intermittent oscillating guiding on Paramount MX+

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Dave Newbury

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Jan 29, 2020, 12:27:13 PM1/29/20
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Having trouble with guiding on a relatively new Paramount MX+.  Much of the time the guiding is in the 2+ RMS a-s range. Sometimes it gets better for a while. The attached graph is a good example of some good guiding (RMS 0.8 a-s range, which is probably as good as expected for local seeing conditions) followed by a dither event and then all heck breaks loose - RMS in 2.5 range.



I've tried much of the obvious ... tighten anything and everything in the imaging train for both scope and guide equip, re-do calibration, check polar alignment (its on a permanent obsy pier), try with and without PEC correction. Have not yet found a root cause.  There are some things I still need to try (listed below) and perhaps I am missing something obvious. Maybe above graph suggests something specific?

A log file from that night is here:
https://openphdguiding.org/logs/dl/PHD2_logs_Q3eD.zip

Things I plan to try next:
  • Recheck mount balance (again!)
  • Lower Aggressiveness - increase Hysteresis
  • Run Guiding assistant
  • Try on-camera guiding
  • Try different guide camera

Appreciate any insights.

DaveNL

Brian Valente

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Jan 29, 2020, 12:53:51 PM1/29/20
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Hi Dave

boy that is sure curious

can I ask about your guide camera and scope setup - what is it and how is it attached? do you have a pic available?

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Brian Valente

Dave Newbury

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Jan 29, 2020, 1:10:39 PM1/29/20
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Hi Brian,
I don't currently have a pic handy, but setup is:

Esprit 150 mounted with clamshell rings. Losmandy bar mounted on top of clamshell rings
Altair 60mm Guidescope in guidescope rings, mounted directly to the top losmandy bar (recently switched from guidescope/rings mounted via shoe on Esprit focuser)
Lodestar X2 guide cam

DaveNL


On Wednesday, 29 January 2020 14:23:51 UTC-3:30, Brian Valente wrote:
Hi Dave

boy that is sure curious

can I ask about your guide camera and scope setup - what is it and how is it attached? do you have a pic available?

On Wed, Jan 29, 2020 at 9:27 AM Dave Newbury <nfn...@gmail.com> wrote:

Having trouble with guiding on a relatively new Paramount MX+.  Much of the time the guiding is in the 2+ RMS a-s range. Sometimes it gets better for a while. The attached graph is a good example of some good guiding (RMS 0.8 a-s range, which is probably as good as expected for local seeing conditions) followed by a dither event and then all heck breaks loose - RMS in 2.5 range.



I've tried much of the obvious ... tighten anything and everything in the imaging train for both scope and guide equip, re-do calibration, check polar alignment (its on a permanent obsy pier), try with and without PEC correction. Have not yet found a root cause.  There are some things I still need to try (listed below) and perhaps I am missing something obvious. Maybe above graph suggests something specific?

A log file from that night is here:
https://openphdguiding.org/logs/dl/PHD2_logs_Q3eD.zip

Things I plan to try next:
  • Recheck mount balance (again!)
  • Lower Aggressiveness - increase Hysteresis
  • Run Guiding assistant
  • Try on-camera guiding
  • Try different guide camera

Appreciate any insights.

DaveNL

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Brian Valente

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Jan 29, 2020, 1:19:39 PM1/29/20
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This is very curious indeed!

i am not sure what's causing this, but it does appear to be linked to when dither happens. It seems the "good guiding" portion is the exception rather than the rule when looking at all your guide runs

Here are some things you can try to see if we can narrow down what's causing this to happen. it looks like you are doing 5 min subs, so i'd say at least 3-4 exposures per test

- turn off dithering and see if it still happens (I don't think it will, but it would be good to confirm)
- guide with the primary scope instead of the guidescope (there are a number of things that could impact it within the guide camera/scope equation, so removing that would help narrow it down if it stopped doing it)
- remove TSX from the equation and try connecting directly to the scope ascom drivers, and keep dither on

Guiding assistant run would be helpful, as there appears to be a fair bit of DEC backlash in your system that PHD can handle well, but I would wait on this until you can hunt down the source of this problem

also I would not change any of your algorithm settings at this point - the changes you are talking about aren't going to impact anything. in fact, i would restore the default values. 



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Dave Newbury

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Jan 29, 2020, 1:51:32 PM1/29/20
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Thanks Brian.  I'll give it a go next clear night (looks like Saturday at the earliest).

Not sure how to avoid TSX as it *is* the ASCOM driver (as best I understand it).  And I'm only using TSX as a means of connecting to the mount, SGP is set to do the rest.


Dave

Brian Valente

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Jan 29, 2020, 2:49:10 PM1/29/20
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Hi Dave

I am by no means a TSX/paramount expert, but my thought was removing anything in the path of communication that could influence tracking or guiding. for example with AP mounts you have APCC inbetween and even though it acts like the ascom driver, it does have influence over the mount performance (in a good way of course :) ). 

I wanted to ensure TSX wasn't inserting any influence on the interaction (such as attempting to correct for things, etc.) - if that's the only way to connect, i would just skip that suggestion

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bw_msgboard

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Jan 29, 2020, 5:59:51 PM1/29/20
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Hi Dave.  As Brian said, this is a bit of a head-scratcher.  As you know, these mounts are usually very stable and people get outstanding results with them under good seeing conditions.  To start, I don’t see anything here to indicate a mechanical problem with the mount itself nor do I see a problem with the guiding.  And I’m not yet convinced it’s well-correlated with dithering.  Look at a few of these sequences that occurred many minutes after a dither:

 

5 minutes after a dither – 4 “spontaneous” displacements of almost 4 arc-sec:

 

 

 

5 minutes after a dither – 3 more big moves of about 3 arc-sec:

 

 

There are no guide commands pushing the scope this way, I think this is something external.  And the Dec motor wasn’t even running for most of these, so we can’t really point to that – that’s another reason I suspect it’s not a problem with the mount itself. You can also see that many of these situations show large displacements on both axes, which again suggests to me that something external to the mount drive system is causing the guide star to move.  

 

You’ve said that you recently changed the mounting arrangement for the guiding gear.  Did you have these problems before doing that?  What about the routing of guide cables, are you sure that nothing there can grab or tug on the gear?  Remember, with your setup, a mechanical displacement of only 4 microns at the camera sensor will cause these large displacements you’re seeing – less than 10% the thickness of a human hair.  It might be instructive to very gently touch or poke the guide camera while guiding is active to get a sense of how little movement can cause a big problem.  Can we assume there was no wind during the session?  What sort of platform is the mount sitting on – any possibility of vibration or mechanical deflection there?  

 

I think the advice Brian gave you is all good.  I would add that the focus of your guide camera looks pretty soft although I don’t see how that would contribute to this sort of problem.  And you should definitely reset the guiding algorithm parameters as he said because those aren’t the problem and the changes could just make things worse.

 

Hope this helps,

Bruce

 

 

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Dave Newbury

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Jan 31, 2020, 4:06:54 PM1/31/20
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Hi Brian,
Understood - and neither am I :=)   It's all relatively new to me (the mount and the software/driver). TSX does have the option to enable/disable PEC and Software Bisque's own "Protrack". I've tried turning those off, but minimal difference that I can see. Perhaps there are other things in there I can look for/disable as well. Will poke around.

Saturday doesn't look so good for clear skies anymore. In fact nothing in the 10 day forecast right now :=(   So might be a while before I get a chance to test again.


Dave
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Dave Newbury

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Feb 1, 2020, 1:47:53 PM2/1/20
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I've tried replying to Bruce's message 3 times and they have all been deleted. Not sure what I am doing wrong?

DaveNL

bw_msgboard

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Feb 1, 2020, 2:03:09 PM2/1/20
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Hi Dave.  Sorry you ran into this.  For no obvious reason, those messages were marooned in the ‘suspect’ message category and we didn’t see them.  I’ve just tried to liberate them so we’ll see what happens.  It’s another instance of this problem:

https://groups.google.com/forum/#!topic/open-phd-guiding/KBZ8H6paoQM

 

Bruce

 

From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Dave Newbury


Sent: Saturday, February 01, 2020 10:48 AM
To: Open PHD Guiding

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bw_msgboard

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Feb 1, 2020, 6:19:56 PM2/1/20
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Hi Dave, see below.

 

From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Dave Newbury


Sent: Friday, January 31, 2020 1:29 PM
To: Open PHD Guiding

Subject: Re: [open-phd-guiding] intermittent oscillating guiding on Paramount MX+

 

Hi Bruce,

Thanks for the detailed reply.

 

The mount is in a dome so wind should not be a problem. I don't think vibrations would be an issue as (i) I generally operate remotely (ii) pier is isolated from dome floor (iii) it is a 10" diameter steel pier in concrete.  I had the same problems before moving the guide scope - in fact purpose of moving it was to attain a more secure/direct mounting arrangement (ie. less connecting parts involved).

 

The external movements without guide commands is interesting. When I first received the scope (last summer) I did have issues with guiding/tracking right from the start. Had some dialogue with Software Bisque about it. They suggested removing the RA worm gear and removing the grease, cleaning and re-greasing. Which I did and it seemed to improve things.

 

Things I still wonder about:

·         I've run a power and usb cable up through the mount (one of the factors in acquiring this mount was for this option). I wonder if there could be cable drag there? When I encountered the RA gear issue mentioned above, I first thought it might be cable drag and I pulled the cables back out - but it did not seem to make any difference

·         I wonder if there still might be some 'play' in the guide scope - it is helical focus type and perhaps this introduces some looseness?

·         Maybe there is still an issue with the RA gear - although problem seems to be with both axes?

Through-mount cabling can cause problems if one of the cables isn’t quite long enough or gets tangled up with another cable in there.  One approach would be to re-point the scope at one of the locations where you had problems, then inspect all the cables and see if one of them is tight or is rubbing over a fixed surface.  I’d pay particular attention to anything that’s connected to the guide camera.  Any sort of “play” in the guide scope – including the focuser – could definitely be a source of these problems.  I kind of doubt the problem is coming from the mount because you can see the problem on both axes at times when Dec isn’t moving at all and RA is just chugging along normally.

These things can be hard to find but you can usually do it by systematically working through all the mechanical interfaces to see what moves.  If it moves, it’s probably bad.  It takes time and isn’t really what you want to be doing, but there it is.  Maybe on a night around full moon… J

Hope you can find it quickly,

Bruce

 

Dave

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Dave Newbury

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Feb 1, 2020, 6:51:05 PM2/1/20
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Hi Bruce,
Thanks again. My third deleted reply was a simple one liner so I think it ended up flagging me more so than the messages.  Hopefully this message gets through :=)

I actually ended up with a 2-hour window of (mostly) clear skies this evening. I fiddled with the guide scope to try and remove any possible wiggle, did some cable mgt and reset the guide algorithm settings Managed to do some testing including a run of the Guiding Assistant. Log here:
https://openphdguiding.org/logs/dl/PHD2_logs_H77h.zip



Dec backlash looks good as does polar alignment.

Per your previous suggestions, I will continue to hunt down possible mechanical issues/root causes. But based on the attached, any further thoughts?

I tried switching to using the main camera and OTA for guiding (QHY128C and Esprit 150) however PHD2 refused to talk to the camera. Tried making sure that nothing else was connected to the camera. And tested that I could connect from Sharpcap or SGP and take a picture (and then disconnect). Kept getting this error "A COM Error occurred ...":

And then the clouds came in ...

Appreciate all your assistance.

Dave

bw_msgboard

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Feb 2, 2020, 4:17:17 PM2/2/20
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Hi Dave.  The GA run is a bit shocking – look at the *unguided* RA behavior here (in red):

 

 

 

 

I don’t know what to make of this, it’s certainly not typical Paramount behavior.  I think your idea of temporarily guiding through the main scope is a good one, it will eliminate many potential sources of the problem.  But I don’t think you want to go down the rabbit hole of trying to figure out the connection problems with the QHY camera – that will just take you further away from the problem at hand.  I think it would be far easier to just move the LodeStar guide camera over to the main scope for testing.  Those cameras have standard nosepieces so it shouldn’t be any trouble to get it mounted and focused.  When you do that, it’s imperative that you create a new PHD2 profile for that experimental setup by running the new-profile-wizard and entering all the new parameters.  Otherwise, the calibrations and guiding parameters will be fouled up and you won’t get meaningful test results.  One of the first things to do after doing a new calibration would be to run the GA again, just like you did in the above graph.  Regardless of the outcome, it will provide useful new information.

 

Good luck,

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Dave Newbury

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Feb 12, 2020, 10:31:59 PM2/12/20
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Based on where this was going, seemed clear this was more likely a mount issue. So I switched to using Paramount's own TSX guiding software (where I was able to use my QHY128 on main OTA) to capture an unguided tracking log. That log still showed unusual activity in the RA, and upon showing to Software Bisque, they are saying the RA Worm block is out-of-spec. It will be replaced under warranty.

Thanks again for everyone's assistance with this.

Cheers,
Dave

Brian Valente

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Feb 12, 2020, 10:34:59 PM2/12/20
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thanks for the update on resolution Dave. hope it gets fixed soon

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bw_msgboard

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Feb 13, 2020, 12:39:43 AM2/13/20
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Hi Dave.  Yes, thanks for letting us know – hopefully they’ll quickly get you a solid replacement mount…

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Dave Newbury

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Mar 9, 2020, 1:03:02 PM3/9/20
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Hi again. Another follow-up. I have received and installed a replacement RA worm block from the mount vendor (covered by warranty!) and have had a chance to retest. Things are noticeably and significantly better!  RA Peak-to-peak is now about 3.0 arc-sec - and a smoother curve. Thanks again for all your help.

For comparison, attached is an updated log file with Guiding Assistant runs.


Capture_PHD2_GA2.JPG



One question I have - previously the GA results consistently showed "Backlash is small, no compensation is needed". However with these new runs (new RA worm block) , it consistently comes back with a recommended backlash compensation value (270-300ms). I have not replaced or touched the Dec worm block. Is this just random result differences? Should I consider implementing the recommended Dec backlash compensation?

Thanks,
Dave
PHD2_GuideLog_2020-03-06_184445.txt

Brian Valente

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Mar 9, 2020, 2:02:43 PM3/9/20
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HI Dave

I suggest turning backlash compensation on and see how your guiding does. If it improves DEC performance, then yes you should leave it on. if Dec performance degrades, then turn it off.

it's worth a shot

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