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Digital Astrophotography is a large and technical subject. There
are some excellent books on the topic.
Many people ask if it is possible to do astronomical digital photography
with their telescopes. The answer is a qualified yes. There are
three basic kinds of digital imaging: Consumer Digital Cameras,
Webcam-type Cameras and CCD Imagers designed for astronomy.
The first two types, Consumer Digital cameras and Webcam-type cameras, are usually not sensitive enough for anything but bright targets like the moon and planets. CCD imagers are relatively expensive when compared with the other two.
Because it is usually not possible to do long exposures with digital photography, several pictures, known as frames, need to be taken and then stacked using a computer to produce a digital image of a dim object like a nebula or galaxy. This technique is called "Frame Stacking".
Another problem with CCD imaging is all CCD produce a certain amount of noise in an image. This can be reduced by cooling the CCD down. Anther technique call "Dark Frame Subtraction" where between imaging frames a frame is taken without gathering any light at the same parameters as the image frames. This "Dark Frame" is subtracted from the image from in post processing on the computer.
Consumer Digital Cameras
Because consumer digital cameras usually do not have removable lens assemblies, the common method of using a consumer digital camera with a telescope is to shoot afocally. Many consumer digital cameras do not even have a filter thread, thus building an adapter to connect the camera to an eyepiece can be a challenge.
Also you will have to find an eyepiece with a wide enough field to reduce or eliminate vignetting.
Webcam-type Cameras
Webcam-type cameras are usually homemade setups in which the lens is remove from a webcam and an 1.25 inch tube is attached (usually made from an old 35mm film canister) to fit in place of an eyepiece. Images are then uploaded directly into a computer and are post-processed with software.
 To
date, consumer versions of these cameras for astronomy are manufactured
by Meade Instruments with their Electronic Eyepieces, which connect directly
to an s-video input like a TV/monitor or VCR, their LPI/Autostar Suite,
their LPI/Autostar Suite, and by Celestron with the NexImage, all which
allows you to image directly into a Windows computer.
 CCD
Imaging
CCD imagers specifically designed for astronomy usually comes with custom hardware and software to drive the whole process. Typically, these systems can be water cooled to bring the temperature of the CCD chips down in order to reduce electronic noise in the image. Many of these systems have built in auto-guiding and filter wheel systems to allow for color imaging. CCD imagers are the most light-sensitive imaging devices and can take stunning, research grade astrophotographs.
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