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Guide log - chasing seeing, backlash, or something else?

astrophotography
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#1 joshschusterman

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Posted 15 May 2019 - 08:28 PM

So I'm really digging in, trying to understand my guiding results using logs. I've read every word of PHD2's guide log analysis tutorial (https://openphdguidi..._Guide_Logs.pdf), and understand it. But I wanted to check my evaluation of a recent session here with more experienced guiders.

 

I set up a few days ago solely for the purposes of improving my guiding. Here are the details...

 

AVX

50mm finder guider

Orion SSAG

8" Newtonian

Bortle 9 (Dallas)

I had just adjusted the gear/worm mesh in both RA and Dec to reduce backlash.

 

Based on what I've learned over the past couple years, and what I read in the PHD2 guid log tutorial, my diagnosis is that I was chasing the seeing too much, and perhaps backlash is some of the problem. Does anyone see something that I'm missing? I'm attaching the guide log for the night. The most puzzling thing is that on one session, the PA error is 0', on anther it's 155', and on another it's 8.2'. But I had set my PA earlier in the night than those sessions (using both Polemaster and PHD2 drift align to check).

 

So... chasing seeing? backlash? PA? Here's a link to a single 20s sub if anyone wants to see it: https://www.dropbox....s__26C.CR2?dl=0

 

Any help is appreciated!

 

 

 

Attached Files



#2 jerahian

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Posted 15 May 2019 - 09:55 PM

Hey Josh-

 

Two things:

  1. When you say you adjusted your RA & Dec gears to reduce backlash, are you talking about having moved the gears in closer to each other to there isn't too much play/space between them?  OR are you talking about backlash compensation via the settings in the hand controller?  OR both?  I ask this because I'm seeing quite a bit of drift in your Dec from your logs, which would highlight a Dec backlash issue to me if you indeed are well polar aligned.  I would really fine tune the backlash compensation on the AVX.  Optionally, you can try backlash compensation via PHD2...just don't enable both.
  2. I wouldn't worry about the run showing 155' PA error.  Something happened during that run which took you ~350" off target, so the stats are all jacked for section 6.  I imagine this was a cable snag or something?  I also wouldn't worry about section 8, where the Dec drift is showing 0.  That obviously is not right, but I'm not sure what happened there.

One thing would be to make sure your mount is on stable ground and not shifting as time goes by (like it may on grass).  If you are stable, then really be sure your PA is dialed in well.

 

Given all that, your total RMS is actually not too bad for an AVX.  You're ranging between 1.27" to 1.60", which is to be expected and not bad on an AVX.  For the AVX, it also depends on where in the sky your were pointing (E vs. W vs. close to the Zenith, etc.).

 

And the end all be all is your stars look nice and round in your raw image!  Ultimately, that is what counts...

 

-Ara



#3 joshschusterman

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Posted 15 May 2019 - 10:23 PM

Thanks Ara! Sorry I should have clarified - I adjusted the gear/worm mechanically, not through the hand controller.

 

You hit it on the head in terms of DEC drift... I'm just wondering what could cause such poor guiding if I'm using both Polemaster and checking it with drift align. The AVX is so sensitive, it seems like even with PA as tight as I can get it with those two tools, I still have worse DEC guiding than most people, not to mention even worse RA guiding. 

 

I suppose the other thing I'm trying to figure out is how much the seeing and urban light pollution is effecting guiding. I wanted to get things as good as I can here before I drive the 2 hours out to the dark site again. Last time I went out there, the guiding was even worse than this session here near downtown Dallas! I'll go ahead and attach that dark site guide log as well. That was Bortle 4 skies, clear night, Polemaster and drift align for PA. Terrible results, hence the tightening of the gears later that week.

 

And I still have slightly elongated stars, all in the same direction across the image field. :-/

Attached Files



#4 jerahian

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Posted 15 May 2019 - 11:50 PM

OK, I would definitely configure your mount with some backlash compensation.  On my AVX, I also adjusted the gears mechanically, and then immediately set the BLC in the hand controller to virtually remove any visually noticeable BL.  I did this with the gear covers off and entered the compensation amounts based on the lag I noticed visually from when each motor gear started moving itself and when it started moving the worm gear.  I based this on a video from Broke Astronomer, which I found helpful.  Without these changes, my Dec would often drift off the PHD2 graph completely.  After the changes, that never happened and my AVX gets between 1"-1.6" total RMS regularly.

 

-Ara



#5 joshschusterman

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Posted 16 May 2019 - 12:12 AM

Will definitely do the compensation on the HC! In fact it was on my to do list, but wanted to get some opinions before I did it.

 

Thanks again Ara!




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