Strange guiding errors with AP1200


Allen Gilchrist
 

Has anyone else seen anything like this before?  During an imaging session with my AP1200, while guiding with an STi on a 400 mm f.l. 80 mm refractor, I noticed an occasional really large guiding error.  It took a few cycles to bring the guidestar back to the center of the guide window, and then all was OK until it happened again.  I started an autoguider log file, and found that the process repeated every 379 sec.  This suggested some problem in the RA drive system, but there were corresponding spikes in the Dec. error log as well.  In fact these errors were larger than those in RA.  It almost looked like something was binding the RA drive every 379 seconds and then, when the drive slipped free, there was an impact on the Dec axis.  Interestingly, after recording five of these events in the autoguider log, the problem vanished.  I continued observing for about another hour and a half but the problem did not return.  I've attached a couple of plots, one showing the problem, and the second one after the problem went away.  The plot scale is in pixels, and each pixel is 3.82 arcseconds.  Any ideas?

Allen


Roland Christen
 

The RA worm turns once every 6.4 minutes and so if there was a piece of dirt embedded on the worm teeth, there would be a jump in that time frame. If it repeats forever, then I would suspect a damaged worm. If it happened just once in the same place in the sky I would suspect a damaged tooth on the main worm wheel.

There could also be a piece of dirt embedded in the final spur gear that's attached to the end of the worm. That can be easily cleaned, but that would show up for every 6.4 minute cycle and would not go away.

The fact that it showed up in Dec is probably due to a bad calibration run. Dec axis probably did not move, but the guide software is interpreting part of the RA error as a Dec error.

I would remove the RA gearbox, clean all the grease off both the worm, the main worm wheel and all the transfer gears inside the gearbox. Then re-grease every thing.

Rolando



-----Original Message-----
From: Allen Gilchrist via groups.io <gilchrist.allen@...>
To: main@ap-gto.groups.io
Sent: Thu, May 6, 2021 4:44 pm
Subject: [ap-gto] Strange guiding errors with AP1200

Has anyone else seen anything like this before?  During an imaging session with my AP1200, while guiding with an STi on a 400 mm f.l. 80 mm refractor, I noticed an occasional really large guiding error.  It took a few cycles to bring the guidestar back to the center of the guide window, and then all was OK until it happened again.  I started an autoguider log file, and found that the process repeated every 379 sec.  This suggested some problem in the RA drive system, but there were corresponding spikes in the Dec. error log as well.  In fact these errors were larger than those in RA.  It almost looked like something was binding the RA drive every 379 seconds and then, when the drive slipped free, there was an impact on the Dec axis.  Interestingly, after recording five of these events in the autoguider log, the problem vanished.  I continued observing for about another hour and a half but the problem did not return.  I've attached a couple of plots, one showing the problem, and the second one after the problem went away.  The plot scale is in pixels, and each pixel is 3.82 arcseconds.  Any ideas?

Allen

--
Roland Christen
Astro-Physics


Allen Gilchrist
 

Rolando,
Thanks for the prompt response.  The 6.4 minutes is close to the 379 seconds I saw.  I was taking 10-second guiding exposures, so the problem could have occurred any time during that interval.  I have the lube kit and instructions, and will follow your advice.  I'll also take a look at the YouTube videos on lubricating and adjusting the gears.  
Allen


Allen Gilchrist
 

Rolando,

I cleaned and lubricated the complete RA drive mechanism, worm-wheel, worm, and spur gears.  I even examined the spur gears with an engraver's loupe, not finding any problems.  I examined the dec drive, and decided to just adjust the gear mesh.  I think the RA cleaning and lubrication cured the problem, but I am not completely sure.  I can't seem to get everything to work at the same time.  Last night, TheSkyX decided not to work with my autoguider, so I had to use PHD2.  I'm not as familiar with the log file that PHD2 produces.  

Allen


Allen Gilchrist
 

Rolando,
I think the gear cleaning and lubrication did the trick.  I figured out how to analyze the PHD2 guiding log.  The plot below shows that the cyclical spikes are gone.  The scales here are different from those posted earlier in this thread.  The earlier guiding data was based on 10 second guide exposures with the guider binned 1x1 while conditions were poorer for the more recent set of measurements based on 10 second guide exposures with the camera binned 2x2.  In any case, the periodic spikes are gone.  


Roland Christen
 

Ok, great!

Rolando

-----Original Message-----
From: Allen Gilchrist via groups.io <gilchrist.allen@...>
To: main@ap-gto.groups.io
Sent: Tue, May 11, 2021 9:40 pm
Subject: Re: [ap-gto] Strange guiding errors with AP1200

Rolando,
I think the gear cleaning and lubrication did the trick.  I figured out how to analyze the PHD2 guiding log.  The plot below shows that the cyclical spikes are gone.  The scales here are different from those posted earlier in this thread.  The earlier guiding data was based on 10 second guide exposures with the guider binned 1x1 while conditions were poorer for the more recent set of measurements based on 10 second guide exposures with the camera binned 2x2.  In any case, the periodic spikes are gone.  


--
Roland Christen
Astro-Physics