[BAA Comets] Astrometrica 4.10.0.431 and UCAC-4 R magnitudes

[Nick James]

I've just moved from Astrometrica version 4.9.1.420 to 4.10.0.431 in 
order to take advantage of its abilty to use the Gaia DR1 catalogue for 
astrometry. I'm still using UCAC-4 for photometry and I use the R band 
option since this is the closest match to my unfiltered CCD.

I was surprised to see that the new version produces R magnitudes that 
are around 0.25 mag brighter than the old one for the same image and 
configuration. Richard Miles has explained that this is a real change. 
Astrometrica used to use the Sloan r' magnitude from UCAC-4 directly as 
Cousins Rc but it now correctly applies a transformation which, on 
everage, corresponds to Rc = r' - 0.24 mag.

Other catalogues containing r' mags (CMC-14, CMC-15) were handled 
correctly so this change only applies to UCAC-4 R mags.

I recommend that you move to this new release however be prepared for a 
zero point shift of around 0.25 mags if you are using UCAC-4 R as your 
photometry reference.

Nick.