What is Periodic Error (PE)? And how do I correct it?

Periodic error (PE) is the name for the inevitable small mechanical errors that occur in the drive gearing as the scope is tracking during long-exposure imaging. 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.

On most Celestron GoTo mounts, we have a Periodic Error Correction (PEC) feature in the hand control which records guiding corrections over one full turn of the worm gear. It then finds a starting point or index of the drive rotation, and plays back your guiding corrections. PEC is a means of reducing errors caused by the mount's worm gear, as it turns to drive the mount.  


You can program PEC through any of the three options:

  • PEC – Your hand control comes with PEC and you can manually correct the periodic errors occurring while slewing using a crosshair eyepiece (aka eyeballing it).
  • Autoguider – Use an autoguider (sold separately) which tracks a star and will adjust automatically correcting the periodic errors as it tracks.
  • PEC Tool – Lastly you can download PEC Tool from Celestron.com and with the help of an autoguider and your computer, you can record multiple cycles of your worm gear, obtain the average course corrections for optimal use, and upload it to your mount.



PEC stores the corrections it receives after one worm gear rotation and plays it back when photographing an object.  The mount thus corrects drive gearing errors.

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.  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.



Autoguiders are special cameras that replace the eyepiece on your telescope.  The camera looks through the scope and watches a star while your mount tracks it.  If the camera detects that the star has moved from its original position, it sends a command to the mount to move the star back to where it was.  This is much faster and much more accurate that you can accomplish by guiding manually using the hand control and a crosshair eyepiece.

Note: Celestron does not supply autoguider software, which automatically counteracts guiding errors as they happen. Two freeware autoguider programs you can find on the Internet are Guide Dog and Meta Guide. Biasing the CGE mount to slightly increase error can actually help such programs. RA can be biased by either dropping or raising the counterweights to slightly imbalance the mount.

PEC Tool

PEC Tool is a "helper utility" which allows you to record and playback PEC data. It also lets you backup, restore, average, and remove drift information from the playback data. The tool averages multiple runs to remove drift. PEC Tool must be used by connecting a PC via your scope’s hand control. With this tool, you can:

  • Seek the worm index
  • Perform training or playback
  • Download the corrections from the mount and view the PE
  • Remove drift from the corrections
  • Save the mount corrections to a file
  • Average multiple training runs
  • Upload corrections back to the mount


Note: PEC and PEC Tool can be used on both Alt/Azm and German Equatorial, though Alt/Azm mounts must have a wedge, such as CPC, CPC Deluxe, and NexStar Evolution. Although NexStar SE 4 & 5 does come with a wedge, running PEC and PEC tool is not necessary.