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Question Dec Dither Overshoot and Long Settle Times on M-Uno

14 May 2022 17:58 - 14 May 2022 18:18 #939 by RickB
Can someone explain this dither behavior on the DEC axis. It appears to go to the position and then overshoot in the other direction which then takes many corrections and a large settle time of 30ses resolve. An RA move settles in about 10 seconds with no overshoot.
Is this normal?
Are you doing any form of controlled acceleration and deceleration to keep this under control?
Are there any settings to minimize this?

Last edit: 14 May 2022 18:18 by RickB. Reason: Fixed settle time #

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15 May 2022 21:41 #940 by Peter_S
Hi Rick,
could you specify your setup (pulse guiding? ST4 guiding?) Do you have the newest firmware and StarGo software installed?
With my M Uno + MGEN3 I get very short = fast dithering. But I experienced weird slew movements (as you describe it when dithering) with overshoot + back and for movements with a previous fw / StarGo version.
Did you check for identical entries in StarGo and your guiding software?
cs Peter

M Uno + remote p. a. kit, M Zero, Esprit APO ED 100, Lacerta MoFoc, Falcon Rot.,
C8 EHD, Lacerta f4 Foto Newton + MoonLite LiteCrawler/Rot., MGEN II + III, W10 64 Mini-PC,

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16 May 2022 12:32 #941 by RickB
I am using PHD2 so I think it is pulse guiding. I see there is an "ST4" checkbox on the StarGo control panel. It was checked and I could find no documentation for that so I left it checked.
I did update everything to the latest version.
I was running with PHD2 settings close to the recommended ones on this website.
My last session I used RA dither only, which worked fine.
Also in this software what is use of the ASCOM control panel. You also need to have the main panel open because many of the functions to control the mount are missing on the ASCOM panel.

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17 May 2022 18:01 #944 by Schaefer
I'm seeing the same behaviour with my m-zero, I just posted images (that are hardly readable due to the forum settings I guess) in the dec backlash thread I started a while ago. Despite this behaviour I have better RMS in Dec than in RA, but of course the dither part is excluded from the RMS calculation. Anyway, I would be interested in understanding this behaviour as it might also be related to the irritating backlash results that the GuidingAssistant was reporting. BTW, did you run the Guiding Assistant and let it measure Dec backlash? How does your Backlash graph look like?

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17 May 2022 22:50 - 17 May 2022 23:08 #945 by RickB
I spent some time today chasing down the DEC lags that I was seeing. I placed a board in the saddle and a dial indicator about 250mm from the DEC axis. I then used the hand control to move the DEC axis. At the 2 fastest speeds the reversal as indicated on the dial appeared instantaneous with the button push and the sound of the motor. On the next slowest speed the button push and motor sound was together but the dial lagged 2-3 second if changing direction. If pressing the button to continue in the same direction then button sound and dial indicator are together.

It appears to take some time for the motor to take up any slack in the belts. This is done so quickly on the 2 faster speeds so you don't notice. The slowest speed had a time of around 13 seconds to reverse the dial.

Maybe this is the lag I'm seeing in both the guide assistant backlash test where it gives up, and the DEC dither delays.

Perhaps a backlash setting could be added that runs the DEC motor faster for few counts to take up the backlash, but only when a direction reversal is requested. Is there something in the software already operating behind the scene to help with backlash.

The M-Uno I have was made in 2016 do I just need new belts.

Last edit: 17 May 2022 23:08 by RickB.

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17 Jun 2022 16:36 #959 by RickB
There were 3 issues with the DEC axis.

1) the drive belts were not the current ones
2) a broken shaft on one of the middle toothed pulleys where it goes into the bearing
3) loose bearings in the top plate

I got replacement drive belts and a pully and also used a bit of epoxy to hold the bearing in place.

The result is much improved. PHD2 guiding assistant reports a backlash on the first few tries of 250 to 500msec. Before the repair PHD2 could not complete the backlash test. Calibration and DEC axis dither are working correctly now.


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