Astronomical photography requires long exposures because celestial objects are very remote and the light received from them on Earth is of a very low level. Another basic problem is that the Earth rotates through 360 degrees once every 24 hours, so everything we see in the sky appears to move.

 The easiest subjects for a beginner are the Moon and the brighter planets. These bodies can be photographed with a modest telescope using relatively short exposure times, hence avoiding the need to track the apparent movement. However, when photographing fainter objects longer exposure periods are required and the apparent movement across the sky produces blurring of the image. A telescope with a drive system that can be used to track movement during the exposure is then required.