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Yet another CGX post...

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#1 KC_Astro_Mutt

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Posted 13 May 2022 - 06:06 PM

So last weekend, had a nice clear night.  I've been chomping at the bit to use my new CGX with the AT130EDT.  Everything worked great, I mostly was testing guiding and snapped a few test photos, with nice results.  Guiding was actually pretty good, but I notice that the RA has twice (and up to 4 times) the RMS error as the DEC.

 

Like I said, guiding is good, but I wanted to investigate the RA swings.  Changing the aggression setting didn't seem to change anything.  I posted on the PHD2 group, and got a response from Bruce that my calibration steps were too short (don't know how to set them, as they are default to PHD2), and I had this spike at 34 seconds in my frequency analysis (whatever that is).  He mentioned that I would have trouble getting good guiding at DEC 0.  I know he's the expert, but if I'm reading it right, it appears the spike is only a .36 arc second error, not really much to worry about.  Is this correct? I have the log viewer, but I don't really have a clue how to use it.  Link to the log is below.

 

https://www.dropbox....2qa4r9S3Ga?dl=0

 

Jeff

 

 

Attached Thumbnails

  • 2022-05-13 17_48_58-PHD2 Log Viewer - PHD2_GuideLog_2022-05-06_195233.txt - PHD2 2.6.11 Windows.png


#2 Jeffmar

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Posted 13 May 2022 - 06:46 PM

My RA is always worse than my DEC. If you have good polar alignment your DEC will hardly need to be  moved at all. That’s just my experience.


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#3 rgsalinger

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Posted 13 May 2022 - 06:48 PM

You use the guiding assistant to get the correct calibration steps. The log says that the steps were too large - not too small - but still you have the wrong numbers. In addition, I would never guide that mount at .8x sidereal, that's asking for trouble and you are compounding the problem by using 1 second exposures. 

 

So, first step is to run the GA and use those settings. Be certain that you are using the multi-star feature as well and that the focal length of the guide camera is set correctly. It looks as if you are using an OAG based on the focal length that you have entered. 

 

Rgrds-Ross


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#4 KC_Astro_Mutt

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Posted 13 May 2022 - 06:48 PM

My RA is always worse than my DEC. If you have good polar alignment your DEC will hardly need to be  moved at all. That’s just my experience.

That may be the case with my CGX.  I've just always been told that it's best if they are close to even in error.

 

Either way, I am getting good guiding.



#5 KC_Astro_Mutt

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Posted 13 May 2022 - 06:53 PM

You use the guiding assistant to get the correct calibration steps. The log says that the steps were too large - not too small - but still you have the wrong numbers. In addition, I would never guide that mount at .8x sidereal, that's asking for trouble and you are compounding the problem by using 1 second exposures. 

 

So, first step is to run the GA and use those settings. Be certain that you are using the multi-star feature as well and that the focal length of the guide camera is set correctly. It looks as if you are using an OAG based on the focal length that you have entered. 

 

Rgrds-Ross

I ran guide assistant 4 times.  I approved all of its suggestions.  I do use multi-star guiding.

 

Aside from that all of my settings in PHD2 are defaults.  The guide rate is whatever PHD2 set it at out of the box.  What should the guide rate be, and where would I set that?  I'll try longer exposures next clear night.

 

Focal length and guide camera are good, yes, I'm using an OAG.

 

Thanks for your help Ross!

 

Jeff



#6 Jeffmar

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Posted 13 May 2022 - 07:19 PM

With my guiding I get a range for .35 to 1 total RMS. One night I was stressing over the 1 reading and a buddy I often go imaging with told me that I could still get good images at 1. He was right. A lot of guiding can be completely out of your control such as when seeing conditions are marginal or when wispy clouds are present. 


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#7 KC_Astro_Mutt

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Posted 13 May 2022 - 07:27 PM

With my guiding I get a range for .35 to 1 total RMS. One night I was stressing over the 1 reading and a buddy I often go imaging with told me that I could still get good images at 1. He was right. A lot of guiding can be completely out of your control such as when seeing conditions are marginal or when wispy clouds are present. 

For sure!  I got great test images.  I'm not really stressing.  It's more of a "why is it like that" thing.  If I get that same guiding consistently, I'll be a happy camper.  Bruce at the PHD2 forum suggested that the 34 second spike was bad, so I brought it up here as he suggested to check with "Celestron or someone on one of the other mount-specific forums... to find out what you can possibly do to mitigate it mechanically", so here I am.  I have a trouble ticket into Celestron.  I suppose I'll get the usual, "it's performing to spec, so it's ok" form response.


Edited by KC_Astro_Mutt, 13 May 2022 - 07:28 PM.


#8 rgsalinger

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Posted 13 May 2022 - 09:32 PM

The recommendation for your mount is .5x sidereal unless it's been changed by Celestron. My advice is to use that. I've never seen PHD GA recommend 1 second exposures either. That cadence is for use when you have a truly awful mount in the first place. Most of the guiding error is seeing, not real mechanical displacement, hence too many corrections won't give a better result and may make things worse IMHO. 

 

Here's the important point. Look at your exposures. I bet that they are just fine. The myth about guiding RMS axis differences comes from situations in which the numbers are much higher than what you are getting. That RMS number is less than 1/2 arc second in RA and 1/2 that in DEC. That gives you such a small error term that you shouldn't see it in the exposures themselves- it's just too small. 

 

When you get guding at .4 DEC and 1.2 RA, then you've got a big enough difference to cause elongated stars, .25 arc seconds as an error term is unlikely to be noticeable. I wouldn't worry about the 34 second .4 arc second spike but you might consider using PPEC or even "real" PEC on the mount to tame it at least a bit. 

 

Rgrds-Ross



#9 KC_Astro_Mutt

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Posted 14 May 2022 - 09:28 AM

The recommendation for your mount is .5x sidereal unless it's been changed by Celestron. My advice is to use that. I've never seen PHD GA recommend 1 second exposures either. That cadence is for use when you have a truly awful mount in the first place. Most of the guiding error is seeing, not real mechanical displacement, hence too many corrections won't give a better result and may make things worse IMHO.

Here's the important point. Look at your exposures. I bet that they are just fine. The myth about guiding RMS axis differences comes from situations in which the numbers are much higher than what you are getting. That RMS number is less than 1/2 arc second in RA and 1/2 that in DEC. That gives you such a small error term that you shouldn't see it in the exposures themselves- it's just too small.

When you get guding at .4 DEC and 1.2 RA, then you've got a big enough difference to cause elongated stars, .25 arc seconds as an error term is unlikely to be noticeable. I wouldn't worry about the 34 second .4 arc second spike but you might consider using PPEC or even "real" PEC on the mount to tame it at least a bit.

Rgrds-Ross


Thanks again Ross. Where would I find Celestron's recommendation on guide rate? It's not in the CGX manual that I could find. I'm currently connecting to the mount thru CPWI software. I assume it's technically the "driver", so perhaps that's where the guide rate settings are. I'll give it a look.

Yes, as I've stated earlier, my current guiding is great, and test images have been outstanding. I'm only here asking questions because the advice I got from the PHD2 guru was that this RA spike at 34 seconds would be a limiting factor to good guiding. I'm inclined to disagree with that, at least at the moment, as the error is sub arc second.

Jeff

#10 KC_Astro_Mutt

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Posted 14 May 2022 - 09:31 AM

So I just got this response from Celestron. I'm not sure I fully comprehend, short of just checking the belt tension.

"Thank you for contacting Celestron Technical Support.
The 34-second periodicity is the period between successive engagements of the teeth on the 14-tooth drive belt on the R.A. worm gear. The 14-tooth belt drives the R.A. gear one revolution every 8 minutes on the CGX. You can remove the cover on the R.A. drive motor by removing four small Philips screws and check the belt tension. If it is noticeably floppy, there are three Allen head screws that secure the adjustable stage and an Allen head jack screw to adjust the tension. It should not be rigidly tight. There should be a slight amount of springiness to the belt. Overtightening it will cause long-term damage.
Good luck!

Celestron Technical Support"

Edited by KC_Astro_Mutt, 14 May 2022 - 09:31 AM.


#11 rgsalinger

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Posted 14 May 2022 - 09:33 AM

If the .5 rate is not in the manual then ask Celestron. I ran my old CGE at that rate and every single other mount I've ever owned except for my AP mounts. AP recommends a faster guide rate for their mounts but that's because they are confident of the quality of the mechanics. 

 

I read that Celestron missive as meaning that if the 34 second spike concerns you, then you can check the belt. I'm surprised that they failed to opine on whether a spike of that magnitude could cause a fall off in image quality. 480/14=34!

 

As I said, though, the guiding looks good and if the subexposures are good then leave well enough alone. At the same time you might get even better results if you tried some form or periodic error control.

 

Rgrds-Ross



#12 KC_Astro_Mutt

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Posted 14 May 2022 - 11:16 AM

If the .5 rate is not in the manual then ask Celestron. I ran my old CGE at that rate and every single other mount I've ever owned except for my AP mounts. AP recommends a faster guide rate for their mounts but that's because they are confident of the quality of the mechanics. 

 

I read that Celestron missive as meaning that if the 34 second spike concerns you, then you can check the belt. I'm surprised that they failed to opine on whether a spike of that magnitude could cause a fall off in image quality. 480/14=34!

 

As I said, though, the guiding looks good and if the subexposures are good then leave well enough alone. At the same time you might get even better results if you tried some form or periodic error control.

 

Rgrds-Ross

I will apply the 0.5 guiding rate next time out, and see how it goes, along with the PPEC RA algorithm in PHD2, and set for longer exposures.

 

Thank you again!

Jeff
 



#13 KC_Astro_Mutt

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Posted 14 May 2022 - 02:19 PM

After much back and forth with Bruce at PHD2, I've re-run the New Profile Wizard.

 

The only thing that changed was the calibration steps.  Went from 700 ms to 350 ms.  PHD2 did start with 0.5 guide speed, but Bruce doubted this.  I've set it back to 0.8 for now.  I also have a question in to Celestron Support to clarify this.

 

I'll be running RA in PPEC to see if that smooths my RA out.  Either way, I've got great guiding already, and hopefully get lots of great photos.


Edited by KC_Astro_Mutt, 14 May 2022 - 02:22 PM.



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