[vsnet-alert 10909] Re: re FWD ATEL 1892

[Michael Linnolt]

Yikes, reliable extrapolation from r' to V is going to be quite poor using IR J/K color differences, and  I would not trust these conversions to any great extent, given all the variables involved with such a major color band shift.  Its tough enough to adjust to Johnson-V from slightly-off photographic visual band color differences even!
 
Mike Linnolt 

----- Original Message -----
From: qso at Safe-mail.net
Date: Monday, January 5, 2009 1:54 pm
Subject: [vsnet-alert 10906]  re FWD ATEL 1892
To: vsnet-alert at ooruri.kusastro.kyoto-u.ac.jp

> ATEL 1892 informs :-
> 
> "The V-band magnitudes of the object were obtained with a 
> comparison star located at RA=12:39:30.08, Dec=+04:39:53.2 
> (V=13.320; NOMAD)."
> 
> Hmmm, around mag 13 is a bit bright for SDSS usage in any conversion.
> 
> I'll have to dig out me own, mindnumbingly unpublished and 
> unpeer reviewed, CMC14 conversion then, V = r' + 0.60(J-Ks)-0.03 
> where r' is from CMC14 and J and Ks from 2MASS.
> 
> That gives the V mag of the calibration star as 13.99, and the 
> two observations quoted in ATEL 1892 then would become 16.4 and 
> 16.8 ish, errors still around 0.1.
> 
> Swift doesn't say when exactly on Jan 2nd the U band was 
> measured, so the potential U_swift-V_cmc ~ +0.4 from the Kanata 
> measure on Jan 2.8 might include some decay in V.
> 
> If the calibration star is V 13.99, that is.
> 
> Incidentally, a bit noisy this relatively faint, but the NSVS 
> data suggest the calibration star is constant enough.
> 
> http://skydot.lanl.gov/nsvs/star.php?num=13207748&mask=15636
> 
> Cheers
> 
> John