Different polar align results

Using SharpCap's Polar Alignment feature
Astro.steve
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Joined: Sat Oct 21, 2017 10:26 am

Different polar align results

Post by Astro.steve » Sat Oct 21, 2017 10:52 am

Hi I have started using polar align feature which I love by the way, easy to use. But I have a strange problem with it. When I rotate the ra 90 degrees one way i can get exellent polar align reading, but is then different when I rotate ra 90 degrees the other direction meaning it's only a fair polar align on the other side.iv managed to equal both sides of the ra out to get a good polar align ready but can't get perfect polar align. Could this be down to cone error or an out of level mount, not sure I trust my bubble level insert on my mount.

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admin
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Re: Different polar align results

Post by admin » Sat Oct 21, 2017 11:15 am

Hi Steve,

the times I have heard of this sort of problem before it's been down to flexure of the guide scope as the mount is rotated - ie the guide scope moves a bit at some point while it is being rotated. This actually means that both polar alignment measurements are going to be a bit out.

What I'd suggest is checking that everything is tight and for snagging or pulling cables and see if that helps. Also you can set a high gain and long exposure and rotate the mount by hand and check that the star trails you see are perfect arcs with no breaks or jumps in them or kinks in them.

If none of that helps then please let me know - we will then capture frames in each rotation position from both attempts and screenshots and I will look into the problem further.

cheers,

Robin

Astro.steve
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Re: Different polar align results

Post by Astro.steve » Sat Oct 21, 2017 6:37 pm

Thanks Robin I will try next time I'm out, will let u know how it goes.
Thanks again
Steve

david_taurus83
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Re: Different polar align results

Post by david_taurus83 » Sun Feb 04, 2018 11:36 am

Hello all. New to the forum. Thought I'd ask in this thread as I have exactly the same problem. I have a 200p on an EQ5 mount, have an extra dovetail bolted onto the top of the 200p rings and a ZWO finder guider bolted solidly to this. Camera is the ZWO ASI120. Sharpcap works fine. It plate solves in the home position fine, I rotated East 90° and it plate solves again. I then adjust as per on screen instructions and get the result down to excellent. When I repeat the procedure to check though, I rotated West for the second step and the error is fair with a result of around 6 minutes. If I correct this and repeat the check back to the East side the error is again mirrored at around 6 minutes! Could this be caused by cone error in both the main scope and also by the ZWO scope?

I'm sure the mount is level. I have spent time getting the tripod head level as the bubble on the mount is questionable.

I'm sure the ZWO has no flexure as it's bolted solidly the the dovetail and all cables are fixed around the main scope dovetail by the USB hub so this is the only point at where the cables need to move and they can't snag.

I intend to use Conesharp to correct any errors in the main scope alignment but I'm stumped on how to address the guide scope. Would I attempt to adjust this by adjusting the guide scope dovetail or using the adjustment screws in the guide scope rings? Has anyone else attempted to fix cone error in their guide scope?

Thanks

David

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Re: Different polar align results

Post by admin » Sun Feb 04, 2018 2:05 pm

Hi David,

The two main causes for this sort of problem (or at least the two I've discovered so far) are flexure/movement and adjusting before you press the 'Next' button to advance to the final stage.

Flexure/Movement:

It's worth checking the position of your camera in the guidescope and whether anything could be causing the cable to pull - even with a firmly attached guidescope a small amount of tension in a cable can pull a camera out of alignment a bit.

The two good tests for this are

1) Use SharpCap 3.1 and rotate gradually 10 degrees at a time with a wait after each step. In 3.1, it draws extra center of rotation markers in deep red showing where the center of rotation was measued at each stage. These should be closely grouped with the final center of rotation marker (bright red). If they are not then you very likely have movement/flex issues somewhere.

2) Get polaris in view, high gain, long exposures, swing the telescope around the RA axis and check that polaris makes a nice smooth arc of a circle. Any distortions or kinks in the trace and again you have flex problems

For the adjustment - just don't adjust until you get to the final stage where the arrow shows. In SharpCap 3.1 I have changed it to deliberately not show the up/down/left/right adjustments until you get to this final stage as adjusting too soon was a common cause of error.

If those suggestions fail then please do the following:

Run the PA twice as you have been, once in each direction.
For each run:
* Save a snapshot of the image used for the first position (press the snapshot button in SharpCap)
* Save a snapshot of the image used for the second position (snapshot button) *and* a screenshot of the whole sharpcap window in the 2nd position
* Save a snapshot image *and* a screenshot after you have finished adjusting

Send those images to me and I will investigate further.


cheers,

Robin

david_taurus83
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Re: Different polar align results

Post by david_taurus83 » Sun Feb 04, 2018 4:05 pm

Hi Robin. Thanks for the reply. I will try again on the next available chance. Have read elsewhere that perhaps releasing the clutch to rotate may cause the mount to move a small bit. Next time I think I will rotate using the hand controller. Will update the thread with results.

Thanks

David

Dirkastro
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Re: Different polar align results

Post by Dirkastro » Sun Jun 10, 2018 8:06 am

I too have this on my mount..its not flexture as I use a polemaster camera and it's fixed to the mount..i assume it to be cone error..

Oshimitsu
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Re: Different polar align results

Post by Oshimitsu » Sun Jun 24, 2018 10:38 am

I have a question. Why is it I can have zeros across the board in sharpcap but when I run the guide assistant in PHD2 the pa error will jump around like crazy? I usually only run it for about 2 minutes but in that time frame it can go from less than 1 arc minute to over 10?

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Re: Different polar align results

Post by admin » Sun Jun 24, 2018 8:41 pm

Hi,

I can only re-iterate that the only times we have tracked down a cause for problems like this, it has turned out to be mechanical (ie something shifts as the mount rotates or the mount itself does not rotate smoothly about a single axis). Cone error should not affect the accuracy of the result as long as the cone error is less than a few degrees.

SharpCap 3.1 helps you check for mechanical problems - please work through the troubleshooting steps on this page - https://www.sharpcap.co.uk/sharpcap/fea ... leshooting - particularly the rotation of the mount in steps of ~15 degrees to see if the calculated RA position remains constant during rotation. If it changes it's highly likely that something is shifting. The page has a pair of screenshots showing the effect of a loose cable pulling on a camera when using the polar alignment compared to a secured cable.

cheers,

Robin

Oshimitsu
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Re: Different polar align results

Post by Oshimitsu » Mon Jun 25, 2018 3:45 am

I'm definitely going to make sure to secure my camera usb next time out. My question really more about why does the pa error in PhD fluctuate during the guide assistant? My Dec will be basically flat the entire 2 minutes while I'm running the assistant, my ra will slowly rise. Is it PE or what?

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