I’m experiencing motion in RA east after slews with my Celestron German GoTo mount. Is this backlash, and how do I correct it?
October 30, 2012
Yes, this is a case of classic gear backlash.
Gear backlash will often show up on the RA axis at rates above rate 2 when moving east. Worm gears will often show backlash of about two arc minutes. Here’s how to fix this issue:
1. Set the anti-backlash correction (under Scope Setup) for the RA axis’ negative direction to 99–its highest value. (In the Southern Hemisphere use the positive direction; the instructions below are for the Northern Hemisphere.) Under Tracking > Mode menu, turn EQ North tracking on, and set the motor speed to rate 3 (or higher). You should now see the following behavior.
2. When you slew left, the telescope will move east, and you will see the field of view moving eastwards through the eyepiece (assuming either an inverted or mirror-reversed view). Be sure to move at least 10 arc-minutes eastward at rate 3 (or higher) before releasing the button. When you release the button, the motor will ramp to a stop and tracking will engage. Just before the motor changes direction, the anti-backlash will kick in briefly and drive through the gear backlash. Because 99 is probably too high a setting, you will overshoot the backlash, and the field of view will jump to the west slightly before tracking engages. Once tracking engages, the field of view will be constant in the eyepiece
3. Adjust the anti-backlash correction for RA negative to 50. Again, slew the mount eastward at rate 3 or higher for at least 10 arc-minutes. If the new value for RA negative is too low, not all of the gear backlash will be corrected. The remaining backlash will be used by the motor after the tracking is engaged in the field of view will appear to briefly continue to move eastwards at sidereal speed. If the new value is still too high, then the gear backlash will be overcorrected and the field of view will again jump to the west after releasing the button, stabilizing once tracking engages.
4. Continue to adjust the backlash setting until there is no visible westward overcorrection. Some users prefer a small amount of under correction but this is a matter of taste. When you are satisfied with the value of the RA negative correction, set the RA positive correction to the same value.
You can also configure the anti-backlash settings during the day on land objects. What you see in the eyepiece will be slightly different, but the basic principle is the same. Slew the mount a significant distance in one direction (at least 2 times the size of the backlash of the mount) and release the slew button. Adjust the backlash setting as high as possible until you see over correction and then reduce the setting until the overcorrection goes away.
When you are done with the RA axis, consider setting up the Dec axis as well. You can start with whatever value you found that worked for RA. Be sure not to overcorrect in Dec, as this may affect your autoguider during its calibration routine.