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Guiding Guidance/Analysis

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#1 Yassinem

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Posted 04 July 2020 - 03:33 PM

I'm diagnosing issues with the guiding on my mount which is a Celestron CGEM II.  Overall getting round stars but you know... I need to get below 1" smile.gif still chasing it.  Anyway looking for any tips, my PHD2 session from my last imaging session is at the link below.  I plan on doing PEC training and hopefully that will resolve the RA errors but the dec worries me.  That night I only had a camera lens and a small 50mm scope.  I don't think the load was more than 10lbs, I know I probalby don't even need to do guiding with this setup but wanted to do it anyway.  

 

Few questions... with my setup I have a side-by-side configuration on one side is my camera and 135mm lens, and on the other is my guide scope and auto guider, does this affect guiding in any way?  Regarding balance I made sure everything was in balance before I connected the cables (power/USB) and it's hanging off one end of my setup specifically the side with the main camera and lens (using my ZWO ASI1600MM as a USB Hub so everything comes off of that).  Would this affect guiding this much?  

 

Anyway I would think I could get very good guiding with this setup given that there's barely any load on the mount, but concerned that I'm still at 1.45 for this session.  LInk below to my PHD guide log, thanks in advance!

 

Setup

  • Celestron CGEM II Mount
  • Rokinon 135mm lens
  • ASI1600MM-Pro w/ Filter Wheel
  • Orion 50mm guide scope
  • Orion SSAG Guide Camera

 

https://drive.google...iew?usp=sharing


Edited by Yassinem, 04 July 2020 - 03:36 PM.


#2 Jim Waters

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Posted 04 July 2020 - 03:47 PM

The PHD2 developers live here.  Post your questions and Log file to the link below.

https://openphdguidi...g/getting-help/

 

Its unlikely you can get below 1 arc-sec with your setup.  The CGEM mounts have crazy DEC backlash.

 

BTW - The Orion SSAG is towards the bottom of the auto-guider list....

 

FYI - Check out this link.

https://astronomy.to...calculators/ccd



#3 Yassinem

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Posted 04 July 2020 - 04:05 PM

Thanks Jim!  I actually just ordered a new autoguider, I've had this one for years and I only use it for this setup, I primarily use my ASI120MC-S for guiding.  Just ordered a QHY5III290C which has smaller pixels and I think will guide better than the 120MC-S.

 

Anyway I'll check out the PHD link, thanks!



#4 Epox75

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Posted 04 July 2020 - 09:48 PM

I have a Celestron CGEM-DX, after different years of use I manage to get good guiding results. The mount is tricky and I had to learn its behaviour. I don't see anything wrong with your settings in PHD besides the fact that the backlash compensation is disabled. This should be done only if your mount has no backlash at all or if you have already set a backlash compensation directly in your mount thru its settings. 

 

A good thing to do, as soon as you have started guiding, would be running the guiding assistant to check what may be the best settings in terms of Minmo of RA and DEC and backlash compensation. 

 

Backlash might be variable thru a session, mine is. I've learnt to read from the graph if the compensation in PHD is too high or too low and if I see that something changes I "help" PHD  by increasing or decreasing the compensation settings so that it reaches the correct amount as soon as possible. 

 

Another alternative, if you have a large amount of backlash and assuming that you have a good polar alignment, would be to guide with DEC in one direction. 

 

Also having a CGEM mount perfectly balanced might be not the best for guiding, especially in DEC. My mount has play on the DEC axis and I've found out that it guides much better when DEC it's slightly unbalanced, this way the play doesn't affect guiding.  

 

Play should be reduced to a minimum on both axis without binding the gears, I do that periodically (once a year, more or less). A good guide to do that can be found here: http://www.astro-bab...slacken it off.  It's for EQ6 and ATLAS but the CGEM-DX has the exact same adjustment method, so I guess the CGEM-II should have it too. 



#5 Yassinem

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Posted 05 July 2020 - 12:42 AM

Thank you!  I ran the guiding assistant and am imaging tonight so we'll see how it works out.  It told me to change my minmo to .1 and .15 for RA and DEC respectively.  I also turned on the DEC backlash compensation and changed the RA algorithm to PPEC on the advice of someone from the PHD2 Forum.  Lets see how this turns out overall, stay tuned!



#6 Jim Waters

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Posted 05 July 2020 - 01:02 AM

If you use PHD2 PPEC make 'sure' you enter the correct CGEM worm period...!  I played around with PPEC and didn't see much improvement.




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