orthogonality error message

[pstra...@gmail.com]

This session is ugly, don't spend too much time on it.

guiding log is at https://openphdguiding.org/logs/dl/PHD2_logs_UmFt.zip. PHD2 seems to have deleted the debug log for that session, sorry.

Adding STC-7 camera with SC2 OAG/camera to my LX200 10" mounted on Moonlite focuser with AP 67 focal reducer.
During the GA session I got high values for PA error and an orthogonality error message I don't l understand. Lots of star lost messages.

Conditions were terrible: high wind, almost full moon, very light overcast.
On 01/16/2022 I had an even worse session, not much use in examining the logs.

All my previous guiding and imaging was done with a  piggybacked Stellarvue refractor, 8300 imaging camera, and Sti on an OAG. PA and guiding were not perfect, but adequate for my purposes.
I would like to find out what I can fix (orthogonality, etc. ) first before I take the step of redoing PA.  I would have to remove both sets of cameras to do a Polemaster alignment.  Drift and other alignment methods haven't worked for me.

Since weather conditions here in Colorado have been really rough, I'm not getting frequent nights I can work on this.

LX200 etc is pier mounted in a Skypod observatory.
If you can help me on the orthogonality issue, that would be great, and can spot anything else I am missing, that would be great.
My plan barring easy solution:
1. Try another STC7 session in better conditions. If still bad
2. Run a Stellarvue/8300/Sti session to see if PA is so far off from that instrument's standpoint.
3. If all else fails, take all cameras down and redo PA with Polemaster.

Pieter Strauss

[wave...@talktalk.net]

Hello Pieter, as an LX200 (Classic) owner, I know how difficult this can be when operating at sub-1 arcsec/px image scale. The calibration can show poor orthogonality, especially from the Meade fork mounts which are notorious for their Dec backlash. From the GuideLog you've provided, it's clear that weather conditions were very poor - maybe too poor to get much useful data. One thing you didn't mention was clearing the Dec backlash before calibrating. If you didn't do that, the results can be rather bad. I'd also strongly suggest getting some basic information on the mount's unguided performance. Do that by running the Guiding Assistant for several worm cycles, say 20-30 minutes or even more. This will provide the Baseline Guiding data and from these, you'll get things like the Dec backlash period, a more accurate polar alignment error and the mount's periodic error. All this is described in the PHD2 manual and various guides available from the PHD2 website. These Meades can be pretty poor in the periodic error department and some PEC work is usually advised (ref PEMpro, etc.)

Hope this helps.

- Jack

[pstra...@gmail.com]

Many thanks, Jack.

My LX200 is a GPS ACF model.

I did clear the Dec backlash -- this is really easy to forget, so I have it bolded and underlined in my things to do tonight list.

This  instrument does have  backlash issues.  Even with the more acceptable calibrations I get when using the piggybacked Stellarvue with STi OAG guiding (but still, it is the LX200 I am guiding), I usually get nastygrams about Dec backlash, and mostly have to guide in one direction.
The LX200 supports PPEC, and I did do a lengthy PEC correction session some time ago. It might very well be a good time to do another one.  I did not yet use Andrew Johansen's PEC editor to smooth out multiple curves, but that too might be a good idea.

I will  try the lengthy GA session to see if it kicks up better data. I usually do 3 minutes.
Beyond that, I think it better to try to get a session in under good conditions, so I can eliminate the fog of wind and clouds.
I still don't understand quite what the orthogonality message is saying.  Is it saying that the LX200's Dec and RA axes are not perpendicular to each other? Or is it saying that the  two axes of the guiding camera chip are not square with the two axes of the LX200? Understanding this would help me greatly.

--Pieter

[bw_msgboard]

Hi Pieter.  Just curious, have you read the discussion in the manual about orthogonality error and the various other calibration alerts?  If so, maybe you can describe what doesn't make sense so we can clarify things.  The discussion assumes you're looking at the graphical result of your calibration which shows how the guide star was moving on the camera sensor.  If you haven't read any of this, the manual is always the place to start.  This particular stuff is covered in the Trouble-shooting section along with many other common problems.

 

https://openphdguiding.org/man/Trouble_shooting.htm#Problems-_Mount_Control

 

 

Bruce

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of pstra...@gmail.com
Sent: Saturday, January 22, 2022 10:39 AM
To: Open PHD Guiding
Subject: [open-phd-guiding] Re: orthogonality error message

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[wave...@talktalk.net]

Hi Pieter, you asked:

I still don't understand quite what the orthogonality message is saying.  Is it saying that the LX200's Dec and RA axes are not perpendicular to each other? Or is it saying that the two axes of the guiding camera chip are not square with the two axes of the LX200? Understanding this would help me greatly.

It may seem counter-intuitive, but I don't think it's saying either.
I'll tell you what I understand about it, but hopefully Bruce, etal. will explain it better to improve both our understanding.

So, taking the last part of your question first. The axes of the guide camera do not need to match with the axes of the mount. It's a fallacy I worked with for a while until it became clear that PHD2 takes account of any angular differences between the camera and mount's axes. Basically, it uses trigonometry to accomodate the difference. From a human perspective, it seems better to have the axes more or less parallel with each other. It's certainly easier to visualise corresponding movements of the star field on-screen, but from PHD2's perspective, it's not relevant.
As to the first part, even though the mount's axes are at right angles mechanically, the star 'trail' generated during calibration to the West and then to the North may not be. I think of it as the guiding or optical orthogonality. Remember while the West - return - North steps are underway, the star field is also moving west at sidereal rate. The resulting star 'trails' are compromised by any backlash in the drives as well as their relative tracking speeds. This is why the RA and Dec speeds are part of PHD2's calibration report. If Dec speed (arcsec/sec) is significantly different from the RA speed, it will contribute to how much the axes appear non-orthogonal. The calibration plot (Tools/Review Calibration Data) illustrates this in a more understandable way.
So, by tweeking the mount parameters (maybe in the ASCOM driver) and reducing Dec backlash before calibration, the mount's optical orthogonality can be improved.
That's as I see it.
- Jack

[wave...@talktalk.net]

Pieter, another factor I perhaps should mention is the guiding image scale. Using SCTs, particularly with off axis guiding can mean you're operating at 1 arcsec/pixel or less. In my experience this adds a layer of difficulty in achieving optical orthogonality. Each contibutory error in the calibration is magnified compared with a shorter FL guidescope.

- Jack

[pstra...@gmail.com]

Jack and Bruce,

Thank you both! 

RTFM.  Guilty!  In my defense, may I say that I have downloaded the PHD2 manual PDF, but it does not appear to be searchable.  Normally I would expect to be able to search for "orthogonal" and teach myself.  With your supplied link I was able to read the relevant section, which answered my questions (along with Jack's explanations).
I would say, that with Dec backlash being a known issue with this mount, and my two sessions taking place under bad and worse conditions, I have no useful data for an analysis. 

So I'll keep plugging away and trying to appease the weather gods.  Will come back to  you when I have  good data -- or find that all is resolved, wouldn't that be nice?

At any rate, I'm very grateful for your help.

--Pieter

PS I would like to download the manual PDF and transfer it to my iPad, my preferred reading vector.  But it only seems to open in a browser, not as a PDF.  Am I missing something here?

[bw_msgboard]

Hi Pieter.  I just opened the PDF version of the manual on the website, did a search on 'orthogonal', and got to all the relevant places including the one describing orthogonality error.  So I don't know why that didn't work for you.  I was using the Adobe version of the Reader but I wouldn't expect that to matter.  FYI, the reference platform for the Help is built into PHD2 which can be run in the daytime (obviously).  The real-time version includes an indexing function that also provides a useful way to find things.

 

Cheers,

Bruce

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of pstra...@gmail.com

Sent: Sunday, January 23, 2022 8:24 AM
To: Open PHD Guiding
Subject: Re: [open-phd-guiding] Re: orthogonality error message

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/0fa57144-cbd0-49c2-9459-646a1a00fce9n%40googlegroups.com.

[Brian Valente]

>>>  But it only seems to open in a browser, not as a PDF. 

That's a setting within your Browser. If you prefer reading in a separate app, you just need to save the PDF and then you can open it in your preferred format. On my iPad, i can also "open with..." and choose Books, where I keep and read manuals like this. 

>>>In my defense, may I say that I have downloaded the PHD2 manual PDF, but it does not appear to be searchable. 

downloading it will make it searchable, although i've been able to search within PDFs viewed in a web browser

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/0fa57144-cbd0-49c2-9459-646a1a00fce9n%40googlegroups.com.

--

Brian 

Brian Valente

portfolio brianvalentephotography.com 

[pstra...@gmail.com]

OK, working now!  Apparently, my brain will work in the day time or night time, but not both. Specifying Adobe Reader in "Open with" functions correctly.  But allowing it to open in Firefox or Chrome gives me a window without a search function that I can find. I'm sure the answer is buried in the depths of the browser configuration.  Thanks again!

[Brian Valente]

>>> But allowing it to open in Firefox or Chrome gives me a window without a search function that I can find.

Use the Find function in your browser, works like a charmin (Ctrl-F for me on chrome/Windows). it's not within the embedded PDF viewer, it's the browser search function

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/3fb600a4-2ad4-4557-b275-b693d2006bb4n%40googlegroups.com.