EQ8-R Pro Gear Backlash Adjustment

My mount was having some problems with calibration and guiding occasionally. I finally decided to just test the backlash on the declination axis by slightly pushing on the telescope while it was mounted. I immediately noted a ton of play in the declination worm gear. This mount is pretty well designed in that it is belt driven, so there’s only one place where backlash could occur: on the worm itself.

The first hint was when I would run calibration in PHD2. It would always report “little south movement.” And while guiding, the declination error would just go crazy sometimes. Finally, after a dither, it would take forever, sometimes a minute, for the declination error to settle. I finally pieced together that this is from a huge amount of backlash in the declination gears.

Luckily, the gear is easy to adjust in this mount. There is an instruction manual for doing it available from Orion, who sells the same mount. The instructions are for the old style of the mount, but it’s pretty much the same.

  1. Remove the plastic cover for the declination motor. There are three small allen-head screws on the top and two large ones recessed in the bottom of the cover.
  2. Slightly loosen the four allen-head screws that clamp the gear to the mount. These are around the motor. Two of them are hard to get to, as they lie right behind the declination motor. I was able to loosen these by grinding down an allen key very short to fit between the screw and the motor.
  3. Loosen the small, recessed, central allen key on the worm gear. This is the locking screw for the gear adjustment.
  4. Adjust the two outer screws to tighten or loosen the gear mesh. It’s good to test the mount in its full rotation as you do this to make sure it does not bind anywhere.

After I tightened the gears so that I could not feel backlash anymore by hand, but the mount still moved properly throughout its entire range, calibration succeeded and my guiding was outstanding. Dithering settled quickly and I no longer had wild oscillations in my declination guiding.