Introduction

Earth rotates at almost exactly 15 degrees per hour (ie 360 degrees divided by 24 hours), causing stars to appear to move across the night sky.

Bright subjects such as the Moon can be photographed using 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, causing images from a statically mounted camera to be 'streaked'.

Streaking can be avoided by mounting the camera/telescope on a motorised equatorial mount. These mounts can be expensive.

Many subjects in the night sky appear too wide to be accommodated within the field of view of a telescope. Ordinary cameras with relatively short focal length lenses are then more appropriate and are typically much smaller and lighter than a telescope based system. Then instead of using an equatorial mount, 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.