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Help with guiding and Goto++ on the Celestron AVX.

Astrophotography Celestron Imaging Mount
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#1 ArandomPilot

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Posted 10 May 2021 - 11:57 AM

Hi all, 

With galaxy season coming to a close and a week of clear skies coming up, I want to solve some of the issues with my mount to get one great image of a Galaxy before the season ends. I am running a Celestron AVX with an Apertura 6" F/4 Newt (9.4lbs) with coma corrector, ZWO 120MM mini and Orion 50mm Guidescope, and the imaging camera is a ZWO 1600MM with EFW mini. I polar align with a Polemaster. My telescope is around 15-17lbs (haven't weighed it yet). I've run a heavier set up before at around 18lbs using a ST-2000XM on the same set up and had better results than this night.

 

This night where I captured my first galaxy, I spent about 2 hours on NGC 3359 (small, IK, but there were so many other problems I just decided to image that since I found it instead of going to M101) but only 45 minutes of it was usable. Here is that image:

NGC 3359

 

 

Problems arose when the about 30 minutes in, the telescope was no longer aimed strait up but was now aimed more west. I went to discord and the people there said backlash issue. Somebody suggested I switch the guiding from North to South and I did and It worked for a about 3 subs before the same thing happened again. Others suggested I off-center my polar alignment by about 2 degrees to compensate for backlash. I really don't want to take the mount appart since I've run the a different set up that's gone further west with no issues, but that was with the 1600mm, 120mm, 50mm guide scope and SpaceCat 51. I'm really not sure what to do there since balance wasn't an issue. 

Guiding issues on NGC 3359
 
The second issue came much earlier in the night and isn't as important as guiding. I was able to platesolve for the first time using NINA but my telescope wouldn't slew there. There were two errors that popped up:
1. My goto platesolve ASTAP constantly failed, I have the G18 star data base uploaded and the correct camera and focal length. For whatever reason it constantly fails. 
2. The mount itself said it needed a star align to have plate solving work. Is this nessisary? The whole point of plate solving is to save time on alignment so I can begin imaging ASAP. Any suggestions help. 
 
Thanks,
Gabe.

 



#2 Forward Scatter

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Posted 10 May 2021 - 01:01 PM

Gabe, 

It looks like classic stiction on the dec axis. It happens occasionally with my AVX. 

 

The PHD2 site has a graph analysis write up and diagnostics pdf here. Look on page 12 and your dec guiding plot looks identical.

 

Try:

  • using lower aggressiveness settings or even guide in only one Dec direction.
  • Balancing the scope perfectly. Try an ammeter to balance the mount by measuring dec slew current load.
  • The dec axis of the AVX is famously stiff as it uses a nylon gasket as a bearing surface. I've replaced it with a teflon gasket and Superlube. On can also back off on the axis shaft tightness a little bit. RocketSparrow has a great write up on this.
  • Adjust the dec motor gear backlash; sometimes this gets too tight or loose. There are several YouTube videos for this.
  • I also sometimes adjust the tension of the screws on the dec motor worm gear mount where it meshes the worm gear to the dec drive shaft.

 

Cheers!


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#3 TelescopeGreg

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Posted 10 May 2021 - 01:34 PM

Hi Gabe,

 

Working in reverse order:  #2 doesn't make sense.  "The mount said", part in particular; where did it say this?  You're right, a big advantage of plate solving is not needing to star align the mount; I haven't done one in about a year.  Question:  How did you initialize the mount?  What I do is to use "Quick align" to get a default (and consistent) sky model, then start the goto / plate solve / sync / re-goto sequence.  I'm not using NINA, but I expect most master sequence programs are similar to this.

 

You can try running ASTAP by itself, loading the FITS image into it and hitting "Solve".  If it works there, then look at how NINA is configured to pass images to ASTAP.

 

I've never had a guiding situation such as what you experienced.  Jay may be on to something with the stickiness of the AVX bearings, but that ramp looks to me like your polar alignment is far enough off that PHD2's correction pulses aren't sufficient to correct for the drift, or you're bouncing back and forth across a large backlash zone.  How are you polar aligning the mount?  I'm using the Celestron Polar Alignment scope. Sometimes the best strategy with the AVX is to get a good polar alignment so that the Dec motor "never" has to run.  It takes careful setup, and some practice getting it right, especially setting the RA properly.  I find it's best to cross-check with a smartphone app.  After that, try using the Guiding Assistant in PHD2 for recommended settings.

 

Good luck!


Edited by TelescopeGreg, 10 May 2021 - 01:38 PM.


#4 Forward Scatter

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Posted 10 May 2021 - 02:45 PM

Greg has a very good point about PA, and I use Shapcap's PA routine as IMO it's about the best one out there, and easy to use. I run it twice to dial in the best PA. 



#5 ArandomPilot

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Posted 10 May 2021 - 03:06 PM

"The mount said", part in particular; where did it say this?

Question:  How did you initialize the mount?  What I do is to use "Quick align" to get a default (and consistent) sky model, then start the goto / plate solve / sync / re-goto sequence.  I'm not using NINA, but I expect most master sequence programs are similar to this.

 

You can try running ASTAP by itself, loading the FITS image into it and hitting "Solve".  If it works there, then look at how NINA is configured to pass images to ASTAP.

 

How are you polar aligning the mount?  I'm using the Celestron Polar Alignment scope. Sometimes the best strategy with the AVX is to get a good polar alignment so that the Dec motor "never" has to run.  It takes careful setup, and some practice getting it right, especially setting the RA properly.  I find it's best to cross-check with a smartphone app.  After that, try using the Guiding Assistant in PHD2 for recommended settings.

 

Good luck!

To answer the first question, the error that the star alignment needed to be complete was on NINA. For my typical alignment, I would normally do a 2 star align, but in this instance, maybe I only partially completed it since I input the time, but never selected to star align. I control the mount via hand control, which is hooked up to my computer. 

 

ASTAP, the plate solve has only worked twice for me when I input an image:

With my SBIG ST-2000XM, it plate solved every image fine, with my ZWO 1600mm, it plate solved only 1 night of subs before errors popped, up. Note that was with the same set up, nothing had changed in focal length, but something changed in the images from the second night onward that makes every image it tries to plate solve very pixilated, including these subs from a few nights ago. In my other software, APT, ASTAP was stuck on 4x4 bin, nothing could pull it out. Even though I use NINA now, it still seems to be stuck on a 4x4 bin, even though NINA is forcing ASTAP to run a 2x2 bin. 

 

As for polar alignment, I use the QHY Polemaster since I can get very accurate results with it. My mount only shifts around by about a few feet, so I'm not concerned with redoing every step, but maybe I have to? 



#6 TelescopeGreg

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Posted 10 May 2021 - 04:07 PM

To answer the first question, the error that the star alignment needed to be complete was on NINA. For my typical alignment, I would normally do a 2 star align, but in this instance, maybe I only partially completed it since I input the time, but never selected to star align. I control the mount via hand control, which is hooked up to my computer. 

 

ASTAP, the plate solve has only worked twice for me when I input an image:

With my SBIG ST-2000XM, it plate solved every image fine, with my ZWO 1600mm, it plate solved only 1 night of subs before errors popped, up. Note that was with the same set up, nothing had changed in focal length, but something changed in the images from the second night onward that makes every image it tries to plate solve very pixilated, including these subs from a few nights ago. In my other software, APT, ASTAP was stuck on 4x4 bin, nothing could pull it out. Even though I use NINA now, it still seems to be stuck on a 4x4 bin, even though NINA is forcing ASTAP to run a 2x2 bin. 

 

As for polar alignment, I use the QHY Polemaster since I can get very accurate results with it. My mount only shifts around by about a few feet, so I'm not concerned with redoing every step, but maybe I have to? 

Yeah, you probably didn't finish the mount start-up sequence.  It's muscle-memory for me now; power-on, enter 3 times, up twice, and enter again.  That last part is where the "two star alignment" is changed to "quick align".  I don't use "last alignment" because I know it's going to be wrong, and prefer a known wrong alignment (default sky map) to an unknown wrong one.

 

So, if the polar alignment is good, then it probably comes down to finding a balance point where the mount (your mount) behaves itself.  Because the bearings (such as they are) are pretty stiff, this can be harder than it looks.  Every mount has its own personality, so try moving things around a bit and see what happens.  I hit upon a "magic spot" a year ago, where guiding started working really well, and marked it with tape.

 

Also, stay away from targets that are high overhead until you get a better feel for what your copy of the mount can do.  I had some of the worst guiding in a year last week when imaging M106.  The mount was just not happy with that one, as it was +/- 30 minutes from the meridian crossing and 80+ degrees in elevation.

 

Oh, and as for ASTAP, a 4x4 bin is huge.  There are settings in both ASTAP and your session manager, so check both for pixel size and telescope focal length.
 


Edited by TelescopeGreg, 10 May 2021 - 04:08 PM.



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