PEC stands for Periodic Error Correction All telescope drive systems have small errors in their gears from the limited machining tolerances possible when manufacturing them Since these errors repeat each time the main gear goes around, it is possible to correct for these errors during one revolution of the gear and then playback the correction from the PEC chip in the telescope's drive This feature is helpful when guiding long-exposure astrophotos and is a feature found only on telescope mounts with the capability of doing long-exposure photography