Taking a few minutes to recover

[Cody Jarrett]

Hi all,

Have an interesting problem and wanted to see if I could get some input. I have an Skywatcher EQM-35 pro mount and after getting a lot better with polar alignments, I've been able to do several imaging sessions with guiding working pretty great. I do have one thing I'm trying to improve though. Occasionally, during a guiding session, or especially right after dithering, the dec takes a long time to get back on track. So there is a visible pattern in the picture attached, or in log viewer, and I see the dec getting off further and further and after a few minutes it catches up. 

I've been reading the Analyzing Phd2 Guide Logs document and I'm wondering if this is backlash related, and something that I can correct? Any ideas and input would be greatly appreciated. Thank you!

Log File from last night:

https://openphdguiding.org/logs/dl/PHD2_logs_E2ed.zip

[wave...@talktalk.net]

Hello cod..., to my beginner's eye, this certainly looks like backlash in the Dec axis. You haven't included a recent Calibration, but I suspect there was a warning about it. It may have recommended unidirectional guiding in Dec (?)

Cheers,

- Jack

[Cody Jarrett]

Ah, that's helpful thanks. I also did another attempt last night with calibrations and guiding assistant. I saw messages stating not enough south movement and other similar errors. I also did start cross tests and I don't see much north/south movement! I did verify that using Manual Guiding, I can send pulses north/south over and over and actually see the dec gears moving. Could the backlash really be that bad or do I have another issue going on? 

Last night: https://openphdguiding.org/logs/dl/PHD2_logs_YxCY.zip

[bw_msgboard]

It looks like you have a very large delay when the direction of Dec guiding reverses.  It's likely that most of this is mechanical backlash - inadequate meshing of the gears in the Dec drive system.  But you also need to be sure that the scope is well-balanced in Dec for all pointing positions and there is no interference or snagging from cables.  You should run the Guiding Assistant and measure the Dec backlash.  Even if the test "fails", which it may well happen, you can look at the graph and get a visual representation of how the mount behaves when it is told to reverse direction in Dec.

 

This will require a mechanical correction.  In the meantime, you can use uni-directional guiding for Dec and use RA-only dithering or spiral mode dithering to minimize the number of reversal delays you encounter.  You should also be aware that the guiding assembly you're using results in a very coarse image scale and is likely to be a source for all kinds of mechanical weakness and instability.  A 4-micron shift of the guide camera will create a 6.5 arc-sec guide star displacement and probably a ruined imaged.

 

Good luck,

Bruce

 

 

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Cody Jarrett
Sent: Wednesday, October 06, 2021 5:24 AM
To: Open PHD Guiding
Subject: [open-phd-guiding] Taking a few minutes to recover

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[Brian Valente]

In the short term, i would definitely recommend doing Bruce's recommendation here:

>>>use RA-only dithering or spiral mode dithering to minimize the number of reversal delays you encounter.  

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Brian 

Brian Valente

portfolio brianvalentephotography.com 

[Cody Jarrett]

Ok this helps me a ton, thank you both! I'm going to focus on the gears and adjustment next and see what happens. Also, @Bruce, you mentioned "be aware that the guiding assembly you're using results in a very coarse image scale", can you help me understand what you mean there a little more? Is it just the EQM-35 pro mount itself, or my guide camera/scope? Thanks

[Cody Jarrett]

I started messing around with the mount and when the power is off to it, I could turn the dec gears and feel a little backlash between the motor and the first gear. I realized though the second gear (the one on the shaft coming out of the head) was loose! The set screws weren't tight and there was slop back and forth on that shaft. So I tightened that up and adjusted the stepper motor more and everything felt much tighter. I couldn't feel any backlash even. So last night I did another session and the first calibration completed without any errors. When I did guiding assistant, it still complained about south movement and the backlash graph did show some movement at least. Still not sure why thats happening, anymore ideas?

I switched over to RA only dither and last night's session looks alot better, but still have some settle failures. 

Logs from last night: https://openphdguiding.org/logs/dl/PHD2_logs_9MCw.zip