Polar alignment and field distortion

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Polar alignment and field distortion

Post by mxpwr » Wed Oct 17, 2018 7:01 am

I have been using polar alignment with sharpcap for a while now.
I use a 50mm F1.8 lens with a 5mp sony IMX326, FOV is around 4degrees and the whole procedure runs usually very smooth.
While it seems to get PA pretty close (below 1minute on both axis), I noticed some strange things with sharpcap's PA and was wondering if errors might come from field distortion in the lens used.

After rotating by 90degrees, i align both axis and then I do a sanity check by rotating forth and back 180. What I see is that often the calculated PA error jumps (by 1 to 2 minutes) when a different star is picked or when the star has moved enough in the image.
Since everything is rock solid on the setup, I would assume that flexing is not the main suspect here, but maybe field distortion.

Would it make sense to add support for distortion polynomials to try to reduce this effect?

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Re: Polar alignment and field distortion

Post by ackram2506 » Mon Jan 28, 2019 8:41 pm


I think I might be seeing the same thing. Recently after performing PA I find I have relatively poor alignment. I tested by doing the PA rotation on the other side of the mount and I generally get fair to poor. Now a friend tells me he's also seeing same thing. Did you resolve this? I'm switching to PolarMaster now.


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Re: Polar alignment and field distortion

Post by admin » Tue Jan 29, 2019 7:55 am


I had a good think about whether distortion could affect the polar alignment results and the conclusion I came to was that the distortion caused by treating the area of the sky around the celestial pole has a flat plane is minimal over the sort of distances from the pole that SharpCap works (less than one part in 10,000 out to 5° from the pole, and much smaller than that even closer to the pole).

However, distortion from short focal length lenses could be an issue as this can sometimes rise to 1% or more. As far as I can tell there's no way to correct for this without knowing in advance the distortion characteristics of the lens (i.e. barrel or pincushion and how much). The correct approach therefore when you suspect this sort of issue is to use an ROI setting in SharpCap to only capture the centre of the image frame and use that the polar alignment meaning that the edges where the vast majority of distortion occurs will be eliminated.

Cheers, Robin

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