Hi Phil,
PI's noise evaluation tends to measure noise in a frame's background. This goes up with increased dark current (e.g higher sensor temp), increased light pollution, increased sky glow, etc. Hence SNR tends to decrease in these cases.
The signal part of SNR is a measure of frame scale or contrast. This goes down with poorer transparency, thin high clouds, etc. Again SNR tends to decrease.
Both noise and signal measures are estimates, so there is room for error. For example if the frames have differing gradients the estimates can be less accurate.
For my own frames I use SNR, FWHM and eccentricity. Star support is less useful to me because my frames are undersampled. In good seeing the smallest stars get rejected as hot pixels. In poor seeing these stars get bloated and so are then counted. So on my frames a higher star support means a poorer frame.
Regards,
Mike
