Mach1GTO RA is lurching back and forth a LOT


When new (November 2017) my Mach1GTO was very smooth. I could shoot 5 minute subs unguided at 480mm all night long. 

Now the RA has regular "lurches" (as I call them). See the PHD2 Guiding assistant pic below.  The RA jumps 2"-8" in the time a 5 second guide exposure is shot, THEN is steady-ish for a while. Of course these jumps are too quick to guide out.  I have run PEMPro, PEC corrected tracking seems to be about 1" RMS.   I have set the RA "backstop" a couple of times with the thumb&forefinger method.  It seems sometimes, in certain areas of the sky, the "lurching" doesn't happen.  But it happens most of the time.

Has anyone else experienced this?  Any ideas of what to try to fix it?

My setup for the following data was:

  • Mach1GTO
  • Permanent pier: 8.625" OD steel pier bolted to 12" diameter 84" deep below ground concrete pier.
  • Celestron EdgeHD 8 at f/10 2032mm focal length. Screenshots below taken at 2032mm, usually I do photography at 1422mm with this scope.
  • Losmandy D dovetail plate mounted to Celestron scope
  • MoonLite focuser
  • ZWO Off Axis Guider
  • ZWO ASI174MM Mini guide camera.
  • 0.59 arc-sec/pixel on the guide camera
  • 2x USB and 2x 18ga power wires through the mount
  • Focus controller, dew controller, and USB hub are mounted on the scope, along with numerous cables.
I have set the "backstop" on the RA motor.  The box with the motor can rock back and forth slightly but it almost takes 2 hands to move it a millimeter or so.
Mach1GTO and GTOCP4 on permanent pier

Join to automatically receive all group messages.