Prime-focus astrophotography usually means you use the camera hooked up to a telescope so that the telescope takes the place of the camera's lens. With this setup, if you use a telescope with a focal length of 500mm, it acts like a camera lens of 500mm of focal length. Technically, even when you use a camera lens, the camera is at "prime focus" of the lens. Generally though, the term "prime focus" means the camera is shooting directly through the telescope with no camera lens on the camera, and no eyepiece in the telescope. The term "prime focus" is used to differentiate between using the scope as a lens, and more complicated optical methods of astrophotography, such as eyepiece projection, or afocal photography. In prime-focus astrophotography, the camera's lens is removed and an adapter is used to place the camera body in the focuser of the telescope where the eyepiece would normally go. This adapter is usually in the form of a T-mount with the camera's specific lens mount and an adapter that fits into the focuser of your telescope. A two-inch adapter and focuser are preferred because a 1.25-inch focuser and adapter will almost certainly vignette the image in a DSLR. For some scopes, such as Schmidt Cassegrains, you may need a special threaded adapter to hook the camera up to the scope. Once you get into shooting through your scope, you will be shooting at longer focal lengths and more magnification. Things like focus and tracking then become more important. More magnification for astronomical subjects also means more magnification for errors and problems. Shooting with a telescope and more focal length allows you to zoom in on a deep-sky object. For smaller objects, this will record much more detail than with a shorter focal length. But with longer focal lengths, you usually also get a slower focal ratio which will require longer exposures. Polar Alignment and Mount Tracking Because we'll be shooting with a longer focal length with more magnification at prime focus of a telescope, our mount's polar alignment and tracking accuracy become more critical. If you're not correctly polar aligned, you may get trailing to the north or south if the exposure is long enough. If you see trailing when you examine your images, check the direction against a star chart or planetarium program. If it's north or south, then you know you have to improve your polar alignment. If the trailing is in the east - west direction, it's probably a question of the mount's tracking accuracy and periodic error. Sometimes you can get crazy trails that are not strictly north - south, or east - west. This can be from wind, or dirt in the gears, or some unknown internal mechanical problem in the mount. It can also be from mirrors shifting or moving during the exposure in Newtonians, Schmidt Cassegrains, or other optical systems that use mirrors. While it is good to keep your exposures as short as possible to keep the number of usable frames as high as possible, the most important thing is to be sure that you expose long enough to get the important faint detail up out of the readout noise of the camera, as we discussed in the section on determining the correct exposure. If your focal length is really long, or your mount's periodic error is really bad, you may not get any frames that do not have trailing in them. In this case, you will either have to switch to a shorter focal length, get a better mount, or start guiding. Compromises The best exposure for your setup will be a compromise between wanting to use a short exposure to minimize trailing due to periodic error and the need to expose long enough to beat the noise in the camera. Take some test exposures to see how long you can expose before you get trailing. Use the shortest exposure you can so that the histogram separates from the left side wall of the histogram box. Then shoot about 20 minutes worth of these exposures. You will find some frames trailed, and hopefully, some not trailed. You want to shoot 20 minutes worth so you are sure you get a full worm rotation. At a reasonably dark observing site, you should be able to shoot an exposure of about 2 to 3 minutes at ISO 1600 at f/6. Shooting 3-minute exposures, you may get one frame out of every three or four that is not trailed. If you have an inexpensive mount, sometimes, for no apparent reason, it may track better or worse. I have a series of 10 frames that I shot of galaxy M83 when it transited in the south where every single frame is trailed. Then I also have a series of 40 frames of M8 and M20, in approximately the same area of sky, but taken a couple of months later, where almost none are trailed. Sometimes one frame will be trailed northeast - southwest, and the next frame will be trailed southeast - northwest. This will really drive you crazy because it is so hard to explain. We just have to accept the fact that we will sometimes get inexplicable behavior in these inexpensive mounts if we are just tracking and not guiding. When you are first starting out, it is really much easier to just let the mount track unguided and pick out the frames that are not trailed. Do not worry about the complexities and added cost of a guidescope, rings, guiding eyepiece, autoguider and computer that are needed for most guiding setups.
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