Re: astronomy lesson

["Dirk Terrell" <terrell@boulder.swri.edu>]

On Sun, 23 Feb 2003 18:22:25 -0600, Michael Koppelman wrote:

>3. How can I start to get an estimate of the interstellar extinction A?

If you have standardized BVRI photometry you can combine it with
possible 2MASS JHK data on the star to do an extinction curve to
measure A_v. Basically you plot the color excess (with E(B-V) scaled to
1) as a function of  reciprocal wavelength (1/microns) and then
extrapolate the curve to zero absorption at infinite wavelength. The
difference between that extrapolated line and the observed color
(scaled by the same amount to make E(B-V)=0) is the extinction for that
filter. The usual relation over most of the sky is that A_v = 3.2 x
E(B-V).

Dirk