High eccentricity value when stars look round?

[HenryNZ]

I wonder what may be giving high eccentricity value when the stars look round.

I have a skywatchers esprit 80 which I think produce fairly round stars with a crop size DSLR. However the eccentricity measures 0.7+.

I comparison I have a FSQ85 which I think actually have less round stars at the corners but it's eccentricity is about 0.5-6

I have a longer focal length AP130GT using same camera my eccentricity is 0.3-0.4.

The fact that I can get nice round stars with 0.3-0.4 with a longer focal length instrument would suggest that guiding etc is not really an issue, right?

Am I missing some setting? The only thing I have not done was to debayer before running the script, but this should affect all three sets of results equally?

[sharkmelley]

With a one-shot-colour sensor you should definitely debayer before measuring eccentricity otherwise the star profiles will look very weird indeed (because different colour pixels will have different brightness) and this might well affect eccentricity measurements.

Using the same camera, my guess is that the eccentricity measurements of the larger FWHM stars from the longer focal length scope will be less affected by the undebayered data than the smaller FWHM stars from the shorter scopes.

Are you producing contour plots of your eccentricity or are you quoting an average across the whole frame?

Mark