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Interpreting the guide log

Equipment Imaging DSO DSLR
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#1 starrycanuck

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Posted 11 August 2022 - 10:10 AM

A couple weeks ago, I was at a Bortle 4 site and captured the North America and Pelican nebula (image here on CN) using the gear in my signature. Seeing was very good that night and overall I'm pretty pleased with the results.

 

That said, looking at the guiding log, I'm hope to figure out what it is telling me and how what I can do to improve.

 

Notes

  • Setup balanced before imaging and all clutches were tight. HEQ5 is unmodified (ie no belt mod) and I have no idea if it has ever been tuned (I'm at least the 3rd owner).
  • Guiding exposure was 3s
  • Polar alignment was within 2' as determined by ASIAIR PA routine.
  • A meridian flip occurred slightly more than half way through the session. Guiding seems to have gotten worse afterwards
  • Looking at the log, guiding star HFD grew from 3.9 to 4.69 after the flip which surprised me.
  • Guiding parameters were not changed during the night but after the flip, guiding degraded from .88 to .97. RA stayed roughly the same pre- and post- but DEC was noticeably worse post (.44 vs. 61) - why?
  • Both pre and post flip, RA shows a roughly 12 minute sinusoidal cycle where guiding constantly pulsing one way, then the other. Is this PEC or a result of poor polar alignment?

 

I've attached PHD2 log viewer graphs of pre and post flip. The log itself is in my Dropbox as it was too big to attach.

 

Appreciate any insights from anyone who has a few minutes to ponder this.

Attached Thumbnails

  • pre-flip guiding.JPG
  • post-flip guiding.JPG


#2 Tapio

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Posted 11 August 2022 - 10:44 AM

Without loking into guide log (someone sure will and can make suggestions) - if you get round and reasonable small stars in your images and in general you are pleased with your images then don't worry too much of guide graphs/logs.

 

That looks what one would expect from this mount and gear and I'm sure it's enough for providing very nice images with your refractor.

If you ever are going bigger scope with more focal length then it can be different matter and may need tweaking (or event change of mount).

 

Worse guiding after flip can be balance issue. These mounts like east heavy balancing and after meridian flip it can be west heavy without intervention.


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#3 starrycanuck

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Posted 12 August 2022 - 11:56 AM

Without loking into guide log (someone sure will and can make suggestions) - if you get round and reasonable small stars in your images and in general you are pleased with your images then don't worry too much of guide graphs/logs.

 

That looks what one would expect from this mount and gear and I'm sure it's enough for providing very nice images with your refractor.

If you ever are going bigger scope with more focal length then it can be different matter and may need tweaking (or event change of mount).

 

Worse guiding after flip can be balance issue. These mounts like east heavy balancing and after meridian flip it can be west heavy without intervention.

Thanks,@tapio, stars in the centre are round and look good so guiding seem to have done it's job. Interesting point about balance ... it was dead on balanced vs. east heavy. Will have to remember that next time I'm out.



#4 Bill G.

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Posted 13 August 2022 - 10:05 PM

starrycanuck,

The sinusoidal waveform looks like PEC so if you can do PEC training on your mount you can probably get rid of a lot of that.

If your mount doesn't support it, then I would reduce exposure time (2 or 2.5'') to get more frequent corrections (assuming seeing allows it) or increase the RA aggression by 5 to 10% to get a little more correction. Almost all RA corrections are taking 3 or 4 pulses or more to correct so more frequent, or more aggressive corrections should help. But, if your mount supports the PEC training, I would do that first and see how it looks. I suspect it will be fine.  If not, then try changing one of the others if still needed.

 

Bill G.



#5 michael8554

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Posted 14 August 2022 - 08:00 AM

Here's the unguided RA from the east post-flip session:

 

14thAug.JPG

 

Period is about 10 minutes, amplitude about 25arcsec pk-pk, so would be worth improving if the mount has PEC.

 

RA and Dec are fairly close  RA =0.72arcsec, Dec = 0.58arcsec.

 

So if your imaging scale is bigger than about 0.8arcsec/pixel you should have round stars.

 

You should Calibrate near Dec = 0 if you can, not Dec = 43.9 deg,

 

 



#6 italic

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Posted 15 August 2022 - 02:22 AM

Something else you can try is the predictive PEC algorithm in PHD instead of the default hysteresis. You have to let it run for at least one full worm cycle while it learns your error cycle, but you won't end up with worse guiding during this period (it behaves like hysteresis), and you'll end up with better guiding as it goes. I'm still new to this algorithm, but I've seen my guide RMS go under 1 arcsec for the first time.



#7 starrycanuck

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Posted 15 August 2022 - 11:13 AM

Thank you all. Re: PEC

 

As I understand it, the HEQ5 Pro does have PEC but it isn't permanent/saved. The challenge is that ZWO has not implemented and or exposed any PHD2 PEC functionality in the ASIAIR Pro so I'm not sure how I can train before each session.. 



#8 michael8554

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Posted 16 August 2022 - 10:21 AM

Sounds like you have as good as you can get with ASIAir's guiding limitations.




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