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Deep-sky subjects are very dim, with surface brightness many orders of magnitudes less than foreground objects. The consequent lack of photons implies long exposure times of up to several hours. However, the distant object will move significantly relative to the camera during an extended exposure period - perhaps by a couple of pixels per second when using moderate focal lengths.

The ideal solution to this problem would be to use a perfect tracking system in combination with optics that produce a perfect image at the plane of focus. However, no such system exists. Polar misalignment, drift, periodic error, frame rotation, mechanical flexing, thermal effects, wind and other factorss combine to produce unacceptable tracking errors. One practical way forward is therefore to divide the long exposure period in to a large number of shorter sub-exposures and then "stack" the resulting images using specialized image processing software.

Short-exposure images can be taken manually, but the process is clearly time consuming and tiresome. Fortunately, software available from major camera manufacturers, and packages such as Images Plus, can be used to automate the process. The computer then instructs the camera to make a sequence of exposures at predetermined intervals over the required period of time. A pause between exposures of a few seconds allows for each image in the sequence to be downloaded.

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