Star Tracker
A motorised mount that rotates the camera to match the Earth's rotation, allowing long exposures without star trailing.
Formula / rule
Sidereal rate = 15.04″/s
Measured in
arcsec/s
In depth
By cancelling the Earth's rotation, a tracker allows exposures of minutes rather than seconds, which transforms deep-sky photography — far more signal at far lower ISO. It must be polar aligned accurately, and any foreground becomes blurred, so landscape astrophotography usually blends a tracked sky with an untracked ground exposure.
Examples in practice
A tracked four-minute exposure at ISO 400 records nebulosity that twenty untracked 20-second frames could not.