Quirky Guiding or a basic error on my part?

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John Deans

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Jun 24, 2021, 8:13:58 PMJun 24
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The linked-to log file is representative of what I have seen over time.  Any insights as to what might underlie inconsistence over a session would be appreciated.

I'm using a 72mm refractor on a wedge-mounted Celestron 8SE mount...a light rig on a low precision mount.  I can have many minutes of very good guiding and good AP results, after which things just seem to go "sideways".  Sometimes stopping guiding and restarting guiding resolves guiding for several more minutes, but not reliably.  Known limitations: I may not be achieving ideal PA, but I think it is close and should be manageable by guiding.  Why might guiding appear to start off very well, only to fail rather suddenly.

Also, I seem to get better guiding near the celestial equator and nearer the meridian, making me wonder if this is a lot about mount balance?

Thoughts are welcome.  Perhaps the logs will show to the observant something fairly obvious.  I don't see it.

Regards, John

https://openphdguiding.org/logs/dl/PHD2_logs_zEDX.zip

Bruce Waddington

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Jun 24, 2021, 11:15:47 PMJun 24
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Hi John.  I think you have a number of mechanical and operational problems that are behind your difficulties.  As you probably know, your limiting condition now is the poor response of your Dec axis.  I think you’re probably on the right track by questioning the balance of the OTA – it may be quite poor.  Here is a particularly bad guiding sequence when you were pointing at Dec=36 degrees:

 

 

 

The red arrows below the x-axis show that the mount is really not responding at all to the Dec guide commands.  This could be caused by a couple of things:

1.       A major imbalance of the OTA in declination that makes it nearly impossible to move the scope in the required direction

2.      An external impediment like a binding or pulling cable that has the same effect

There’s no way for us to know the difference, it’s something you will need to track down.  Balancing a scope on a fork mount can be a challenging process.  If you loosen both the azimuth and altitude clutches, you should be able to point the scope in any sky position without having the OTA then move on its own.  If you can’t do that, you are going to have guiding problems. 

 

In contrast, here’s a guiding sequence that occurred right after the first one – but at a Dec of -18 degrees:

 

 

This has a very different appearance.  The mount is still slow to react to guide commands but mostly when the direction of the corrections has changed.  It isn’t nearly as unresponsive as the first graph.  This looks more like the typical Dec backlash found in these mounts.  The Guiding Assistant will measure that for you – have you done that?  The big difference between these two sessions is what makes me thing you’ve got a large imbalance problem or cables that are interfering with the guiding assembly.

 

The upper arrow in the first graph shows an event where the OTA spontaneously moved by about 7 arc-sec.  This wasn’t caused by guiding or by the mount drive system, it was caused by something external to the mount.  Judging from your image scale, I’m guessing you’re using one of the flimsy finder-scope arrangements as a guide scope.  These are notorious for not providing the rigidity necessary for good guiding and there are many places that can become slightly loose and allow the guide scope/camera to shift by a small amount.  Or, as mentioned earlier, you might have a cable tugging on the guide camera.  The amount of movement is tiny – probably less than 10 microns or 20% of the thickness of a human hair.  This is one of the many problems with these types of guide scope arrangements –a perfect storm of flimsy mechanics and very tiny pixels in the guide camera.  So a 1-pixel movement of the guide camera on your setup translates into a 6 arc-sec glitch.  Finding and eliminating these problems is going to take some work, not something that you can do while imaging or just by hoping that things get better.  But you can probably explore the balance problems during the day, especially if the current balance is way off the mark.

 

Finally, the Guiding Assistant keeps warning you to get a better focus on your guide camera – and I think you should, it’s probably pretty bad.  Your typical star sizes are over 4 px in half-flux-diameter but I think with your setup you should be well below that, probably well below 3 px.  You need to use a measurement tool to reach good focus – a Bahtinov mask, the PHD2 Star Profile tool, or an app like SharpCap.  You can’t just look at the star image on the display and get a critical focus.  While you’re tracking all this down, you *must* stop fooling around with the guiding parameters – these aren’t guiding problems.  So restore all those settings to their default values and try to work through the mechanical issues.  On a positive note, you can quit worrying about polar alignment – yours was plenty good enough and isn’t the cause of any of these problems.

 

Good luck,

Bruce

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John Deans

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Jun 25, 2021, 6:36:22 PMJun 25
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Bruce, this is incredibly valuable feedback and insights.  I have my list and will tackle this, since actually, your suggestions in themselves are straight forward to tackle.  Yes, lots of careful work, but still straight forward.  The only part I don't quite get is where the log shows a lot of tweaking.  I recall only using the guiding assistant and following the suggestions.  

I will report back.

Bruce Waddington

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Jun 25, 2021, 8:48:03 PMJun 25
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Hi John.  This was what provoked my comments about fiddling around with the guiding parameters:

 

 

These aren’t big deals but I try to discourage this sort of thing when the user is just getting going and should be concentrating on equipment shortcomings.  If the gear is working correctly and the new-profile-wizard has been run, the default guiding parameters in PHD2 will produce generally good results.  At that point, changes should be made only with a clear understanding of the why and what and even then, only carefully.  When the changes are being made blindly and in desperation, they usually just make things worse.

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John Deans

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Sep 14, 2021, 6:35:34 PMSep 14
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Perhaps a basic question, but one that I am asking.  Your guidance is appreciated!

I have had much better guiding since reconfiguring my mount so that nothing is located too far away from the mount itself.  (In order to attach a small refractor to the SE mount, I was using an adapter that likely resulted in a lot of strain on the DEC moving parts.)

So, using an SE mount - or any other mount for that matter - should it be necessary to recalibrate any time I move the rig to point to a new target that appreciably changes the load balance , on the RA access in particular?  Just looking at how the even very modest weight of a 72mm refractor + DSLR + StarSense accessory + guide scope and camera (which are way under the quoted mount limits) places the load, suggests to me that this is required.  My guiding results seem to suggest this too.

I appreciate your comments.

Brian Valente

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Sep 14, 2021, 6:57:12 PMSep 14
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Hi John

it's hard to respond meaningfully to a question without some data. If you have a few pictures you could post of your setup, that would probably help clarify

Generally speaking :

- you should not need to recalibrate unless you are using ST-4 style connection (which you really shouldn't be anyways - use pulse guiding via ascom/indi instead). this is totally independent of your telescope balancing
- if you are finding your setup is differently balanced at different sky locations, then i would say your scope isn't properly balanced, or you have things that are shifting about
- I think there's some misconception that lighter telescopes are somehow less prone to balance or tracking issues. In fact, too little weight on a mount can be an issue as well. I don't know enough about your setup and your mount load capacity to comment much further on it. 





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Brian 



Brian Valente
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