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

In order to perform PEC, you must first index or reference a point on the worm gear. 

Second you must train or guide your scope for at least a full rotation of the worm gear (eight minutes).  Training tells the mount’s PEC function the periodic errors that exist in the worm.  As you train, the mount is recording the corrections you are making.  So whenever you train the mount, you are recording the errors into the PEC function’s memory.

Last you will play back the recorded training to correct the errors guided out during training.  Playback is performed while actually using the telescope for imaging or visual purposes.