Richard – Thanks for having a go at this challenge. Comphot estimates a coma diameter of 29.6 arcmin from these images. The attached file is one of the diagnostic plots that it produces and this one shows the diameter that it has determined. The estimator basically generates a set of circular rings centred on the photocentre and keeps going out until the level in the ring falls to less than 0.8*sigma above the median sky level where sigma is the standard deviation of the noise in the background image. This threshold is rather arbitrary but it seems to work reasonably well.
BTW this green frame is one of the components of the final image here:
https://britastro.org/observations/observation.php?id=20230123_023135_df98af0b86eb5907
The colour image nicely separates the green coma from the whitish dust tail.
