An astronomical barn door is a simple DIY tool to enable a camera to track the motion of the stars while taking long exposures running to several minutes. Without some form of tracking, stars will rapidly smear into trails, ruining your attempts at astrophotography.
There are many designs of barn doors, but the simplest is the original Haig design, consisting of two bits of wood driven apart by a bolt that is turned once per minute. This design is adequate for single exposures or a series of exposures totaling 10 to 15 minutes, after which incremental "tangent error" causes stars to form noticeable trails.
Barn Door Clock enables a Haig-design barn door tracker to track celestial objects for up to three hours without any incremental tangent error. The clock slowly speeds up to correct for tangent error.
This allows simple, lightweight manual barn door trackers to have the same tracking accuracy as larger, more complex barn doors. Simply input the number of seconds initially required for each cycle of the bolt, the initial angle of the barn door, and the number of prompts per cycle the clock should display. With only a little creativity, the clock can give other rates, like half-sidereal or lunar rate.
The clock also enables you to take rests. In-between sets of exposures, tap the Rest button and quickly dial two cycles ahead (advancing to the "twelve o'clock" position). The clock will beep when you need to resume precise tracking.
The clock design is night-vision friendly.
For more information on building your own barn door tracker, this design demonstrates the simplicity of the concept: