Deviation issues in DEC control on my AVX

[Paul E]

Hi there.

Since I started astrophotography I have always had varying issues to a degree with deviation in DEC control on my AVX mount.

I carried out all checks and possible causes when setting up. It came to a point where I became disillusioned and stopped for a while. I started again during lockdown but concentrated astro with my DSLR camera before purchasing a dedicated astro camera and setting up the camera train.

On many occasions when I set up using PHD2, the East/West calibration steps completed normally but when it came to North/South calibration steps, the South steps to ‘return’ were always slow and usually not completed. 

My last resort to try and fix the DEC issues was to get my AVX tuned (StellardriveTM)  which I did a couple of months ago and only now (due to weather) have I been able to try out the mount again.

Initial tracking was excellent but I then had a DEC issue again. The DEC guiding suddenly deviates and then gradually settles back. I tried changing various settings during guiding to see if this corrected the issue but to no avail.

I tried the guiding assistant but again due to the South step not moving, the guiding assistant couldn’t complete although it did give pointers to improve guiding which I used.

Through CPWI program, I had previously completed the PEC training of the mount.

I would be extremely grateful for any advice/solutions to my problem. i.e Hardware issue? / balance? / stiction ?

I have attached links to my PHD logs and to 2 videos showing the DEC deviation  during guiding below.

The time on the 2nd video when the DEC starts to suddenly deviate for the 2nd time is approximately 01-00hrs.

Kind regards

Paul

PHD2 log file
https://openphdguiding.org/logs/dl/PHD2_logs_vmub.zip

2 videos of guiding (Onedrive)

https://1drv.ms/u/s!ArJQWFQC7gxAouREZkf4T_jZV4kxUw?e=UHokNi

Equipment:-

Celestron AVX (on a fixed pier) – Latitude 54 deg N (Newcastle / UK)

OTA :- Edge HD 8” / 0.7 reducer. 

Main camera :- ZWO ASI 294MM

Guide camera:- Lodestar x2 (using a OAG)

Celestron focuser / Starsense camera.

Initial Polar alignment was carried out using the QHY Polemaster and then an ASPA was carried out using CPWI program. I also tried PA again using the PA routine on the ZWO ASI air Pro to confirm accuracy of PA.

Target that night was NGC 869 (to adjust and verify guiding focus on the lodestar and ensure enough guide stars for narrow fov)

[wave...@talktalk.net]

Hello Paul, I'm also a learner but I'll take a shot at your Guide Log and tell you what I reckon.

When I read that you found the North-South calibration step didn't complete, I immediately thought 'backlash'. And that may well be part of the equation, but the PHD2 plot does suggest there's another factor called static friction (stiction) involved. Here's one of the points at which the Dec axis is persuaded to move, resists somewhat, then overshoots before being coaxed back. This shows Dec axis only.

This is a mechanical issue that PHD2 is trying to cope with by issuing lots of rapid pulses. These eventually force the axis back but it overshoots and the pulses repeat in the opposite direction.

One way to overcome excessive backlash and stiction in the Dec axis is to adopt unidirectional guiding. My old LX200 mount has around 4000ms of backlash, but one way guiding copes with it very well.

Hope this is somewhere near the mark - which helps me learn too.

- Jack

[Paul E]

Hi Jack. 

Many thanks for your reply. That graph looks very similar to my DEC output!

Yeah my initial thoughts was either backlash or stiction as when I loosen the DEC clutch on the AVX and rotate the DEC, its quite stiff compared to RA. (To be honest, this has always been the case since I purchased the AVX even after it was tuned)

However what I can't understand is that DEC pulses were being sent and corrections made for a while without issue. The time interval between the two DEC variations on the video was approximately 8m & 45 secs. I didn't get enough data/time to suggest that there is a 'sticking' point in DEC or whether its variable stiction (if that's possible) 

I did increase the DEC pulse to 3000ms and DEC aggression to 95%  and also tried different DEC command settings  (North/South/Auto)  but the clouds started to roll in so didn't have time to observe any changes within the time frame.

Hopefully the next time I have clear skies, I'll have time to change settings and observe any changes and see if it points to something in particular.

Regards

Paul

[wave...@talktalk.net]

Hello again Paul, the trace I posted IS yours. I simply selected and enlarged one of the sections where the Dec trace goes awry. Must admit I don't now much about stiction. What I know I read in Bruce Waddington's Tutorial on Analysising Guide Logs (from the PHD2 website homepage).

I'm not sure that you an alter the guide pulse lengths when guiding. PHD2 determines each pulse length separately. You can change it for the calibration steps but that doesn't carry over into the guiding phase.

To determine which direction to guide in Dec, run the Guiding Assistant for 20-30 minutes and note which direction the Dec trace trends. You obviously select the opposite direction for Unidirectional guiding. That's how I cope with a 4 second slop in the Dec gearbox.

- Jack

[Paul E]

Hi Jack. lol....I thought it looked similar!

With regards to guiding assistant....good point...I'll try that next time (my experience with  PHD2 is limited to say the least).

Regards

Paul

[bw_msgboard]

Hi Paul.  The discussion so far has focused on the slow and sticky response you get on the Dec axis.  That's fine but I think an easier and more important question is what's causing these huge Dec excursions in the first place.  Here's a typical example (Dec in green):

 

 

These are huge excursions to the north and they are not caused by guiding.  Here's a close-up view of one of them:

 

 

You can see there was no guiding activity at all at the point the guide star suddenly plunged to the north.  All the subsequent guide pulses were simply a reaction to that.  So the Dec motor wasn't running at all when the problem occurred, therefore it has nothing to do with stiction or backlash or any of that.  Something external to the mount caused this problem.  A likely explanation is that you've got a problem with the routing of the cables - something is snagging or dragging over a stationary surface and is gently tugging on the guiding assembly.  We talk about these as "huge" excursions but in reality they represent unwanted movement of about 25 microns at the guide camera sensor - that's about 1/2 the thickness of a human hair.  You can read more about these "gremlins" in the document Jack referenced.

 

https://openphdguiding.org/tutorial-analyzing-phd2-guiding-results/

 

It can take some time and effort to track these down and fix them.  Other sources of this kind of problem include wind gusts, movement of the mirror in a movable-mirror telescope, looseness in fittings on the back of the scope such as the focuser - the list unfortunately goes on.  But if you can eliminate the source of the problem, you will see a lot of improvement.  Fiddling around with the guiding parameters is a wasted exercise at this point so you should revert to all the default settings until you get further along.

 

Good luck,

Bruce

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Paul E
Sent: Thursday, August 26, 2021 3:55 AM
To: Open PHD Guiding
Subject: [open-phd-guiding] Deviation issues in DEC control on my AVX

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[Paul E]

Hi Bruce.

Many thanks for your help. That is very insightful of you with regard to the absence of pulses just before the deviation.....something that I would never have realised.

In a way I hope that this is the cause of the sudden jumps in deviation as it means its an inexpensive fix. 

I also took some stills of the guiding (see below) when  the issue arose and would appreciate your view/verification that your reason for the jumps confirms this.

Stills of DEC deviation

Generally I let most of the cables hang as the pier has a small footprint underneath the mount (self build bespoke mount/foundations) and there is no specific part of the mount/pier that I can think of that the cables would snag on.

However, with my setup, there is an opportunity for the cables to rub together and temporarily catch the cable-to-cable connection points as the mount moves over time.

The  actual cable to the guide cam (loadstar x2) is taped to the camera at the point of connection to ensure the connector doesn't move, however the rest of the cable, which is quite long, I let hang. From this point the other end is connected to the usb cable in a 'U' shaped configuration. So in retrospect this may itself cause movement, however small.

I'll tidy the cabling up and  check connections etc. to eliminate any possible causes the next time I am imaging and I'll read through the article on interpreting guide results.

Kind regards

Paul

[bw_msgboard]

Hi Paul.  I don't have any reason to doubt what I told you.  Since you're going to be trouble-shooting this stuff, I think it would be a good time to become acquainted with the tools we use to understand what's going on.  Start by installing the LogViewer tool:

 

https://openphdguiding.org/phd2-log-viewer/

 

Then take a look at the tutorial for how to analyze the data.  You can work through the simple examples then pay special attention to the section on "Sudden Large Excursions" starting on page 6.  I could just as well have used your log data for that example. :-)

 

https://openphdguiding.org/tutorial-analyzing-phd2-guiding-results/

 

With cabling issues, a common problem comes from the use of cable ties or ridged plastic cord covers.  These things create surfaces that aren't smooth so they are capable of briefly catching on any static surface they rub against.  Hanging cables are also a problem because they can represent enough unsupported mass to pull on the equipment by tiny amounts.  It's typical for the problems to show up in particular pointing positions even while most other positions are fine.  In the log data we're looking at, you'll see you were pointing at Dec locations between 40 and 57 degrees from the west side of pier.  You'll probably have to spend some time on it, this stuff is no fun.

 

Bruce

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Paul E

Sent: Thursday, August 26, 2021 1:52 PM
To: Open PHD Guiding
Subject: Re: [open-phd-guiding] Deviation issues in DEC control on my AVX

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/21c884e1-04ce-43d2-a77e-c3b7184462bbn%40googlegroups.com.

[Paul E]

Hi Bruce.

Thanks for your invaluable help (and also to Jack).

I've installed the log viewer tool and I'll look at my data for that imaging session to get a better understanding of how to interpret the phd logs, taking pointers from the guiding tutorial.

Your absolutely right with regards to the pointing position of the mount, because if I recall when I first started out astro imaging using an EQ mount, DEC control always seemed to be worse at low declination angles and this was a factor in becoming frustrated and disillusioned every time it occurred....which was often!

Now, with your help and a better understanding of the factors that can cause this I can (hopefully) reduce/limit these issues.

Kind regards

Paul