Most motorized mounts offer the ability to perform Periodic Error Correction (PEC). The periodic error is due to the internal mechanics of the mount. The magnitude of the periodic error varies from frame to frame. Some very high quality frames have minimal periodic error. Obviously, the price of the mount increases with the tracking quality. You can invest more than $10 on a very high quality frame. 

You can increase the tracking quality of your mount by performing a PEC. Tracking will be more accurate, even when autoguiding. In visual observation, PEC is not necessary because even if there are tracking errors, the object will always remain in the center of the field of view of the eyepiece. On the other hand, during long exposure times (over 30 seconds) for deep sky photography, the periodic error of the mount can cause trailing stars to appear instead of round.

This procedure will briefly describe how to perform a CEP with the frame I am using (CGEM). Subsequently, this procedure can be adapted to your equipment. To carry out the PEC, I use a camera CCD ZWO ASI 120MM installed on on an optical splitter and mounted on the main telescope (Celestron Edge HD 800) in autoguiding on a star. I am using the software PHD Guiding 2 for computerized autoguiding. The telescope is self-guided during a complete cycle of the toothed wheel of the CGEM mount, which lasts 8 minutes. So everything is fully automated. All guidance corrections, made by the autoguiding software, will be stored in the mount software, which will subsequently reduce the periodic error and increase the tracking accuracy of the objects to be imaged. It should be noted that the PEC does not eliminate the use of autoguiding during the acquisition of deep sky objects. Object homing will be more accurate with a working PEC.

  1. The telescope is in an equatorial position oriented precisely on the celestial North Pole and the engines are operational.
  2. An RJ-12 cable connects the module GPUSB to the autoguider port on the mount and another USB cable is used to connect the module to the computer. If your guiding camera has an autoguiding port, then you must use this port to connect the camera to the autoguiding port of the mount using the cable provided by the camera manufacturer.
  3. The guide camera is connected to the computer by a USB cable.
  4. The guide camera and the module GPUSB are configured with software PHD Guiding 2 (or other software) to autoguide the telescope.
  5. Choose a faint guide star south at around 40o above the horizon. Preferably east of the meridian.
  6. Aim the telescope at it using the finderscope.
  7. Center the star chosen on the software guidance screen. Use the mouse to select it as the guide star and activate the telescope's autoguiding function. Wait for guidance calibration
  8. Stop autoguiding.
  9. Using the mount lever, select Utility.
  10. Select PEC.
  11. Select All time lap record and wait for the PEC to initialize. At this time, the mount is waiting for the PEC to start.
  12. Resume autoguiding and wait for it to stabilize.
  13. To push on All time lap record.
  14. After a 5 second delay, the PEC registers. The recording time lasts 8 minutes. The end of the recording is carried out automatically and the display of the controller returns to the previous screen. The PEC is automatically saved in the controller memory.

Always activate PEC before starting imaging

  1. Return to the main screen (UNDO to come back).
  2. Push the button Menu.
  3. Select Utility.
  4. Select PEC.
  5. Select Playback to reactivate PEC.

Richard Beauregard
Sky Astro - CCD

Revised 2021/01/14