Losmandy G11 Periodic Error Testing and PEC Programming

One of the things I was hoping for with my G11 mount was better periodic error than my old Celestron CGEM. The periodic error on my CGEM was pretty huge. Even after moving to OnStep with belt-drive motors replacing the gearboxes, it still had a periodic error around +/-40 arc seconds. Thankfully, the errors were fairly smooth so autoguiding worked well to tame them.

New G11 mounts are advertised with a periodic error around +/-5 arc seconds. My G11 is a 2007-vintage mount and they weren’t quite as good back then. Still, my mount has the newer high precision brass worm gears instead of the old stainless steel ones. Looking around the interwebs before I bought it, it seemed like G11s with high precision brass worm gears and the original two-piece worm blocks had around +/-9 arc seconds of periodic error. That is, as long as the worm block alignment and mesh were good.

There are free programs to analyze periodic error, so I tried a few of those. I used PHD2, PHD2 Log Viewer and PEMPro Log Viewer to get an initial look at this mount’s performance. EQMod’s PECPrep is another free tool and it can also do periodic error correction (PEC) programming for some mounts. Unfortunately, there aren’t any free tools that will program PEC for the Losmandy Gemini 1 or Gemini 2 mounts. PEMPro is, for now, the only software that will program PEC for the G11. (That is, aside from doing the less beneficial programming built into the Gemini-based mounts.) PEMPro comes with a pretty hefty price tag, but it does have a 60 day free trial.

The previous owner of my G11 hadn’t done any periodic error analysis recently, so I was crossing my fingers for good performance. In line with what others have measured with similar vintage mounts, I was glad to see that my G11 measured right at +/-9 arc seconds of periodic error.

Using PEMPro and Brian Valente’s great Gemini PEMPro guide, I spent a couple nights learning and programming PEC on my G11. I was happy to see that my mount didn’t have much response on the dreaded 76 second peak. With the new PEC curve enabled, I was able to get the mount down to the +/-4 arc second range.

Not perfect, but that’s about 10x better than my CGEM’s performance. This is only part of the final “goodness” though. PEC results plus autoguiding work together to make pictures look great. That’ll be the real test.

One thing that I thought was weird was that sometimes I needed to invert the PEC curve in PEMPro and sometimes I didn’t. I haven’t figured out why yet, but for now I program PEC then test and invert if I need to.

After programming PEC with PEMPro, I did some autoguiding tests with PHD2. RA results were promising – in line with the limits of atmospheric seeing. Dec backlash was the biggest thing of note. Some PHD2 calibration runs failed when calculating dec backlash, so that’s not good. Generally dec only needs to autoguide in one direction, but with my telescope mounted on my permanent pier I have my polar alignment pretty dialed in. This results in dec autoguiding needing a little bump in both directions every now and then. With a big (uncalculatable) backlash, this makes autoguiding sloppy/laggy in dec. This was a bit of a let down since my CGEM showed a dec backlash of zero after I did the OnStep mod.

I researched G11 (plus G8 & GM811) declination backlash a bit. Many of the comments I found online were from people with newer mounts than mine with one-piece worm blocks or spring-loaded worm gears, so those didn’t all apply. However, even some of the online discussion about the original G11 two-piece worm block dec backlash hinted that PHD2’s dec backlash compensation was enough to tame the G11’s dec backlash.

Since I wasn’t seeing perfection yet, I kept digging and found some people talking about using the Gemini TVC (Time Variable Compensation) along with PHD dec backlash analysis to get better results. Initially my TVC was set at 5 (maybe the default?). I bumped it to 10 and then 20. At TVC = 20, PHD2 measured dec backlash at 250ms and guiding seemed improved. My next step is to bump up TVC more (30 maybe?) and compare results.

I still could do some mechanical work and adjust the worm spacing on RA & dec. It’s way too costly to upgrade my mount to the newer high torque, tucked motors and spring loaded worm assemblies from Losmandy, but I’m tempted to design and make my own spring loaded setup. We’ll see how the photos look though.


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