A_b

     Compute B band interstellar extinction according to the RC2.

     The predicted B band extinction is computed as a function of
     Galactic position  using the 21 parameter function given by
     deVaucouleurs in the 2nd Reference Catalog of Galaxies (RC2).   Note
     that this formula was not used for the RC3 and that reddenings
     were instead obtained from the Burstein-Heiles 21 cm maps.

     result = A_b( l2, b2)

     l2 = Galactic longitude (degrees), scalar or vector
     b2 = Galactic latitude  (degrees), scalar or vector

     RESULT - Interstellar extinction Ab in magnitudes, same number of
             elements as input l2 and b2 parameters

     The controversial aspect of the deVaucouleurs reddening curve
     is that it predicts an extinction of about 0.2 at the poles
     The parameters used here differ from the ones printed in the RC2
     but are the ones actually used for entries in the catalog
     (see Rowan-Robinson 1985)
     This procedure is mainly of historical interest only, and reddening
     is now better determined using dust maps, such as those available at
     http://astro.berkeley.edu/davis/dust/index.html

     Written by R. Cornett and W. Landsman, STX October 1987
     Vectorized code      W. Landsman   STX    December 1992
     Converted to IDL V5.0   W. Landsman   September 1997

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