Unexplained Jerks In RA???-Help!

[Kamran Ashraf]

Hi everyone!
I have a GEM45G mount carrying GSO 8" Newt and ASI 533MC cam. I am using dec cable management provided on mount.

Here is my guide log-

https://openphdguiding.org/logs/dl/PHD2_logs_ZrG3.zip

I have had done various analysis regarding PEC etc i ll share chat GPT link here-
https://chatgpt.com/share/675942fb-2fa0-8013-9add-fd7ef1d24128

Frequency Spectrum Analysis:

• The RA error shows dominant periodicities at frequencies (Hz):
  0.00298, 0.00068, 0.00196, 0.00162, 0.00111, 0.00179, 0.00051, 0.00247, 0.00102, 0.00060.

• The DEC error spectrum highlights these frequencies:
  0.00034, 0.00162, 0.00485, 0.00008, 0.00503, 0.00511, 0.00545, 0.00264, 0.00519, 0.00537.

chap gpt says its performing good in its class, well i cant decifier it -

11. Conclusion

• Quantitative Judgments: The total guiding RMS error of 0.152 arcseconds is within acceptable limits, but DEC performance can be optimized.
• Qualitative Judgments: The mount's performance is solid for its class. Fine-tuning and PEC would elevate it to a more premium category.
• Comparison: Compared to similar mounts, this GEM45G performs slightly above average but shows typical DEC backlash issues.
• Rating: 7.5/10
  

-______________

while photographing i am consistently seeing unexplained KICKS AND JUMPS in RA ! no analyis talks about that-?
can someone help me out whats going on in here?

(i am uploading another log but hbis time kindly ignore the STAR LOST events - my image sequencer stopped the tracking. )

(

https://openphdguiding.org/logs/dl/PHD2_logs_bEEi.zip

)

i am also uploading raw images from 2 sessions 

(kindly ignore the coma and other optical imperfections)

[Bruce Waddington]

Overall, you should have been getting reasonably round stars during these guiding sessions although their FWHM values were probably higher than you want.  Your typical overall guiding RMS was usually around 1 arc-sec or somewhat less with fairly similar values for RA and Dec.  So if you are getting seriously elongated stars in your main images, something else is wrong.  It looks to me like the seeing conditions were probably not very good based on the Dec behavior, in which case it's not a good time to be trying to refine mechanical performance.  All of this assumes that the high frequency fluctuations aren't the result of the embedded guide camera in the mount.  That device puts extreme demands on mechanical stability and complete absence of vibration.  An unwanted movement of 1.9 microns - from any source - will result in these apparent 3 arc-sec guide star excursions you're looking at.  For reference, a human hair is about 50 microns thick. 

In terms of the RA tracking accuracy itself, there are two prominent peaks of tracking error with periods of 200 and 400 seconds:

This could be improved with a conventional periodic error correction using an app like PemPro, or you could try using the PPEC guiding algorithm in PHD2 for RA.  In the latter case, set the 'period length' to 200 and disable auto-adjust.  I think this might reduce the size of the RA excursions you're talking about. You won't be able to post full-size images on the forum, you will need to use a cloud storage service of some kind with publicly accessible links.

Regards,

Bruce

[Kamran Ashraf]

Thanks a lot Bruce for the comprehensive response. 
will u please confirm that the VIBRATIONS u r refereeing to, are supposedly coming from the inbulit nature of the GUIDE SCOPE AND CAM ( iguider of GEM45 (G))?

And, are 3" excursions not big enough if overall RMS is <=1" ?

[Bruce Waddington]

I don't know if you have vibration problems or not, I'm only suggesting it's a possibility.  If they're present at all, they could be transmitted from the drive system, the pier or tripod, etc.  I brought it up because you have a lot of high-frequency movement in both RA and Dec.  As I said, this could be purely because of seeing, there's no way for me to tell.  Since you are using 1-second exposures - and I don't know why that is - you will be more affected by seeing effects. I think there are inherent shortcomings in this built-in guider approach, mostly because of its coarse image scale but also because it can be prone to differential flexure and vibration.  I think if you look around on other forums, you will find that many people have stopped using it in favor of better guiding arrangements, notably OAGs and higher quality guide cameras.

In terms of the overall guiding results, only you can judge that based on your imaging results.  If you are happy with the star sizes and shapes, then that's good enough.  I do think that you can reduce some of the RA spikes by using PPEC.

Bruce

[Kamran Ashraf]

Thank u Bruce!

I was using short exp because guiding assistant recommended short and the DRIFT LIMITING EXP WAS ONLY 0.8s. Plus i am getting bigger deviation on using longer exp say 1.5 s.

Thanks for suggesting the PPEC algo , i ll do as you suggested and see if the prob is solved.

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[Bruce Waddington]

Whoa!  You're hitting me over the head with GA recommendations?  Ouch! <lol>  Well done!

Cheers,

Bruce