PHD2 strange behaviour, especially after dither

[Tnarius]

I'm new to PHD2 and have just been using it on default settings after a calibration and also a run of the Guiding Assistant, I am using a Celestron C8 with f6.3 reducer on an HEQ5 mount (tried with PEC active and inactive) and I am guiding with a ZWO 280mm Guidescope and ASI120MM mini.

I have 2 queries I am hoping I can get advice on:

1. I am getting the graph below which seems to have a surprisingly regular oscillation in RA error which I cannot get rid of and looks different to other graphs I've seen where corrections seem much more random
2. I am seeing a strange behaviour when dithering, although I would expect a spike one way or another, the counter movement seems odd and happens after the dither has finished and once the next exposure has started

Any suggestions on what to try to address this would be appreciated please, I'm afraid I didn't record any logs, I can do that next time I have a clear night if this graph isn't enough to troubleshoot from.

[alacant]

Hi

Do you mean that your images are unsatisfactory and you have (accidentally?) deleted the logs? What is the problem with the images?

A few thoughts if your images are not what you are expecting.

Guess: RA wobble is the gear mesh period and DEC excursions are backlash, but without the logs, it's just that.

Dither hits the mount hard and suddenly. Even a well adjusted mount will need time to recover.

Cheers

Edited February 28, 2021 by alacant

[Tnarius]

I did some more troubleshooting tonight before the clouds came in and a Cross Test and Backlash Graph from the Guiding Assistant clearly shows I do indeed have a backlash issue I need to get sorted before I do anything else. Looks like I'll be taking the HEQ-5 apart this weekend...

[alacant]

12 hours ago, Tnarius said:

taking the HEQ-5 apart

Just before you do, there maybe an easy fix. The guide speed defaults to 0.1 sidereal. What do you have set?

Always a good idea though to dismantle, clean, re-grease and set tolerances;)

Cheers

Edited March 2, 2021 by alacant

[Tnarius]

Thanks for thinking of that but I spotted that in the set-up guides and I'm at 0.7 currently having tried various settings between 0.5 and 1.0. There is definitely backlash to sort as I can feel the slop in the mount by hand now I know to look for it.

[michael8554]

PHD2 always automatically records a GuideLog and a DebugLog, stored in the PHD2 folder.

Post a GuideLog that includes a Calibration and GA run.

You can see PHD2 struggling with Dec after the dither due to the Dec Backlash - did you accept the Backlash Compensation value offered by the GA run ?

1.5 seconds exposure might be just a tad too fast for the conditions, causing a bit of "Chasing the Seeing" in RA.

If the HEQ5 has Permanent PEC and it's a good model then use it, perhaps with the PHD2 PPEC RA algorithm.

Always clear the remaining Dec Backlash before Calibrating, by nudging north until you see the guidestar move.

Michael

[Tnarius]

Thanks guys, I've followed through with a tuning of the mechanics & gear meshing and I'm pleased to say that although still having a little more Dec backlash than I would like, it's much better now and my images are noticeably better...thanks

[Grant Fribbens]

I think it may help with longer exposures on the guide camera otherwise you will be fighting the seeing. I usually go for 2.5s to 3 s. This is after reading this article https://astrobackyard.com/phd2-guiding/

Edited March 7, 2021 by Grant Fribbens

[Tnarius]

Yes, I'm up to 2.5 now which I'm sure is helping too.