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

[Roger Dymock]

Hi Nick,

I found out about the Astrometrica r' vs R some months ago when working on using my DSLR camera for RGB photometry. Having confirmed this with Richard I then developed an equation for the transform. There was one in existence but, for some reason Herbert Raab could not explain, it wouldn't work with Astrometrica. My formula is R=r'-0.108(B-V) -0.1322

I have been writing a paper, with much help from Richard, and will continue with it as soon as I have written the Comphot instructions.
To verify my equation I;
- compared the R mags of Landolt stars with the R mags calculated using my equation with the UCAC-4 catalogue
- imaged Landolt stars with SSON and measured their magnitudes using the latest version of Astrometrica.

A later version of Astrometrica will introduce (B-V) limits to exclude very red or very blue stars from the astrometric reduction.

Regards
Roger Dymock
www.britastro.org/projectalcock
Skype; rogerdymock67
Tel; 023 92647986

-----Original Message-----
From: Comets-disc [mailto:comets-disc-bounces at lists.britastro.org] On Behalf Of Nick James
Sent: 07 March 2017 07:38
To: BAA Comets discussion list <comets-disc at britastro.org>
Subject: [BAA Comets] Astrometrica 4.10.0.431 and UCAC-4 R magnitudes

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.

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