Recent Erratic Guiding

[Juno16]

 Hi Forum,

I have enjoyed good guiding with my Skywatcher HEQ5 mount for the past few years.

Just after receiving the mount, I installed the Rowan belt modification kit and adjusted the DEC and RA worms and according to the PHD2 Guiding Assistant, the DEC backlash is very low.

Just the past few sessions, the guiding has been very erratic. I have gone from my usual total rms error 0.6-0.7” to 0.8-1.0”. It is important to note that the targets were mostly due east crossing the meridian close to the zenith.

The recent results of 0.8-1.0” are a mix of good guiding of 0.6” to poor guiding excursions to 1.3”. Before this issue, I did have an occasional guiding excursion, but they usually calmed down to result in a very good (for me) average total rms error (as reported by the PHD2 Log Viewer).

I check my subs with Pixinsight’s Blink and until recently, I discarded very few subs because of mishapen stars. More recently, I am deleting 20-50 subs (30 seconds integration) out of a 600 sub session.

I uploaded a two guide logs. One from a recent night where I calibrated PHD2. I believe that it also includes a GA run following the calibration, and another log from last night.

https://openphdguiding.org/logs/dl/PHD2_logs_gMFN.zip

Just to note, I calibrate near the intersection of the Celestial Equator and Meridian and also run the GA there too.

When I started to have issues, I ran the GA (also near the intersection of the Celestial Equator and Meridian) and haven’t really seen little or any difference in the recommended settings from before I had these issues.

I am using a belt modded Skywatcher HEQ mount with a Explore Scientific ED102 (reduced to 571mm). I use a Orion Star Shoot Autoguider camera on a Orion 50mm guide scope. I also use a ZWO EAF and a ZWO ASI 533MC camera.

I am using the GreenSwampServer ascom driver for the mount and guide camera.

I know that my seeing is usually fair-poor. I tried changing the guide camera exposure time from 4.0-2.0 seconds without able to stabilize the guiding. Guide camera exposure time is all that I did actively during the sessions to help the guiding.

NOTE: Visually, the imaging train including the guide camera/scope seem solid with no movement.

Thank you for any direction that you can give me and I am very grateful for the time that you’ve spent looking at this data.

Thanks again!

Jim

[Bruce Waddington]

Hi Jim.  I think there are a number of things contributing to your problems.  If we look at the GA run you did in December), it looks like the seeing was quite poor:

Even with 2-s exposures, you're getting lots of high-frequency excursions of 1.5 arc-sec (even worse in the January log).  This is exaggerated by the very coarse guider image scale you're using, 6.6 arc-sec/pixel.  That means these "big" excursions could be caused by mechanical movement of the guide camera sensor of 2.5 microns - a human hair is about 50 microns thick.  So your tolerance level for any kind of mechanical shift is near-zero. You're also getting low SNR values for the stars despite the reasonable exposure times and what appears to be a good focus.  Are you working in a light-polluted area?  This is limiting your ability to get the full benefits of multi-star guiding in some sessions.  Next, the min-move recommendation made by the GA was 0.15 pixels, which would normally be considered fairly low.  But again, this translates into a 0.9 arc-sec excursion - so guide corrections won't even be executed for any excursions smaller than this.  I think the coarse guider scale is a significant problem and is made worse under poor seeing conditions.  I think you should revisit your choice of guide camera and guide scope - if you can halve the guider image scale, things should become easier for you.  I think one good option would be a considerably better guide camera coupled with an OAG.

If we look at long windows of guiding, the Dec and RA rms values are fairly similar which means you should be getting round stars.  They might be bloated but they should be round.  If you are only taking 30-sec exposures, it's surprising to me that you are getting the rejection rates you're talking about.  I think you should identify specific times when you got elongated stars and see how the guiding and tracking behaved during just that interval because there are intervals where the mount response isn't good.  In this section, there is pretty clear evidence of stiction on the Dec axis:

You can see that direction reversals take a considerable amount of time to have an effect but once the axis starts moving, it over-shoots.  You can also see how the min-move value has you running un-guided in Dec for periods of time.  Perhaps in your effort to eliminate Dec backlash, you have created binding in the drive system.  Backlash is much easier to deal with than stiction from a guiding perspective.

Until you can improve the situation with your guider image scale, you can try using min-move values of 0.1 px.  But with these seeing conditions, I think you may also need to adjust your expectations.

Regards,

Bruce

[Juno16]

 Hi Bruce,

Thank you very much for the analysis that you have done on my logs.

I did read your message before I turned in last night, but I wanted to re-read your message again as there is lots of information shared.

Yes, I am in a solid Bortle 7. Especially in this part of the sky (east) and low altitude, the lp is significant. I did have trouble even after the meridian, where I might have a B5 level of lp, so as you confirmed, my seeing is usually very challenging.

Interesting information about the guiding image scale. I remember many years back when I purchased the Orion StarShoot autoguider package, for some reason, I was shooting for under 5:1 (image scale guider/image scale of imaging train). So, I thought that I was okay at 6.6/1.358=4.8.

This setup has done well for me, but as you said, My seeing conditions definitely push this setup and I will look into a larger guide scope, but more specifically an oag as it would save weight.

I guess that in my overzealous DEC worm adjustments, I certainly could have gone a bit too far and didn’t consider binding since manually rotating the DEC worm gear seemed very smooth.

I have to say that I like to see the GA report zero or very low DEC backlash!

I truly cannot thank you enough for this analysis. I will definitely work on your suggestions and adjust my expectations.

Thanks again for your time and analysis and may you and your family have a wonderful 2024!

Jim