On 5/16/2025 12:32 AM, Oberon510 via groups.io wrote:
Hi,
I have been getting some complaints from PHD2 that my Dec backlash is high and I used this video to adjust the spring loaded worm gear ".
It didn't make much difference and now I am about to clean and regrease the worm gear but hoping for some more advice/pointers on other things to look for or do as part of the process.
Any guidance appreciated.
Thanks,
Ken
Hi Ken,
In my experience with my GM811, I was never able to get my raw (uncompensated) DEC backlash below about 2000mS (two seconds) by merely adjusting / tweeking / re-greasing.? And that was after carefully doing the worm bearing squeeze and setting a value for TVC.? Sometimes it was better.... and sometimes it was much much worse.? Pointing location and balance had major impact on the amount of backlash.
You can adjust the value of the TVC parameter in Gemini2, which will compensate for a fixed amount of backlash.? TVC is actually my preferred backlash compensation because it runs at about 10X the guide speed (unlike PHD2's compensation which runs in real time) so (for example) 2000mS of backlash compensation occurs in about 200mS using TVC.? But there's a BIG catch..... if your uncompensated backlash changes over time and ever drops below the amount of TVC compensation, the system will oscillate.? In my experience, the total system backlash was variable enough that even with a carefully chosen value for TVC, PHD2's backlash compensation (which was in the area of 2000mS give or take a lot) would occasionally fall to near zero and DEC would oscillate.? Reduce TVC and try again and you eventually end up with a very low value for TVC....
So in desperation I did a deep dive into the system and identified several suspect areas, most of which were inherent design choices or machining limitations.
I made several mechanical changes to the DEC (and RA) drives (some simple and some not so simple), run my clutches very loose, and my backlash is now very consistent.? I also made and installed a 3rd axis counterweight to compensate for the fact that my filter wheel shifts the center of mass of the scope off the DEC axis slightly. After all this, and with a carefully chosen value for TVC, PHD2's backlash compensation CONSISTENTLY runs between about 200mS and 800mS.? If it ever rises above 800mS I know it is time to re-adjust the "spring loaded worm" pressure, which is typically required twice a year due to seasonal temperature changes.
If there is any interest (and if it is allowed here) I can go into more detail.? All but one of my modifications are non-destructive, fully reversible and of varying difficulty.? But the bottom line is the combination of bearing choices and machining tolerances in the GM811 added up to quite a bit of backlash (and significant 74 second periodic error too).? After my modifications.... "it just works" and I get very good tracking and guiding.? A bad night for me is anything above 0.6" RMS.? On very stable nights (Detroit metro area) I have seen it in the high 0.3" RMS range.? My 240 second periodic error is almost un-measurable.? But more importantly, it does that night after night regardless of where I am pointed.
Maybe a longer answer than you were looking for, but it is a complicated subject.
Paul
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Paul Goelz
Rochester Hills, MI USA
pgoelz@...
www.pgoelz.com
