Comparing Images

[groberts]

Last night I was looking to add more subs to my M51 project, now running on-and-off since the end of last month.  To my eye the sky didn't look too bad, though not as good as other nights recently and when I started imaging the PHD2 results varied between ugly and none i.e. poor seeing.  After waiting for an hour or so + playing around with PHD2 settings my sense was that there was indeed some thin, high cloud causing this problem and I abandoned for an early night and some much needed sleep! 

However, during the time I did capture  x10 x 180 secs red subs which on processing strangely don't look too bad and I'm wondering afterall (a) whether I should have continued last night and (b) if these can be included with the previous subs data that will ultimately form the final image? Attached are two stretched and highly magnified r-sub stacks - Top = last night , Bottom = some from last month.

Apart for any aesthetics (subjective issues) is there any empirical test that can be applied to such data or resulting stacks that can help to quantitatively compare their respective merits?

Graham 

     

[wimvb]

I don't know about PS, but in Pixinsight you'd use subframe selector to measure and plot star profiles (fwhm and eccentricity) and backgroynd level on a number of subs. Any deteriorating conditions are easily revealed that way. You'd then set approval/rejection conditions and assign weights based on those measurements, which can be used during stacking. 

[groberts]

Thanks Wim, that sounds like the sort of thing I was thinking of - I have PixInsight but have yet to tackle its use, this is another reason to do so!

Graham