CGX - frequent elongated stars in RA, 2" vs DEC 0.5"

[Stewart Tansley]

Celestron CGX, currently carrying Celestron EdgeHD 8 reduced 0.7x, OAG, ASI174MM-mini guide cam. 1488mm f/l, 5.86um pixels, 0.81 arc-sec/px.

I’m sure I have a few things going on (cables, polar alignment, maybe balance – all being refined), but I continue to have consistent substantially elongated stars in RA on ~30-50% of my 60-sec subs. RMS shows ~1.5-2” in RA, cf. ~0.5” DEC.

The scatter plot below summarizes nicely what is going on, but obviously not the cause – random extended movement in RA. It’s like something’s loose?

There’s no significant (in FFT analysis) oscillation, though there are peaks at 34.2s (1”) and 26.7s (0.7”)  -- in particular, the worm cycle (~480s) is down at a rather nice 0.1”. The mount has just been Hypertuned (to reduce my variables), but the elongation was happening before that too. So consistent. I always follow the Guiding Assistant recommendations.

Anyone seen something like this and knows a fix?

I will post a properly baselined log if this is not just obvious from the scatter.

[bw_msgboard]

You'll need to post your log files - anything short of that is just guess-work.

 

Bruce

---

From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Stewart Tansley
Sent: Saturday, September 11, 2021 1:20 PM
To: Open PHD Guiding
Subject: [open-phd-guiding] CGX - frequent elongated stars in RA, 2" vs DEC 0.5"

--
You received this message because you are subscribed to the Google Groups "Open PHD Guiding" group.
To unsubscribe from this group and stop receiving emails from it, send an email to open-phd-guidi...@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/8f21fb6f-f498-4207-9617-33dfce18c1e0n%40googlegroups.com.

[Stewart Tansley]

Here you go, thanks Bruce - https://openphdguiding.org/logs/dl/PHD2_logs_mLXr.zip

[bw_msgboard]

Hi Stewart. The native RA tracking on your mount looks pretty rough to me, I'm not surprised you're getting so much tracking error.  Here's a look at the raw RA behavior of the mount:

 

 

You can see there's a nearly 40 arc-sec peak-peak long period error here.  Of more concern is all the rapid and steep errors on the individual sections of this periodic error.  The frequency analysis helps to understand this:

 

 

The large error is approximately the native worm period, so that can be improved with a periodic error correction - and its actually handled pretty well by PHD2 already.  The higher frequency errors that occur at 26, 34, 80, and 235 seconds are more difficult to correct, especially the 2.5 arc-sec error at 34 seconds. Those errors are too abrupt to be handled well by normal guiding so they impose a limit on what you can expect.  Your RA RMS during this period was over 1.5 arc-sec and I think most of that is due to these higher frequency components. 

 

I think it would be worth asking Celestron if this is all you can expect, I suspect it should be capable of better performance.  You can also ask on other forums where there are Celestron mount experts available. The "hyper-tuning" may have made it worse, I really don't know.  Personally, I've never seen the value of these "tuning" operations unless the person doing them can provide before and after views of the performance at the level of accuracy needed for auto-guiding.  Of course, if the star elongation in your final images is tolerable to you, you can just keep on running the way things are.

 

Good luck,

Bruce

---

From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of Stewart Tansley

Sent: Saturday, September 11, 2021 11:25 PM
To: Open PHD Guiding
Subject: Re: [open-phd-guiding] CGX - frequent elongated stars in RA, 2" vs DEC 0.5"

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/04e635ed-8abc-46dc-9819-892298c9d099n%40googlegroups.com.

[Stewart Tansley]

Super-helpful analysis and guidance, very very kind of you, thank you Bruce! 

Celestron is a good suggestion, but most practical solution is likely to dedicate this one to planetary and visual, and perhaps wide angle focal lengths, <500mm. For planetary, it carries my Edge11 well. Will continue saving for a Premium mount!