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Hi Andy,
Here's a map of FWHM and Eccentricity (values around 0.42 and less look nicely round) from your image.
A couple of interesting things: FWHMs are worse in the centre of the field which is the opposite of what you usually see and star roundness gets worse as you go from the right hand side of the image to the left. The FWHM thing could be caused by field curvature and focus. Maybe a little tilt contributing to the variation in Eccentricity too?
It's hard to say too much because that's not a very good FOV for making measurements. You'd be better off with a field with stars from edge to edge and no nebulosity.
Cheers,
Rick.
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