Do I need to use the PECTool program to use PEC with my Alt/Azm?

No, you do not need this program or any other program to take advantage of your scope’s periodic error correction (PEC) function.

PEC is a means of recording 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 the CPC’s tracking for astro-imaging.

Note: The scope should be used with a wedge and polar-aligned for PEC to work best.


PEC 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 and can be rerecorded to be changed. The corrections are then played back when imaging an object and the mount thus corrects the drive gearing errors. This is the basic PEC function, and it’s already programmed into the scope and the hand control.

PECTool is a helper application that manages the correction data in the mount. 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


PECTool also displays periodic error. It must be used by connecting a PC via your scope’s hand control with Celestron's NexStar RS-232 (serial) cable.

PECTool is a free download off Celestron’s website at:

Detailed instructions on its use are in the help files which can be accessed after it is installed on your PC.

Updated 12/23/13