[vsnet-alert 10911] Re: re FWD ATEL 1892

[Brian Skiff]

On Mon, 2009-01-05 at 14:27 -0800, Michael Linnolt wrote:
> 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 


     This sort of adjustment, using 2MASS J-K colors to get a
rough visible-light color (like B-V or V-R), is okay as long as:
1)  the field is well outside the Milky Way, so there are minimal
effects from reddening, and 2)  the star(s) involved are "ordinary",
i.e. somewhere between say A0 and K5 giant or dwarf --- no M dwarfs,
no OB stars, no supergiants.  If the star is faint enough to have
SDSS data (also the right part of the sky), then the transformation
error going from r' to V is < 0.01 mag, better than the actual
accuracy of the data itself.
     If r' is available from CMC14 or SDSS, I'd skip the conversion
and just use r' directly.


\Brian