Periodic Error Correction
Periodic error in worm gears is caused by slight imperfections in the worm. As the worm turns the error manifests itself in a periodic drift of a target star to the east and west from centre. This can be a large error in the range of 50-60 arcseconds or a very small error in the range of 5-10 arcseconds. Periodic Error Correction in a drive allows the drive to be 'taught' where the errors in the worm are and how much to compensate for these errors.

If you are looking at a target star through a reticule eyepiece you can watch the star migrate from east to west across the crosshairs as the worm turns. In the EQ6 there are 180 teeth to the worm gear and the worm turns once every 8 minutes. In the HEQ5 there are 135 teeth in the worm gear and the worm turns once every 10 minutes and 40 seconds. The calculation for this is as follows:

    Minutes in a day * 60 / number of teeth in the gear

If the star drifts to the east, then the mount is being driven too fast. Conversly, if the star drifts to the west the mount is being driven too slowly. In the illustration North is at the top and West is to the left. This is the orientation as seen in a Schmidt-Cassegrain with a star diagonal.



With the Rajiva updated electronics it is possible to remove a good portion of the periodic error manifested by the gears.

Method
  • If you have a barlow use it. Image quality is not important but accuracy is
  • Turn on and focus on your illuminated reticule eyepiece
  • Align the crosshairs so that one pair is aligned east/west
  • Select a suitable guide star near the meridian and celestial equator
  • Find west in the field. You can do this by momentarily turning off the drive and watching the star drift
  • Ensure that your buttons for guiding in RA are correct (West is West and East is East)
  • Turn on PEC training. To do this select 16x and click all four buttons at once. You will see the LED flashing 3 times. Click the 'Up' declination button
  • The LED will flash green. Guide the star to the centre of the reticule
  • Over the space of TWO cycles of the worm (16 minutes for an EQ6 or 21 minutes 20 seconds for an HEQ5) guide to keep the star in the centre of the reticule
  • At the end again select 16x, click all four buttons and click the 'Up' declination button to turn off PEC training

You should do this with all of your imaging gear installed with the mount properly polar aligned.

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