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My ES/Losmandy G11 mount's periodic error (PE) is higher than I expect, what can I do to Improve that?


There are several things that you can do to improve any unexpected periodic or non-periodic error that you have detected in your mount. This can be a complex issue, but when dealing specifically with the ES/Losmandy G11 PMC-Eight mount, there are a few specific areas to investigate.

First, before troubleshooting the problem, make sure you have properly identified the issue using an auto-guiding system which can record the PE of your mount. You need to take data using your auto-guiding system with the guide corrections disabled so that you record the native or raw PE of your mount.  Typically, any PE will not show up through visual use of your telescope when observing a star or planet, it will only be noticeable when doing long-exposure photography.

You should take the time to make sure there are no other problems with your guide camera or guiding software settings that would cause an issue with your guiding performance.  If you have a consistent problem with your auto-guiding and with your PE, here are two areas that when maladjusted can cause the PE to be larger than expected. These areas can also be the source of Non-Periodic error:

  1. Worm/Wheel Alignment (gear mesh  
  2. Motor/Worm Shaft Alignment

First, the worm gear needs to be aligned correctly with the wheel gear, AKA the ring gear so that there is the proper amount of gear lash in your mount's drive.  The alignment of the worm and wheel needs to be set to minimize the amount of gear lash, but also needs to be set loose enough to keep from binding. See the PMC-Eight knowledge base article "What is that noisy ratchet sound I hear from my mount? Why won't my mount move?" for more information about adjusting your G11 mount's gears. 

Second, the G11 mount uses a direct-coupled motor assembly that requires a precise alignment between the motor shaft and worm shaft. The primary cause of large PE in your G11 mount may be that the motor/worm shaft's need to be aligned properly and not be shifted.   This shift can cause the torque on the worm gear bearings to increase and cause the PE to increase accordingly.  A shift in the alignment can also in some instances cause the motor/worm helical-coil coupler to act like a spring and store energy as it slowly turns. At a certain point while turning slowly, the energy will be released and cause a spike in the tracking rate. This non-periodic spike will ruin any image that you are taking at the time of the spike.

If after removing the coupler from your motor/worm, and reattaching the motor, you can see a significant amount of offset (1 mm) then you should reattach the coupler and attempt to adjust the motor assembly / worm block position to bring the shafts back into alignment.  If you are not able to adjust the motor assembly sufficiently to correct the alignment, then you should photograph the motor assembly showing the offset alignment and contact Customer Service at or at 1-866-252-3811.   When emailing Customer Service, be sure to include any photographs you have taken and any auto-guiding log data you have taken showing the large PE.  After examining the data, Customer Service will contact you on how to proceed. This may entail sending your mount back to us, or just the motor assembly.

You may be considering getting an expensive motor/worm coupler to use on your mount but our testing, of both an expensive helical-coil coupler and the standard coupler we deliver with our G11 mount, has shown that there is no significant difference in PE performance between the two couplers.