I’m experiencing motion to the east after slews with my Celestron Alt-Az GoTo mount. Is this backlash, and how do I correct it?
October 30, 2012
Yes, it is classic gear backlash. Gear backlash will often show up on the AZ axis at rates above rate 2 when moving east.
Here’s how to fix this issue:
1. Start by properly aligning the mount so it will be tracking.
2. Set the anti-backlash correction (under Scope Setup) for the AZ axis’ negative direction to 99, its highest value. (In the Southern Hemisphere use the positive direction; the instructions below are for the Northern Hemisphere.)
3. Point the scope to a spot near the meridian and near the celestial equator. Set the motor speed to rate 3 (or higher). You should now see the following behavior.
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 the 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.
4. Adjust the anti-backlash correction for AZ negative to 50. Again, slew the mount eastward at rate 3 or higher for at least 10 arc-minutes. If the new value for AZ negative is too low, then not all 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.
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 AZ negative correction, set the AZ 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 two times the size of the backlash of the mount) and then release the slew button. Adjust the backlash setting as high as possible until you see over correction, and then come back down until the overcorrection goes away.
When you are done with the AZ axis, consider setting up the ALT axis as well. You can start with whatever value you found worked for AZ.