What to do when Photometric Color Calibration Doesn't...

[Linwood]

So "not enough sources" looks probable. The APASS catalog is heavily biased to the galactic plane (to be fair, there are a lot more stars there), while M101 is about as far from the galactic plane as it is possible to get.

I guess you mean "due to drizzle".

Thanks, it's very helpful to know that error is essentially "give up and stop trying for this image", rather than "keep trying with different parameters".

And yes, drizzle... I really need that setting on my browser for "Are you sure you want to post something after midnight, you know your brain is not really working, right?".

 

[fredvanner]

Give up with PCC, but ColourCalibration will do quite a decent job with this image (with ROI previews for background and white):

It is interesting to note that Gaia EDR3 (using Vizier online search) finds 1949 stars within 20' of the centre, of which 1855 have catalog values for R, G and B flux (Gaia G, BP and RP filters) - including 112 brighter than 16mag. It would be nice to build a Gaia-based fall-back photometric catalog to fill in the APASS gaps (any volunteers...?)