What is periodic error (PE)? What is PEC?

Periodic error (PE) is the name for the inevitable small mechanical errors that occur in the drive gearing as 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 recording your guiding corrections.


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

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

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

PEC is a means of reducing errors caused by the mount's worm gear, as it turns to drive the mount.  

In order to perform PEC:


1. You must first index or reference a point on the worm gear.  


2. Train or guide your scope for at least a full rotation of the worm gear (eight minutes).  As you train, the mount is recording the corrections you are making, so you are recording the errors into the PEC function’s memory.

3. Playback the recorded training to correct the errors guided out during training. Playback is performed while actually using the telescope for imaging or visual purposes.

Updated 12/20/13