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The fit RMS is simply the fit to a stellar psf curve. Astrometrica, in general, does not use photometric catalogues (it does have one that is being tested for photometric accuracy but it has very limited coverage). If you look on the second image you will note that the dmag values for the images range from 0.2mag to 0.3mag. (This gives you an indication of the level of accuracy in your images using whatever catalogue you are using). Some areas in the astrometric catalogues are known to be well off in magniutde (up to 2 mags worse case).
How well have you accounted for linearity in your images? How about saturated stars? You need to realise that Astrometrica was not designed with photometry in mind.
Cheers
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