Astrophotography comes in many forms, from star trails and wide-field Milky Way shots to high-resolution images of the planets in our solar system. The equipment needed to simply view distant objects can be purchased fairly inexpensively, but getting good photographs of them can cost considerably more. There are a few ways to capture objects in deep space with a fairly low-cost setup, though and I'm going to describe one in this article.
This method involves pairing your DSLR with a telescope type that's typically considered impractical for planetary photos. It's commonly called a Dobsonian telescope, in reference to the type of mount it uses.
A large-aperture (high-magnification) reflector telescope is mounted on a low platform that pivots on the azimuth axis and incorporates a yoke that allows the altitude axis to be adjusted. Both axes are adjusted by hand to align the scope with the object to be viewed. This type of mount allows amateur astronomers a great option for viewing celestial objects without the cost of mounts designed for automatic tracking.
It's that lack of tracking capability that makes most astrophotographers put the Dobsonian telescope in the “viewing only” category. Because the light from a deep space object (DSO) is dim in relation to closer objects like the moon, longer exposures are required and tracking helps keep an object centered to avoid blurring. However, there's another way, using multiple images. Since you can get your hands on a Dobsonian telescope with a 6” aperture (1200mm focal length @ f/8) for about $300, it's a great way to try your hand at DSO imaging cheaply.
There are a couple of ways to take a photo of the view through a telescope. The cheapest (and most difficult) is afocal capture, in which you simply focus the camera on the image in the eyepiece, either with a very steady hand or by mounting the camera on a tripod or a special bracket attached to the optical tube. It's important to keep the camera lens aligned with the viewing lens on the scope to avoid vignetting. Prime focus capturing involves mounting the camera body in place of the viewing lens, via an adapter and reducer. This can add considerably to the cost of the setup, however.
Bright objects, such as dense star clusters can be photographed this way, but your results will vary greatly, since the exposure time may be too long to avoid trailing and blurring. The planets, however, are comparatively quite bright and can be imaged well by taking a series of photos or recording a video as they cross the field of view, then stacking the photos or movie frames. This method of creating a high-resolution composite is known as planetary imaging, and it's easily accomplished with a free software application named Registax.
A detailed guide to the process of taking and processing the images is beyond the scope of this article, but there are several great guides available. One that I recommend is A Guide to DSLR Planetary Imaging, a very helpful guide on CD-ROM. The point of this article, which I hope you've seen, is that you don't have to invest a lot to get started in planetary astrophotography. Enjoy!