Off by 2 arc minutes? Help?

Using SharpCap's Polar Alignment feature
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jeffreyhorne
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Off by 2 arc minutes? Help?

Post by jeffreyhorne » Thu Mar 26, 2020 7:36 pm

I’ve been using Sharpcap’s polar alignment routine for about a year now, and I really love how easy it is! I can always get both up/down & left/right to within under 10 arc seconds (often under 5!) in just a few minutes.

My guiding has never been perfect, though, usually struggling the most with RA, with about 1.7 total error. (EQ6r-pro)

I’ve tried using the atmospheric correction setting, and tried not using it. I’m using a 50mm Orion guidescope with an Orion SSAG pro (basically an asi120mm).

Last night I let PHD2 guiding assistant run for a while, and it said I was off by a little over 2 arc minutes. The funny thing is, I got the best guiding I’d ever had, with a total error of about 1.4.

This makes me think that my polar alignment is usually off by more than 2 arc minutes, and I’m wondering why. Does anybody know of anything glaringly obvious that I’m not doing? Should I run the routine 2 or 3 times in a row, for more accuracy?

I’ve read the instructions a lot of times, and I’m pretty sure I’m doing everything how I’m supposed to...I follow the on-screen instructions and make sure my last frame is solved before proceeding to next steps.)

Thanks in advance!

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admin
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Re: Off by 2 arc minutes? Help?

Post by admin » Thu Mar 26, 2020 8:01 pm

Hi,

the most likely cause of a significant error is flexing of the guide scope/camera mounting as you rotate the mount. It doesn't have to flex by very much to give a noticeable impact – in fact even a dangling USB cable can pull on the camera enough to give a noticeable error if the cameras and firmly mounted. Guidance on checking for this and other common problems can be found here : https://www.sharpcap.co.uk/sharpcap/fea ... leshooting

Hope this helps, Robin

jeffreyhorne
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Re: Off by 2 arc minutes? Help?

Post by jeffreyhorne » Thu Mar 26, 2020 9:26 pm

Thanks, Robin!

I have my cable attached to the side of the scope, at a good distance so there isn’t any movement of the cable when I’m loving the scope. I’ve also checked all of the fittings quite a few times, to make sure that nothing on the telescope or guidescope was loose.

I’m going to try starting at about 35° on one side, and moving to 35° on the other, and see if it makes a difference. :D

jeffreyhorne
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Re: Off by 2 arc minutes? Help?

Post by jeffreyhorne » Fri Mar 27, 2020 1:06 am

My imaging scope has a FoV of 3.5° x 2.3°... I guess I could try polar alignment with my imaging scope?

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oopfan
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Re: Off by 2 arc minutes? Help?

Post by oopfan » Fri Mar 27, 2020 4:35 am

Hi Jeffrey,

Do you have something like this:

https://www.telescope.com/Orion-Mini-50 ... /99607.uts

Me too.

It's got three spring-loaded adjustment screws that unfortunately shift a bit as the scope tracks a target. I've witnessed 2 arc-minutes of offset during the course of a 3-hour session that gets progressively worse the closer I get to the meridian. The culprit are those adjustment screws. I fixed the problem by taking JB-Weld Steel Stick and wedging it in and around those screws where they make contact with the guide scope. Essentially I locked it in. Guess what? I no longer have a problem!

Brian

jeffreyhorne
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Re: Off by 2 arc minutes? Help?

Post by jeffreyhorne » Fri Mar 27, 2020 12:57 pm

oopfan wrote:
Fri Mar 27, 2020 4:35 am
Hi Jeffrey,

Do you have something like this:

https://www.telescope.com/Orion-Mini-50 ... /99607.uts

Me too.

It's got three spring-loaded adjustment screws that unfortunately shift a bit as the scope tracks a target. I've witnessed 2 arc-minutes of offset during the course of a 3-hour session that gets progressively worse the closer I get to the meridian. The culprit are those adjustment screws. I fixed the problem by taking JB-Weld Steel Stick and wedging it in and around those screws where they make contact with the guide scope. Essentially I locked it in. Guess what? I no longer have a problem!

Brian
That’s the exact guidescope! Thank you so much, Brian. So helpful! I’m going to try just using my imaging scope first, and then I’m going to try your solution. Maybe both together at some point, since the issue you mentioned probably affects guiding even after polar alignment is nailed.

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Re: Off by 2 arc minutes? Help?

Post by oopfan » Fri Mar 27, 2020 1:35 pm

Jeffrey,

There is another weak point in their design: the single shoe that mates to the main scope. To test, grab the stem and give it a firm twist to see if you feel any movement. If you do then loosen the thumbscrew and slide the scope out of the shoe. Take a screwdriver or Allen Wrench and make sure the screws are tight. (If you find that they loosen over time then apply some Loctite Thread Lock.) Next, slide the scope back in the shoe and make sure you give the thumbscrew a good tightening without stripping the threads.

Brian

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