Different results depending on direction of rotation of the mount

Using SharpCap's Polar Alignment feature
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Vaujean
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Joined: Sat May 23, 2020 6:47 am

Different results depending on direction of rotation of the mount

Post by Vaujean » Sat May 23, 2020 7:15 am

I have three questions:
1) I realized that if in stage 2 I turned my mount 90 ° I obtained a very different result depending on the direction of rotation of my mount (Célestron CGEPro):
1°10'49 "in one direction (rotation of + 6h = + 90 °) and 1'30" in the other (rotation of -6h = -90 °). These results are an average after several measurements which are well reproducible.
Thank you for telling me what can be the cause of this very large gap.
2) I use on my frame a C14 and an ED80 guide scope which are practically aligned at a few minutes angle. Polar Align works better (larger field) when using the guide scope. Can we use the guide telescope or the C14 interchangeably?
3) By using the polar alignment function of "Prism 10" (King's method) I get significantly different results (when SharpCap gives 1'30 ", Prism gives 7'12" - average values ​​after several measurements). Who to believe? Sharpcap or Prism?

Thank you very much in advance for your answers and congratulations on your software.
Setup: C14 with focal reducer (f = 2635 mm) + ASI 1600mm (0.9 "/ pixel in bin3) and guide ED80 (f = 480mm) + ASI 178mm (2" / pixel)
Mount: CGEPro controlled with CPWI or Maxpoint

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admin
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Re: Different results depending on direction of rotation of the mount

Post by admin » Sat May 23, 2020 7:49 pm

Hi,

Have you had a look at this page which gives some pointers on what might be causing this sort of error and what to do to correct it?

https://www.sharpcap.co.uk/sharpcap/fea ... leshooting

The most common cause of the sort of problem you are seeing is if something is shifting as the telescope is rotated. Something as minor as a dangling USB cable can cause this sort of issue if your camera and guide scope are not firmly attached.

Cheers, Robin

Vaujean
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Re: Different results depending on direction of rotation of the mount

Post by Vaujean » Sun May 24, 2020 7:05 am

Thank you for your reply. But unfortunately it does not help me much! I would have liked specific answers to the three questions:
1) does the direction of rotation between step 1 and step 2 matter?
2) can the polar alignment be done indifferently with the guide telescope or the telescope?
3) Have you compared the results of your method with those given by other methods (King or Bigourdan)?
It would be very nice of you if you took a little time to answer my questions specifically. I will read the page of the link sent carefully and I will start my measurements again.
Yesterday I used polar alignment with CPWI (software mount Celestron) and SharpCap gave me 40 '!! It is quite disheartening .
Have a good day.
Sincerely.

chongo228
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Re: Different results depending on direction of rotation of the mount

Post by chongo228 » Sun May 24, 2020 2:38 pm

From what I know you can use the guide scope. Many DSLR users polar align with the guide scope.

Reading that you have a C14 got me thinking. I will be taking my SCT off the forks later this year to put on my Atlas Pro. Could mirror flop change the results? It would explain why you get different results when rotating different ways.

Have you tried comparing the guide scope results to the C14 results?

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oopfan
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Re: Different results depending on direction of rotation of the mount

Post by oopfan » Sun May 24, 2020 2:50 pm

"Who to believe? Sharpcap or Prism?"
I would believe the one that results in the least declination drift.

You sound like a seasoned imager so my recent experience will not pertain to you but may help others:

For several months I have set my tripod in the exact same spot, perhaps off by a millimeter or two. Increasingly I would experience severe declination drift. The steps I used with SharpCap Polar Alignment never deviated but when I was finished and then slewed to the first object, I apparently lost PA. After having used SharpCap's PA for several years I've learned to trust it. The next session I decided to plant the tripod in a different spot. The improvement in tracking was amazing! Only a few arc-seconds of declination drift in 3 hours. I had a theory which led me to try the new spot: I theorized that the azimuth locking bolts made a minuscule depression in the metal surfaces that the mount wanted to return to whenever I started to "man handle" the telescope. The depressions became deeper the more I planted the tripod in the same spot. After moving to the new spot the azimuth locking bolts were given a fresh metal surface to grab onto. What an amazing difference!

Brian

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Re: Different results depending on direction of rotation of the mount

Post by admin » Sun May 24, 2020 8:58 pm

Hi,

Sorry not to give specific answers – the direction of rotation doesn't matter, nor does the use of the guide scope or the main scope (except in terms of needing a suitable field of view). It also doesn't matter if the scope is not perfectly aligned with the RA axis.

Testing against drift alignment results gives good agreement, and the results (round stars, minimal drift when guiding) speak for themselves.

cheers,

Robin

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Re: Different results depending on direction of rotation of the mount

Post by umasscrew39 » Thu May 28, 2020 8:58 pm

Vaujean

I have a C11" SCT and mirror shift and mirror flop are inherent in its design. Some SCTs are worse than others for various reasons. Your C14" could have these effects even worse given the larger primary mirror vs. the C11". I am not saying one of these is absolutely the cause of your varying results but they can contribute to it inanition to the other reasons Robin has mentioned. Flexure on a large SCT is very common as well, especially using a guide scope or a piggybacked smaller refractor which is not recommended for guiding, polar alignment, etc. Also make sure your mount is functioning properly. So, to address your specific issue, I have tested my polar alignment in my permanent observatory with my C11" EdgeHD on an AP1100 mount using SharpCap, QHY Polemaster, and classic drift alignment (still considered the gold standard). The results were almost identical. I did my final PA using SharpCap and have been imaging with no issues for months. Thus, I think SharpCap is producing a good, reproducible PA.

Regards,
Bruce

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