NexStar Resource Site

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In May 2005, Celestron released a utility that greatly aids Periodic Error Correction (PEC) recordings for the NexStar GPS, CPC, CGEM, CGE and CGE Pro mounts.  Note that Celestron has not updated this tool in years so no guarantees it will work with all current or future mounts.

PECTool is available free from the Support Downloads section of their web site:
Click the Drivers and Software category link, then Show More towards the bottom of the page and search for PEC Tool. Download the "zip" file, extract it to a folder and run the installer.

PECTool requires motor control firmware versions equal or higher than:

  • GPS - 4.06 (beta version 40.40 does not support PECTool)
  • CPC - 5.07
  • CGE - 5.07 (beta versions 41.36 or lower do not support PECTool)
  • CGEM - all available versions are compatible
  • CGE Pro - all available versions are compatible

Read this for upgrading motor control firmware.

PECTool lets you record several PEC runs, average them and then upload the averaged PEC recording to the mount. The result is much lower periodic error than you can achieve with a single PEC recording.

A common point of confusion is that you must then use PECTool every time you set up your scope.  On the contrary, the PEC recording is stored in the motor control section of the mount, so, after you upload the PEC recording to the mount you do not need to run PECTool to use the recording (using the recording to reduce periodic error is called "playback").  Currently though, if you use the hand control (versions 4.03 or lower on the GPS and CPC and 3.01 on the CGE) when you start PEC playback using the hand control, you cannot use the hand control without exiting playback.  You can start PECTool, start playback and then exit PECTool.  Playback will continue in the background and you can still use the hand control for other functions, GoTo, for example.  You can also use my program NexStar Control Pad  to start PEC playback while leaving the hand control free for other uses.  Newer versions of the hand control will likely allow you to start PEC playback and continue using the hand control for other functions.

Another other useful feature of PECTool is that you can now save your current PEC recording. This is useful since upgrading the motor control firmware erase your PEC recording in the mount. The averaged file you upload to the mount is of course your current PEC recording, but it is likely by the time you next upgrade your motor control you will forget which of those files on your PC is the PEC recording you are currently using.  Use PECTool's "Download Data From Mount" button to download (save) your current PEC recording, upgrade your MC firmware and then use PECTool's "Upload Data To Mount" button to restore the PEC recording.


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Michael Swanson
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