What is periodic error?

PE is the back-and-forth tracking error caused by imperfections in the mount's worm gear. It repeats every worm cycle (typically 4 to 10 minutes).

Use periodic error correction (PEC), autoguiding, or a mount with a high-quality worm gear. Belt-driven mounts often have lower PE.

Is autoguiding always necessary?

For long focal lengths or multi-minute exposures, usually yes. At short focal lengths (under 200 mm) many mounts track well enough unguided.

Determine the maximum acceptable periodic error of a mount: PE_max = pixel_scale * acceptable_drift_px, in arcseconds.

Pixel Scale

arcsec/px

Acceptable Drift

Worm Period

Max Periodic Error

1.5400 arcsec

Max Drift Rate0.1925 arcsec/min

Max Periodic Error vs Worm Period

For unguided astrophotography, the mount's periodic error (PE) must be small enough that stars do not trail beyond the pixel scale.

PE_max = pixel_scale * drift_pixels

*pixel_scale* = arcsec/pixel from sensor and focal length

*drift_pixels* = acceptable pixel drift (typically 1 to 2 px)

The drift rate limit is PE_max divided by the worm-gear period.

0.77 arcsec/px scale, 2 px acceptable drift, 480 s worm period.