Median eccentricity

[astrodoc71]

Just a general question on what values, in the image analysis scripts that measure this, mean more "roundness". I'm pretty sure that lower values e.g. 0.3-0.5 are "better" but just trying to understand the concept better.
Thanks
Dave

[bulrichl]

Hi Dave,

in the reference documentation of SubframeSelector, Mike gave the definition of eccentricity:

https://pixinsight.com/doc/scripts/SubframeSelector/SubframeSelector.html#subframe_property_Eccentricity

---------- Citation ----------
"Eccentricity is a measure of star profile distortion. Given an elliptical star profile with major axis diameter a and minor axis diameter b where a is greater than or equal to b,
the star profile eccentricity equals (1 - b² / a²)^0.5, the star profile aspect ratio equals b / a and the star profile flatness equals a / b - 1. A distortion with an eccentricity
less than about 0.42 is not perceptible to most people."
-------------------------------

Bernd

[astrodoc71]

Aha! I guess I would have seen it under documentation in the subframe selector script but I only looked at the FWHM eccentricity script which does not have any attached documentation. Thanks!
Dave