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Introduction
Earth rotates at almost exactly 15 degrees per hour (ie 360 degrees divided by 24 hours). Astronomical camera / telescope assemblies are typically fixed onto mounts that can accurately counteract this rotation, and so record stars correctly as points of light.
Many subjects in the night sky are too wide to be accommodated within the field of view of a telescope. In such cases, ordinary cameras and lenses are more appropriate. Bright subjects such as the Moon require only short exposure periods, so that a statically mounted camera can be used to effectively 'freeze' the effects of rotation of the Earth.
Less bright subjects require longer exposure periods, so that images would appear to be 'streaked', but for many wide sky subjects, it is not neccessary to use highly accurate and expensive mounts.
An effective and economical solution can be found in a home made 'Barn Door' mount.
This article reviews some background literature and descibes my own implementations and experiences of Barn Door mounts.
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