Polar Alignment Error

[Dan McC]

The PHD2-reported polar alignment error of my RST135e increases dramatically over the course of an evening. In a recent session it started at 1’ 56” and roughly seven hours later it was at 50.2’.

Here is a summary of the Polar Alignment error reported by PHD2 over the course of the session for the major runs:

1’56” NINA’s 3 point polar alignment at 18:48

6.1’ PHD2 log section 8 at18:53

20.5 PHD2 log section 10 at 18:57

5.5’ PHD2 log section 12 at 20:48

27.3’ PHD2 log section 16 at 21:38

43.5 PHD2 log section 18 at 22:58

50.2 PHD2 log section 19 at 00:46

I guess if I was getting good results I would not care, but this night was typical and I had to discard about 1/3 of the subs, and these were 30s at a fairly wide angle so should not really be very challenging for the mount.

My setup is a Borg 55mm refractor at 280mm with a QHY268, weight is about 9 lbs, well under the mount’s capabilities. Guiding is a 30mm f4 guidescope with an ASI120mm guide camera. Tripod is levelled with a carpenter’s level, resting on concrete pavers not grass or dirt.

Without any success I have tried:

Two different tripods

Two different cameras

Two different telescopes

Two different computers to control

I also use an Skywatcher EQR6 Pro in the same location and it does not exhibit this problem - its reported polar alignment error will bounce around within a couple arc-min.

So I am puzzled why I am seeing such dramatic deterioration in the Polar Alignment error? And related to that also is why does my reported RMS error not blow up also? How can PHD2 even guide if my PA error is 50’?

Thanks!

Dan

[Dan McC]

The NINA 3PPA.

[bw_msgboard]

Hi Dan.  The polar misalignment estimate is based only on the rate of declination drift.  So the real question is what is causing such large and variable declination drift.  One possibility is that either the guiding assembly or the whole OTA is sagging or shifting by small amounts as the scope moves.  I would look first at the guide scope/guide camera and see what it's doing.  The comparatively flimsy mechanics of the guiding assembly coupled with the 4 micron pixels in the guide camera make it easy to have these problems.  The guiding sessions shows all kinds of mechanical instabilities that are likely to ruin images (Dec is green):

 

 

These large guide star excursions aren't caused by guiding, and the Dec motor wasn't running when they began. These are equivalent to 4 micron movements of the guide camera sensor, less than 10% of the thickness of a human hair. I think it's these spikes that are ruining your images, not the drift per se.  I think you've probably got something loose in your setup somewhere.  If this was my situation, I would replace that whole finder-scope arrangement in favor of a longer focal length guide scope with a very rigid dovetail type mounting plate and clamshell rings, all mounted close to the main scope.  Either that, or move to an off-axis guider.

 

Hope you can track it down,

Bruce

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Dan McC
Sent: Thursday, January 13, 2022 7:43 PM
To: Open PHD Guiding
Subject: [open-phd-guiding] Polar Alignment Error

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[Dan McC]

Thank you Bruce!

I watch the posts on this forum and I see you give this advise frequently. I am a bit embarrassed, in hindsight it seems obvious that this is a likely culprit. Especially with the frequent DEC excursions and the fact that I have swapped out everything else but the guiding assembly.

But I do have a longer focal length guide assembly with clamshell style mounting on my larger rig, so I can easily swap it in and try it on this rig. If that proves unsuccessful then I will investigate an OAG setup.

I will also re-examine the connection of the OTA to the mount to verify that it is tight.

Cheers,

Dan

[bw_msgboard]

No problem, Dan.  I'm probably starting to sound like a broken record about these finder-scope assemblies but they really can be a problem.  I think it comes back to the reality of sensors with tiny pixels (thank you cell phones) and some geniuses in marketing who decided they could put those on a finder-scope and flog it as an inexpensive guide scope.  Well, inexpensive is one thing, effective is quite another. <lol>

 

Cheers,

Bruce

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From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Dan McC

Sent: Saturday, January 15, 2022 9:05 AM
To: Open PHD Guiding
Subject: Re: [open-phd-guiding] Polar Alignment Error

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/1e88a916-5a2b-44de-beed-d02166766fean%40googlegroups.com.