Different polar align result on the opposite side of the mount

Using SharpCap's Polar Alignment feature
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Rab722
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Different polar align result on the opposite side of the mount

Post by Rab722 » Mon Jun 29, 2020 3:32 pm

I have a Celestron 8se and ZWO 533 on a newer Losmandy G11. I have been using the Sharpcap polar align routine. I can go through the steps and get an excellent result but if I then swing my RA axis 180 degrees to the other side of the mount the result is reported as just fair or poor. I do suspect my polar align is off because Sharpcap is showing a drift when stacking the frames.

I would expect a close result on both sides of the mount. I suspect my DEC axis is not perfectly aligned with the mount axis. If this is the issue maybe polar align routine needs an intermediate step to get the DEC axis collinear with the RA axis.

Rick

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Re: Different polar align result on the opposite side of the mount

Post by admin » Mon Jun 29, 2020 7:44 pm

Hi Rick,

the declination axis shouldn't come into it as you don't change the declination during the alignment procedure. The alignment between the guide scope and the RA axis is also not critical (you don't want to be five or 10° off, but a degree or two of misalignment will have no effect on the results).

The most likely cause of different results in the situation you describe is something shifting as the mount is rotated – most likely the guide scope or guide camera as the weight distribution on it changes. In particular a dangling cable can be quite enough to cause this sort of problem if there is any potential flexibility in the mounting of the guide scope.

If you rotate very slowly during the procedure stopping after every 15° or so, you should see a number of dark red crosses drawn on the screen. If everything is fixed perfectly then those crosses should all be in the same place and should be in the same place as the final brighter red cross that shows the position of the RA axis. If anything is shifting as you rotate then you will see these crosses being in different locations on screen – if that's the case then tighten everything up and restrain all cables to see if that helps.

Cheers, Robin

Rab722
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Joined: Mon Mar 02, 2020 12:20 pm

Re: Different polar align result on the opposite side of the mount

Post by Rab722 » Mon Jun 29, 2020 8:39 pm

I am using the main C8 with Hyperstar to polar align so flexure probably isnt a problem but perhaps main mirror movement could cause a shift. I asked this on Cloudy Nights and some people only go to 45 degrees on either side for the second movement and then go 45 on the other side to check. I will try that next.

Rick

Rab722
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Joined: Mon Mar 02, 2020 12:20 pm

Re: Different polar align result on the opposite side of the mount

Post by Rab722 » Tue Jun 30, 2020 12:12 pm

I had better results last night. On the Losmandy mount I was releasing the RA axis and swinging it manually and then locking it down. I noticed the reading could change depending on how tightly I turned the clutch knob. When I instead tightened the clutch to start and used the motors to slew to +/- 45 degrees the results were much closer.


Rick

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Re: Different polar align result on the opposite side of the mount

Post by admin » Tue Jun 30, 2020 8:05 pm

Hi,

yes the 45° to either side is a good trick as it gets you the 90° of rotation without changing the loading quite so much. In fact you can get away with less than 90° rotation – 50 to 60° would properly do and some people are working with 20 to 30°.

Cheers, Robin

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