Cannot Solve Repeated Excursions

[Jeff Rothstein]

https://openphdguiding.org/logs/dl/PHD2_logs_eMna.zip

Hi all.  I've worked to get PHD2 working on my rig for several months, without any consistent success.  I regularly receive Star Lost Star Mass Changed messages which coincide with large excursions in Dec (according to PHD2) that ruin the current image.  I run NINA and PHD2 through ASCOM.  I am running PHD2 v2.6.10dev3.

My mount is a Losmandy GM-8 from 2007 with Gemini-2 carrying a 4" refractor and guidescope, total weight about 21 lbs.  I run all software through ASCOM. I've been through all the PHD2 documentation on troubleshooting and best practices, and had the help of Brian Valente and other thoughtful folks on the Losmandy groups.io forum, but I haven't been able to eliminate all the gremlins.

Let me ask about just one of the two in the zip file I uploaded, from Jan 7.  After a frustrating week of excursions every few minutes over several nights, I went back to the beginning to create a guiding baseline per PHD2's documentation.  I created a new profile (and yes, all the data I entered was double-checked) and loaded my previous dark library (only a few weeks old and I had not moved the camera, guidescope or guidescope focus).  I carefully aligned the mount using Drift Align.  Ran GA after that, and it said my error was .5 or .6 arc-min.  I struggled a bit to recalibrate at the CE and Meridian (see Log Secs 19-22).  Several times the star was lost as the south steps began, and once the star did not return to the crosshairs after the step direction reversed.  I ran GA again (Log Sec 23) and received only min-move recommendations, which I applied.

I then started to create the baseline, but I lost the guide star for the first time after a minute (Log 24).  I kept restarting guiding and logged about 45 minutes (Logs 25-34), with each log section reflecting a lost star. 

I have been outside with the scope when the star is lost. Without wind or vibration that I can detect, the star simply jumps from the crosshairs to about halfway across that window.

This problem does not happen when I am not running PHD2.  I ran 90s images for several hours one night this week and none reflected this star jumping problem.  (They did show elliptical stars, which I suspect is due to the different RA and Dec errors that my PHD2 logs consistently reflect).

I've checked every physical issue highlighted in the Best Practices and other docs, and am now at a loss for what else to check on my mount so that it will play nicely with PHD2.  Thanks in advance for any suggestions that you have, and if you need any additional information, please let me know.

Kind regards,

Jeff

[bw_msgboard]

Hi Jeff.  Why don't you send us a photo of your guiding assembly - cables, guide scope, guide camera, and mounting arrangement.  I suspect your problem doesn't have anything to do with the mount or with guiding.

 

Regards,

Bruce

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Jeff Rothstein
Sent: Sunday, January 09, 2022 7:09 AM
To: Open PHD Guiding
Subject: [open-phd-guiding] Cannot Solve Repeated Excursions

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[Jeff Rothstein]

Many thanks for your note, Bruce, that would be good news indeed.  Recent photo attached.  The scope here is facing East. Given my yard’s layout and the light dome of Tucson to the S, I try to image from about 30 degrees altitude in the E and NE to the meridian, and then pick up another target.  When the scope is on the W side of the mount, as here, the cables all hang free as near as I can tell.

Best,

Jeff

[bw_msgboard]

Hi Jeff.  Well, this looks pretty bad but before we get into all that - I think you have Dec backlash compensation enabled in the mount.  That will kill you.  The Jan 2 log shows that you get into these huge Dec oscillations whenever there's a direction reversal in Dec guiding.  You should be absolutely sure you have disabled that before we get into the other issues with the guiding setup.  For now, leave Dec backlash compensation disabled in both the mount and in PHD2.

 

Cheers,

Bruce

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Jeff Rothstein

Sent: Sunday, January 09, 2022 11:56 AM
To: Open PHD Guiding
Subject: Re: [open-phd-guiding] Cannot Solve Repeated Excursions

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/f71c19a7-d15d-47a0-aec2-b86f26944e17n%40googlegroups.com.

[Jeff Rothstein]

Thanks, Bruce.  You are right, the Gemini-2's Dec backlash compensation, TVC (which I thought I'd checked earlier, but...) was set to 500.  I've reset it to 0.  Dec backlash comp in PHD2 is off.

Best,

Jeff

[Jeff Rothstein]

Hi Bruce.  You mentioned things looking "pretty bad" in general.  Are there other issues I ought to look at before starting to test again?  I have time; the forecast doesn't look good for the next 10 days.

Thanks,

Jeff

[bw_msgboard]

Hi Jeff.  I'll just point out some things that I wouldn't want on a guiding assembly and places where I think you might create problems. 

 

 

1.  These tall mounting blocks put the optical axis of the guide scope far away from the RA/Dec axis and the main scope and they can probably flex a bit. Ideally, the guide scope should be mounted close to the main OTA.

2.  3-point mounting rings are a frequent source of problems.  The thumb-screws are usually tipped with Delrin and it's very hard to keep them tight enough to prevent tiny movement of the guide scope. 

3.  Hanging cables swing around and can drag across other surfaces.  If the cables are in ribbed cable-covers, the little ridges on the covers can briefly catch-and-release when they rub across another surface. In this picture, if you slewed the scope north from this location, don't the cables come into contact with the scope and with each other?

4.  Where does this cable go when you do a meridian flip?    Isn't it going to rub against the top of the tripod?

 

Best practice on this stuff is to avoid hanging cables and do a careful job of routing them.  That can be hard to do because what works in one sky location can bind in another.  You can hopefully find lots of examples in other forums.  I'm not saying that what you have can't work, only that it looks a bit dicey to me.  Remember that your guide camera will "see" a movement of only 4 microns and that will translate to a guiding excursion of 16 arc-sec. 

 

Regards,

Bruce

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Jeff Rothstein

Sent: Wednesday, January 12, 2022 4:35 PM

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[Brian Valente]

Hi Jeff

I only just saw this thread, sorry

As Bruce pointed out, based on your guidelogs i think the high TVC value was killing you and causing lost star events. I can see that everything was going along merrily and then the first dec reversal caused your guidestar to fly off by 10" or more

However, don't throw out the baby with the bathwater: You should enable PHD auto backlash compensation, start with a value of 800 and max 2500 and let PHD adaptive algorithm find where it needs to be

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/F89D6CC094B148D78B5F5050CD0002D3%40HomeDesktop.

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Brian 

Brian Valente

portfolio brianvalentephotography.com 

[Jeff Rothstein]

Thank you, Bruce and Brian, for the constructive thoughts.  I've already been thinking about how to manage the cables better and will get to work on that.  I see an opportunity to move the Powerbox, get rid of the guidescope risers, and route some of the cables along the mount.  To work!

Best,

Jeff