learn that “webcams” – the inexpensive video cameras sold for interactive video chatting on personal computers – can be used for astrophotography. More than that – they are the best tool for a certain class of astrophotography, namely photographing the moon and the bright planets (Mercury, Venus, Mars, Jupiter, and Saturn).
Hooked to a telescope, one of these cameras is used to take a several-minutes-long video of the target. Since a video consists of multiple short exposures (usually 10 to 20 frames per second), the individual exposures are quite short, minimizing the effects of telescope shake or imperfect tracking. A several-minute video will be composed of many hundred, or even a few thousand, individual exposures. Computer software can then be used to automatically reject poor frames and to align and mathematically combine the good ones into a single picture that is of much higher quality than any individual exposure.
- The short exposure times of individual frames allows less-precise mounts to be used.
- Mount tracking is less critical. Although the target may drift in the field of view in a several-minutes-long video, software can align all the frames.
- Taking hundreds of frames greatly increases the probability that many of them will correspond to fleeting moments of excellent seeing. Poor frames can be discarded.
- Combining multiple frames greatly increases the resolution of the image, revealing details that cannot be seen with the naked eye or in individual exposures.
However, there are limitations to this technique as well:
- Most important, it works only for very bright targets. The moon and the bright planets will work, but deep space targets will not.
- You need to work out a way to mount the camera to the computer. Fortunately, most of these cameras have a threaded hole to hold the lens assembly, and an adaptor can be purchased to replace the lens with a 1.25″ tube that fits like an eyepiece into a telescope. Astronomy supply stores carry these adaptors.
- You need a computer at the telescope to drive the camera, and power for the computer.
- Some trial-and-error is necessary to learn the appropriate exposure lengths, and to learn to accurately focus the image.
- The software for selecting and combining frames is complex and will take some time and effort to master. (It is free, however.)
- Good computer skills and available time for image processing are needed – the result is not “instant gratification”.
You will see cameras of this type packaged in three main ways:
Commercial “Lunar and Planetary” cameras: Cameras that are virtually identical to WebCams can be purchased from major telescope manufacturers, packaged for astrophotography use. Purchased this way, they will have a 1.25″ eyepiece tube installed and are usually bundled with appropriate software.
A Special Case: Integrating Live Video Cameras for Astronomy
It’s worth mentioning one more class of video camera: the small video camera that produces a video signal, not a digital signal. These cameras are designed to be hooked to a small portable monitor or television and used for “live” viewing of the telescope image. They have an internal ability to combine small numbers of frames to enhance resolution. (For example they may take 30 frames a second, but combine groups of 5, producing an enhanced 6 fps image.) They produce a “live” image on the monitor that shows more detail than can be seen with your eye at an eyepiece. They are primarily intended to produce an “enhanced live viewing experience”, but they can be used for photography either with a “frame grabber” feature on a video card, or by recording video and then stacking frames with a program such as Registax, as is done with a webcam.
I have one of these cameras and find it a great aid at star parties, or with visitors in the observatory, to let them easily view objects, since several people can see the screen at the same time, so I can point things out to them. However, I don’t use it for photography and it’s outside the scope of this article. If you would like to use such a camera for photography, solve the problem of capturing the video frames to your computer, then follow an approach similar to that discussed for webcams here.
Specialized Astronomy CCD Cameras
The final section in our tour of cameras will be a look at the specialized CCD cameras that are made specifically for astrophotography and that can’t be used for anything else.
Comparison to Normal Cameras
When you first look at an astronomical CCD camera you are likely to think, “where is the camera?”. Astronomical CCD cameras look nothing like normal household cameras and are, indeed, very different devices. In one sense, they are very simple: they are just a CCD chip and the circuitry to make it work.
- There is no viewfinder;
- There is no focus;
- There is no exposure control mechanism (aperture diaphragm or shutter speed);
- There is no lens and no way to mount one (or, if there is, it is complex and is only for specialized uses);
- There is no shutter release button. In fact there are no controls of any kind, and may be no shutter.
- There is no built-in memory or built-in power supply: the camera must be connected to a computer to work.
Why buy such a limited and specialized device instead of a DSLR that can be used for other purposes?
- The CCD is optimized for dim subjects. It is far more sensitive to light, and is designed to produce little noise during long exposures;
- The CCD usually also responds to a broader range of frequencies of light, especially at the red end of the spectrum;
- There are provisions for keeping the CCD chip cool to further reduce noise;
- There is circuitry optimized for getting data off the chip and into your computer quickly (e.g. USB2, FireWire, or USB3 connections).
- Astronomical CCDs are smaller, lighter, and simpler than DSLRs, so they are more compact and easier to mount and balance on a telescope;
- The lack of mechanisms means they are more reliable and they don’t tend to misbehave in the cold;
Features and Specifications
There are many technical specifications associated with astronomical CCD cameras, and it can be confusing. Some of the most important specifications include:
The number of pixels on the chip is a good basic measure of chip capability. All else being the same, more is usually better; but all else is rarely the same. The size of the chip, the number of pixels in each dimension, and the size of the pixels are more important.
Smaller pixels on a chip of a given size mean that more pixels are covering your target, which will result in higher-resolution imaging. On the other hand, larger pixels are usually more sensitive to light, and don’t result in a loss of resolution if used with the larger image produced by longer-focal-length telescopes.
For example, consider two hypothetical 4-megapixel chips, each being square, 2 megapixels on a side. One is chip is 1 centimetre square, while the other is 2 centimetres square.
While these are both “4 megapixel cameras”, they have very different capabilities and are suited to different types of telescopes. The larger chip can image larger objects with a given telescope, or can handle a wider-field telescope for a given target. The smaller chip produces a higher-resolution image of a small section of sky, so may be better for small targets. The larger chip, because it has larger pixels, may also have better sensitivity.
Beginner-level cameras will make most of these decisions for you by offering a reasonable number of pixels (1 to 4 megapixels) of moderate size. As you become more advanced, you may start choosing specific camera specifications to match the specific telescope you will be using. Sky and Telescope has a good article on matching chip specifications to your optics.
CCD chips produce electronic noise, and lower-noise chips are preferable to noisy ones. Unfortunately there are many kinds of noise, measured in a variety of non-standard ways, so you are looking for words saying “low noise” or evidence of low noise, usually provided by showing nice noise-free dark frames.
Two fairly common technical measures are Readout Noise and Dark Current, both of which should be low numbers.
CCD chips produce more noise at higher temperatures, so almost all astronomical CCD cameras have some provision for cooling the chip. The cooling features will include:
Most CCD cameras have the option of binning: combining a set of adjacent camera pixels into a single pixel produced as output. For example, a square of 4 pixels on the CCD chip might be reported as one pixel containing their combined value. This example, creating a square 2 pixels by 2 pixels, would be called “2×2 binning”.
Binning has a variety of purposes and is a useful feature. While reducing the resolution of the image, it increases the sensitivity and improves the signal to noise ratio. By increasing the sensitivity, it allows shorter exposures and by producing fewer bits of image, it permits faster downloads during fast exposures while hunting for objects or focusing, or allows some parts of a multi-exposure image to be exposed differently than others. For example, it is common to combine a full-resolution monochrome image with binned images taken through colour filters to produce a colour image faster.
Binning normally applies only to monochrome cameras. Binning a One-Shot Colour camera would be combining adjacent pixels that are filtered for different colours, which would eliminate the ability to “debayer” the image to produce colour. You might still bin an OSC image for other purposes, such as focusing or guiding.
When taking long exposures, some CCDs react to bright stars in an otherwise dim field of view by producing a horizontal or vertical streak of light, which can spoil the image. This error is called blooming. Some cameras incorporate antiblooming circuits which reduce or eliminate this.
While this seems desirable, it is actually controversial. Anti-blooming features reduce the sensitivity of the chip and there are other ways to reduce blooming, such as combining multiple shorter exposures. Many astrophotographers prefer not to buy anti-blooming features in their cameras for these reasons. Beginners may like Antiblooming to avoid having images containing bright stars spoiled.
Apogee Instruments, a manufacturer of high-end CCD cameras, has a good article on blooming vs. antiblooming.
Even with a high-end mount, long exposures will produce streaked stars because of minor variations in the mount tracking. The solution to this is guiding: sending the mount frequent instructions to do minor corrections in its tracking. We’ll discuss guiding in depth in a later article.
Guiding requires a separate CCD chip, either in a separate camera, or as a separate chip in the main camera, that takes small images every few seconds and then reports them to the guiding software, which calculates any needed corrections and sends them to the mount.
There are many approaches to guiding, involving separate cameras or the main camera, and separate guiding telescopes or the main telescope. When reading camera specifications, you will notice that many are described as “self-guiding”, which means there is a second, small, CCD imaging chip inside the camera that is used for the guiding images.
Guiding commands sent to the mount require a special electrical signal, and a special relay box needs to be connected between the computer and the mount to produce this signal. Many self-guided cameras contain this relay box, so the camera can send the guiding signal to the mount, requiring one less box and one less cable in your setup. This is usually called an “ST-4 signal”, after the protocol used with the original ST-4 autoguider pioneered by SBIG.
CCD cameras must be used with computer control, and the collected images are sent to the computer. Since images can be quite large, the download speed is important. Most modern cameras use USB2 or USB3 connections, which are quite fast. Older cameras use only USB1 and very old cameras (inexpensive on the used market) may use a simple serial port connection.
A camera with only a serial port connection may be a problem depending on your needs:
- New computers usually do not have a serial port connector, so you will need a serial-to-USB adaptor; and
- Serial connections are slow – so slow that you may not be able to use a serial-connected camera for applications, such as autoguiding, where multiple exposures must be produced and downloaded quickly.
CCD cameras don’t use a shutter to begin and end the exposure – that is done electronically by the camera, by simply switching on and off the circuit to collect light. However, astrophotographers need the ability to take an image of complete blackness, to create “dark frames” for noise cancellation. One way to do this is to take an exposure with the dust cap of the telescope installed. Some cameras, however, have a built-in shutter that can be closed under computer control. This is used only for taking dark frames, not as part of regular imaging, but is a great convenience.
Another approach for conveniently taking dark frames is, on a camera with an automatic filter wheel, to have one position in the filter wheel that is completely black, and to take exposures with that “filter” selected.
CCD cameras range in price from $200 to $15,000, and you might well wonder what an extra $10,000 gets you in a camera. Typically, the expense of high-end cameras buys:
- Larger chips containing more pixels
- Very high sensitivity combined with very low noise
- Regulated multi-stage cooling
- Built-in autoguiding chips and control circuits
- Built-in filter wheels
As a beginner, you can get good results with cameras in the $few-hundred to $2000 range, especially if you are willing to buy good-quality used equipment.
Connecting Camera to Telescope
You can take basic snapshots of the moon by just hand-holding your camera carefully in position at the telescope and using a fast exposure. The results won’t be great, but it will work.
- Hold it in exactly the right position, both left-right and up-down with relation to the telescope, and perfectly parallel (or orthogonal, depending on how you think about it) with the telescope optics; and
- Hold it at a distance from the telescope or eyepiece where focus can be achieved; and
- Hold it securely in position with no slippage or vibration whatever, even when the telescope is pointed to different parts of the sky.
Telescopes offer several ways to connect a camera. Cameras may be less standard: CCD and DSLR cameras are fairly simple to connect, but point-and-shoot cameras were not designed for this, and may need more complex setups.
There are only a few options on the telescope for where you would attach a camera.
You will almost certainly be attaching your camera to the telescope at the telescope eyepiece location, either with an eyepiece in place or without.
- With an eyepiece in place, you will need a mechanism that clamps over the eyepiece and provides a camera attachment. You’ll do this for afocal and eyepiece projection photography.
- Without an eyepiece in place, you will attach a tube to the camera so that it simulates an eyepiece, and just slide this into the telescope’s normal eyepiece holder. This is used for prime focus photography.
The mounting method you use on the camera depends very much on the type of camera.
For afocal photography, nothing is attached to the camera at all – the camera and its built-in lens will be just held in place at the eyepiece with a clamp.
For all other modes, with DSLRs or CCD cameras, the lens is removed and the camera, without lens, is attached to the telescope. Fortunately there are some widely-accepted standards on how to do this.
For DSLRs there is an elegant solution. All manufacturers sell a T-Ring Adaptor: a narrow metal ring with a T-Mount thread on one end and whatever shapes are necessary to fit into the camera’s lens holder on the other end. For example, you would buy a T-Ring for Canon, a T-Ring for Nikon, etc. All of these have the same T-Thread on one end, and their intended camera’s proprietary connection on the other.
Another, less common, connection that you may encounter is the C-Mount. This is a standard threaded connection of a smaller diameter than the T-Mount: one inch in diameter with a 32tpi thread.
C-threads are common on small video cameras and other small cameras developed for industrial use, so you may encounter them if you are using video cameras or webcams for lunar or planetary imaging. If you have one, you can get appropriately threaded eyepiece tubes for connection to a telescope, or you can use a C-thread to T-thread adaptor.
Specific Connection Methods
Now we can discuss typical connection methods you are likely to use for specific applications.
You will need a mechanical jig that can be attached firmly to the telescope, holds the camera firmly, and allows adjustment in 3 dimensions so you can precisely line up the camera lens with the eyepiece. An Internet search on “digital camera telescope adaptor” will find many sources.
One example is shown here. The adaptor is clamped around the eyepiece, and vernier controls allow precise adjustment of the camera in two dimensions, while the standard tripod screw adjusts it in the third dimension.
If you have a DSLR, the preferred way to connect it for wide-field, deep sky objects is Prime Focus, where the telescope, with no eyepiece, acts as the camera’s lens. We already discussed this method when introducing the T-Mount system above.
- Purchase a 1.25″ or 2″ eyepiece tube (depending on which size eyepiece your telescope’s focuser accepts) with a T-thread on one end.
Prime focus of a DSLR (above) produces a low-power, wide-field view. One of the ways to increase magnification for imaging smaller objects is Eyepiece Projection, in which the camera, without lens, is mounted behind a telescope eyepiece, just like your eye would be placed. Then you can vary eyepieces to vary the magnification of the image in the camera.
Your next challenge, after getting the camera connected to the telescope, will be getting the system to come into focus. If you are doing eyepiece projection or afocal photography this will be no problem – just set the camera’s focus to “infinity” or “distance” and then use the telescope’s normal focus knob.
If you are doing prime focus photography, you may find it is not possible to move the telescope focuser far enough to focus. The problem is that the telescope is set up to bring light to focus where an eyepiece would be in normal use – so the length of the “light path” to the focus point is calculated to include the telescope, a diagonal (for refractors or SCTs), and an eyepiece. If you take the eyepiece and diagonal out of the system to connect a camera, you have made the light path shorter, so you will have to crank the focuser “out” farther – possibly farther than it is able to go.
This is usually not a problem on SCTs, because the focus can be moved through a very large range of positions. It is often a problem on refractors and reflectors, and could be a problem on an SCT if you have added additional accessories (such as external focusers or filter wheels) that add more distance to the light path.
If you need to get the camera farther from the telescope to achieve focus, the solution is extension tubes. If you carefully hand-hold an eyepiece or camera in free space behind the telescope, looking for the point of focus, you should be able experimentally determine approximately how much extra distance you need to add. Usually you will find you need 50 to 100 mm. You then add an extension tube of the appropriate length, and you will be able to achieve focus. An extension tube is just a hollow metal tube, containing no optics, with appropriate fittings to go in the telescope’s eyepiece holder and accept the camera connection at the other end. The camera end of the extension tube might, itself, have an eyepiece hole and set screw, or it might end in a T-Mount thread, allowing direct connection to the camera.
If you need to get the camera closer to the telescope to achieve focus, you may have a problem. You might be able to use fewer, or thinner, intervening accessories; or you may have to abandon the approach and try something different.
As you go through various telescope and camera phases, you will probably build up quite a collection of adaptors and extension tubes. Keep them – you will need a variety of combinations as you work through the hobby.
By the way, your diagonal acts as an extension, since it adds length to the light path. However, most imagers prefer not to include a diagonal in the light path because
- the additional glass absorbs or scatters a certain amount of light, and because
- the right-angle turn puts the weight of the camera off the centre line of the telescope, possibly creating balance problems.
Modifying Focal Length
One final topic is worth considering as we discuss all the bits and pieces that you may be connecting together on your telescope. You will occasionally find your telescope is producing the wrong amount of magnification for an imaging task. It may magnify too much if you are trying to image a large galaxy or nebula, or it may magnify too little if you are trying to image a small object like a planet.
The size of object you can image is a function of the size of the CCD chip in your camera, and the focal length of your telescope. You can’t easily change the size of your CCD chip, but you can easily change the effective focal length of your telescope.
A focal reducer is an additional optical lens that you place in the light path, that has the effect of reducing the focal length of the telescope by some factor. 0.6x reduction is very common. So, if you put a 0.6x focal reducer on the back of a 1000mm focal-length telescope the result is a 600mm telescope.
A focal reducer has two other important advantages too:
- It reduces the focal ratio of the telescope as well as the focal length. So, with a 0.6x focal reducer, an f/10 telescope becomes an f/6 telescope. This yields shorter exposure times (in this example over twice as short).
- Most reducers also have the effect of flattening the field – i.e. reducing the tendency for stars at the edge of the field to be out of focus when the centre of the field is in focus. Because of this effect, reducers are often called “flatteners” or “correctors”, or some combination of these words.
Focal reducers are especially popular with users of SCTs, since these tend to have very long focal lengths and slow focal ratios. Many reducers are designed to thread directly onto the rear cell of an SCT.
A Barlow Lens is the opposite of a focal reducer: it increases the effective focal ratio of the telescope by a given factor – usually 1.5x, 2x, 3x, 4x, or 5x. It also increases the focal ratio, so longer exposures are required.
A Barlow lens is an excellent way to increase the magnification of a given telescope enough to get good sized images of planets, and may yield better results than the more complex Eyepiece Projection method of increasing magnification.
Supporting the Weight
You may encounter another surprising problem as you venture into astrophotography.
The camera and connector system we described above is potentially quite heavy. On a refractor or SCT, this weight is pulling down on the bottom of the telescope. On a reflector, it is pulling off to the side, which may be downward depending on the orientation of the telescope. This may cause you two problems:
- It will dramatically change the balance point of your telescope. You will need to re-balance the scope on the mount, and may need to change or add additional counterweights.
- You may discover that your focuser can’t hold all this weight in position – it may slip, with the tube gradually creeping outward as the weight of the camera pulls it down. If you find that your images will not stay in focus, check if your focuser is slipping. There is usually some way to adjust the tension on a focuser to compensate for this. On Crayford focusers there is often a small knurled thumbscrew to easily adjust tension, while other focusers may requiring tightening set-screws with a tool, or even adding shimming material such as a strip of Scotch tape on the focuser tube.
Like other aspects of equipment selection for astronomy, picking a camera for astrophotography is a case of making choices and compromises, trading off price, convenience, quality, and ease of use, all while trying to best match the camera to your intended use.
There is a lot to learn so, all else being the same, you would be better to start with a simple camera and master some basic skills before investing in a complex and expensive instrument.
- Use an inexpensive webcam to do lunar and planetary imaging.
- This allows you to learn basic setup, focusing, and computer skills including image stacking and image processing.
- Exposure times and precision of your mount tracking aren’t so important at this phase
- Next, use a DSLR in prime focus mode to begin imaging DSOs.
- The DSLR has other uses in your life, so the investment is spread out over several applications.
- Colour imaging is automatic with the DSLR so you don’t have to learn the skills of combining filtered images yet.
- You will learn the art of focusing on faint objects, and begin developing the skills needed to take long exposures, including improving the tracking of your mount.
- Consider learning autoguiding now, using a separate guide camera (or defer till the next step if you will be buying a self-guiding CCD camera).
- Finally, and this step is optional, consider a dedicated Astronomy CCD camera for more detailed long exposure imaging of faint objects.
- Since you already have a DSLR for easy colour photography, you might consider a monochrome CCD, with separate colour filters, to explore the full range of applications of astrophotography.
- You will spend substantial time learning to manage exposure lengths and combine images from multiple exposures.
- You will almost certainly need to do additional work on the tracking accuracy of your mount.
- Master autoguiding for long exposures.
- If you have a monochrome camera, learn to make composite colour images using filters.
The next topic in our introduction to astrophotography equipment will be a discussion of Mounts for Astrophotography.