Re: Mach1GTO RA is lurching back and forth a LOT

Roland Christen

If you did a balance with the worm gear backed off, you may have backed off the worm in a forbidden position. The ONLY configuration that you can back off the worm is in Park3. If you tried to back off the worm in any other position you may have accidentally scraped the worm teeth across the worm wheel and thus damaged both worm and worm wheel.

This scraping happens when you are unbalanced a lot and you disengage the worm with the lever. The axis will move suddenly and you will hear a scraping sound as the teeth run over each other. That is why we strongly recommend first doing a rough balance using the clutches, followed by a fine balance with the worm disengaged in Park3 (scope and counterweight vertical) and then moved to where the scope and counterweight are horizontal. Under no circumstances should you ever disengage the worm while counter shaft is horizontal. Same is true for the Dec axis.


-----Original Message-----
From: fd <fd@...>
To: main <>
Sent: Wed, Apr 29, 2020 12:08 pm
Subject: [ap-gto] 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

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