EQM-35/PHD2 Guiding and LARGE scale dec issues (Logs attached)

[Guest]

I have been plagued by DEC guiding issues for the last few weeks. I was getting regular dec spikes and made some adjustments and now I am now I am getting irregular massive dec issues.

After the initial issues I both updated PHD2 to the latest version and made sure the mount motor controller firmware was the latest version.

Before the last logs from last night I adjusted the worm gear tensioning to remove most of the slack but the issues still remain but not as regular but at a greater magnitude. I include the guide logs and debug logs for the last three nights imaging and wondered if anyone with better knowledge than I had any ideas.

PHD2_GuideLog_2020-08-03_222402.txtPHD2_GuideLog_2020-08-01_214216.txtPHD2_GuideLog_2020-08-03_222402.txt

PHD2_DebugLog_2020-07-31_223456.txtPHD2_DebugLog_2020-08-01_214216.txtPHD2_DebugLog_2020-08-03_222402.txt

 

PHD2_GuideLog_2020-07-31_223456.txt

Edited August 4, 2020 by Guest

[alacant]

Hi

The debug log gives some good clues:

grep meridian PHD2_DebugLog_2020-08-03_222402.txt 
22:45:15.720 00.000 3984 Alert: PHD2 is not able to make sufficient corrections in Dec.  If you have just done a meridian flip, check to see if the 'Reverse Dec output option' on the Advanced Dialog guiding tab is wrong.  Otherwise, check for cable snags, try re-doing your calibration, and check for problems with the mount mechanics.

HTH
 

[Guest]

Thanks @alacant. The issues still occurred after a number of re-calibrations, there were no cable snags and I overhauled the dec gearing before the latest log above and can confirm all dec gear play was adjusted out.

[alacant]

Mmm. It also suggests...

Quote

check to see if the 'Reverse Dec output option' on the Advanced Dialog guiding tab is wrong.

For eqmod it should be unchecked but you say you updated the fw, so whatever it's set at, set it to the opposite and try again. Cheers.

Edited August 4, 2020 by alacant

[MarkAR]

There's the possibility that if it's adjusted too tight there might be binding issues with the gear. Just a long shot I know. 

[Guest]

I did test a number of very long slews to see if there was any binding and the slews were smooth and accurate. I will look again. I have also just noticed that there is play in the helical focusser on the guide scope do have ordered a replacement that uses the finder shoe. The single mount screw for the guide scope won't be particularly stable either.

I will also test with reversing the Dec the next session.

[MarkAR]

Watched a video by Amy Astro the other day and she had loads of guiding issues because of play in helical focusers. Resolved the issue by getting rid of it.

https://www.altairastro.com/precision-camera-focuser-for-auto-guiding-with-altair-60mm--80mm-raci-finder-scope-86-p.asp

[Guest]

4 minutes ago, MarkAR said:

Watched a video by Amy Astro the other day and she had loads of guiding issues because of play in helical focusers. Resolved the issue by getting rid of it.

I went for the Altair MG32 https://www.harrisontelescopes.co.uk/acatalog/altair-mg32qrb.html#SID=1729 of which I have heard good reports.

Of note is this from the Altair website:

Quote

Zero flex, and your guide camera will stay in perfect focus forever: 
The camera is threaded into the optical housing, so you don't need to worry about differential flex. The finder scope front lens is rotated to focus, then the locking ring is tightened up. You are now focused forever.

 

[almcl]

I hope your replacement guidescope solves the problem, but if it doesn't, you might find that getting rid of the scope rings with their plastic tipped screws and clamping the guidescope firmly to the dovetail with, for example, clamshell clamps and then clamping the dovetail to both the main scope's rings will help reduce the mechanical flexure.  

There are several occasions in your guide logs where the DEC performance is actually better than the RA - I haven't looked at all the logs, but here's one example:

When the Dec goes wrong, it's large (the largest peak below is almost 20 arc seconds) and pretty rapid and, as the DEC motor was not running, not down to software.  Here's an example:

There is still some backlash (it takes 15 pulses before the small deviation at 00:53 comes back within limits, for example) but this doesn't seem to be the main problem.

[Guest]

Thank you @almcl. Despite the fact that the mount is not the best, I was hoping it would tide me over for a year until I can justify an EQ6-R Pro and then the EQM-35 would then become a dark site mount for me. I have taken some very solid 600 subs with this setup and I felt that the magnitude of the dec errors could surely not be backlash alone so its good to get feedback that indicates the same thing.

There will certainly be less flexure in the new guide scope and I will initially mount it on the finder shoe as I exclusively plate solve now so have no need for a red dot etc. If there is still issues then I will take your advise and attach to both scope rings.

Appreciate you taking the time to look at the logs.

[Guest]

I think the most telling session was from last night after I had adjusted the slack in the dec gearing:

I am no expert with PHD2 log viewer but with perfectly adequate guiding for over half an hour including dithering only to get that spike at the end has to be something other than dec backlash.

[geoflewis]

Hi Chris, I took a look at the logs in your session last night and saw that several (but not all) of those huge Dec excursions occurred right after a dither, so it looks as if PHD2 was unable to recover the size of Dec dither component, so perhaps you could try dithering in RA only, to see if that makes any difference. I didn't see any GA (Guiding Assistant) runs, so you might also want to do that with backlash measurement enabled and see what that reports.

Some of the other Dec run-aways happened as soon as the mount got outside of the limits set, with Dec never being recovered, i.e. Dec was lost as soon as PHD2 tried to recover it, so as @alacant identified from the debug log, PHD2 was not able to make sufficient corrections in Dec. The long slews that you did to check for binding, are not really a good test for the level of binding that might be experienced by the very small guide pulses, so I wouldn't discount the gears being overtightened based on those slews alone.

I agree with @almcl  that those 3 point guide rings are a potential disaster for mounting your guide scope (trust me I've experience how bad they can be), so I'd also suggest replacing them with something more solid. I'm sure that you are aware that even movement of a few microns in the guiding assembly can destroy your guiding - we are talking margins about the width of a human hair, so not something that you can easily observe.

Good luck resolving these persistent issues.

 

Edited August 5, 2020 by geoflewis

[Guest]

Thanks @geoflewis, really useful. The lateral play in the helical focusser and guide camera to guide scope is truly shocking, not to mention the play in the single threaded connection holding it to the mount. I can literally move the guide scope and camera side to side (dec axis) by ~5mm with very little force by holding the guide camera so feel this has to have some impact on the issue. My new Altair SG32 flex free guide scope hopefully arrives today with screw thread focussing and camera to scope connection also by screw thread. I will be eager to see what effect this has on the issue.

It is worth mentioning, and apologies for omitting this before, but after the big dec excursions, my target was no longer in my main scope. In Carte du Ciel the mount had moved a not inconsiderable amount which necessitated slewing back. This leads me to believe that the correction pulses to try and correct the huge errors were actioned successfully by the mount but took the mount off course rather than trying to keep it on course. To me this indicates the huge spikes were the star moving in teh guide scope but not in the main scope?

Once I have quantified the improvement, I will potentially:

1. Check the tension of the worm gear engagement to ensure it is not binding
2. Potentially limit dithering to RA only (Though interestingly I turned off dithering completely before the last 1-2 dec issues the night before last)
3. My new guide scope also comes with 3 point guide rings, so I will order have just ordered a https://www.altairastro.com/altair-gpcam-photo-tripod-clamp-25-p.asp to make it even more stable
4. I will run some guiding assistant runs next time I do get any issues 

Really do appreciate all the wonderful help, this forum offers me so much help and support and I am truly grateful  

Edited August 5, 2020 by Guest

[Guest]

Following excellent advice in this thread, my new flexure reducing guiding setup ready for first light!

[MarkAR]

Fingers crossed for tonight.

[Guest]

Unfortunately the new guide scope did not improve tracking enough so this morning I decided to take a mallet to the proverbial nut...

[MarkAR]

Caught your decision on the other thread, the EQ6 is a good choice.

[geoflewis]

14 minutes ago, Chris Wright said:

Unfortunately the new guide scope did not improve tracking enough so this morning I decided to take a mallet to the proverbial nut...

Hi Chris, I think that's a good shout and I sure hope that it gives the performance you want. 🤞

[michael8554]

On 06/08/2020 at 14:39, Chris Wright said:

Following excellent advice in this thread, my new flexure reducing guiding setup ready for first light!

Three dangling cables = Diff Flex

Tie them neatly to the lower ring, then a smooth curve to another tie high on the tripod where it can't be tangled with the counterweight etc.

Michael