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Periodic error (PE) is the name for the inevitable small mechanical errors that occur as the drive gearing in the scope turns. Since these errors are part of the scope’s drive and are predictable, they can be evaluated and counteracted to improve tracking for astro-imaging.

PE correction (PEC) on most Celestron German equatorial GOTO mounts does this over an 8-minute interval, finding a starting point or index of the drive rotation, and then recording your guiding corrections.

This information is stored in the mount.  It can be rerecorded to be changed.

The corrections are played back when photographing an object and the mount thus corrects the drive gearing errors.

This basic PEC function is already programmed into the scope and the hand control.