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PostPosted: Thu Oct 15, 2009 1:50 pm 
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Joined: Sat Jul 24, 2004 12:12 pm
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Location: Flackwell Heath, Bucks, UK
Imaging with a DSLR through the telescope

The principle of taking photographs through your telescope with your DSLR is quite simple: just use the telescope in place of the camera lens, and snap away. Simple! But as soon as you try it, you will start to discover the problems.

Attaching the camera
The simplest way is to use a T-adapter. Many modern telescopes have an eyepiece holder with an external thread which is the same as that adopted byTamron many years ago when designing lenses to fit a wide variety of cameras. It is 42 mm in diameter but has a different thread from the old Pentax 42 mm lens fitting. You can get T-adapters for most DSLRs to fit this thread.

If your telescope doesn't have this thread, you can get a 1¼-inch adapter with the thread. http://www.telescopehouse.com/acatalog/Revelation_T_Adaptor_1_25__.html is one example.

You will need to rebalance the telescope once you have attached the camera. Meade ETX telescopes in particular cannot be balanced, the only alternative being to tighten up the declination clamp rather more than is wise. You can buy additional weights to fix to the ends of Schmidt-Cassegrains to help balance them. Don't expect an axis clamp to be able to cope with the extra weight of your camera – it may work to start with, but you'll cause additional wear on both the clamp and, more importantly, the motors, which will always be straining to move more weight than they were designed for.

Focusing
This is not as straightforward as you might wish. DLSRs invariably use autofocus, and lack the focusing aids of film SLR cameras. But your telescope has to be focused manually and judging when a star is in perfect focus can be tricky. You might even find that you can't bring a star to focus at all, particularly with the cheaper Newtonians. The popular SkyWatcher 130, for example, doesn't focus close enough to the mirror to achieve focus with a camera attached.

If you can't adjust the focus far enough out, you could add a short extension tube or a star diagonal, or if you are using a 1¼-inch adapter just don't push the adapter right in but tighten up the thumbscrew to hold it in place. But if you can't focus far enough in, all you can try is to use a Barlow lens in the system as well. This increases the effective focal length of the system, giving more magnified images, but it often allows you to focus.

If your camera has Live View (which shows the actual image seen through the system), use it to help you to focus. On many cameras, such as the Canon 40D, the maximum zoom of Live View has some interpolation and never appears as sharp as the final image so you still have to guess the exact focus point, but it is more precise than using the basic optical viewfinder.

However, the best solution, if your camera manufacturer provides the software, is to shoot 'tethered' to a computer. This allows you to see the Live View image on the computer screen, but without any interpolation, so you can focus critically, but even if you don't have Live View you can still take a shot and check the focus. Shooting tethered has other advantages: you fire the camera from the computer, so you won't jog it, you may be able to operate the camera from indoors, and you have the whole storage capacity of the computer rather than just your memory card. The drawback is that it means yet another black wire to trip over in the dark, and of course you need the computer near the camera, which generally means using a laptop.

Otherwise, use a cable release to trigger your shots so as not to jog the camera when shooting. Some cameras use a conventional mechanical cable release, which is comparatively inexpensive, while others opt for their own electrical releases, which cost more. Alternatively, set the camera's timer so that the shutter goes off either two or ten seconds after you press the shutter release, giving vibrations a chance to settle down.

Shooting
Now you are ready to start taking photos. But as you know, objects move through the field of view quickly unless your telescope is driven. In the case of non-driven telescopes, you will be restricted to the Moon and possibly Jupiter and Venus, which are sufficiently bright that you can give exposure times faster than about 1/100 second. But for longer exposures, the telescope must be motor driven, ideally on an equatorial mount. If you have an altazimuth, you'll find that after a short time stars at the edge of the field of view will start to trail around the centre of the field of view – called field rotation. In this case you may be restricted to exposure times of just a few seconds.

Assuming that you have a driven equatorial mount, you will soon discover its limitations. A drive that keeps Jupiter, say, in the field of view for many minutes at a time for visual observing does not need to be very accurate. It will probably suffer from periodic error, which is a regular variation caused by errors in the machining and alignment of the worm and wheel drive mechanism. Typically, you will see a periodic movement of a few arc seconds every eight minutes or so. The other error is the polar alignment of the equatorial mount. If this is not spot on, stars will trail even in the absence of periodic error.

Periodic error correction (PEC) is provided by many mountings, but you need to follow a star for the duration of the periodic error, using an eyepiece with crosswires which itself is a considerable extra expense, correcting the error as you go. You may need to do this every night you observe, if the telescope is not left in situ during the day. Polar alignment is time-consuming and requires care, and is also lost unless you keep the mount in place. But you can get away without doing PEC every night, and with only good-enough polar alignment, by using an autoguider. But using an autoguiding system is a subject in itself and will not be covered here.

Other issues
As well as autoguiding, you may now need to tackle the other major issues of astrophotography -- getting rid of light pollution on your images, and image processing to bring out the faint detail. These will hopefully be the subject of articles to come.

Robin Scagell
15 October 2009


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