FLUX2MAG

     Convert from flux (ergs/s/cm^2/A) to magnitudes.

     Use MAG2FLUX() for the opposite direction.

     mag = flux2mag( flux, [ zero_pt, ABwave=  ] )

     flux - scalar or vector flux vector, in erg cm-2 s-1 A-1

     zero_pt - scalar giving the zero point level of the magnitude.
               If not supplied then zero_pt = 21.1 (Code et al 1976)
               Ignored if the ABwave keyword is supplied

     ABwave - wavelength scalar or vector in Angstroms.   If supplied, then
           FLUX2MAG() returns Oke AB magnitudes (Oke & Gunn 1983, ApJ, 266,
           713).

     mag - magnitude vector.   If the ABwave keyword is set then mag
           is given by the expression
           ABMAG = -2.5*alog10(f) - 5*alog10(ABwave) - 2.406
           Otherwise, mag is given by the expression
           mag = -2.5*alog10(flux) - zero_pt

       Suppose one is given wavelength and flux vectors, w (in Angstroms) and
       f (in erg cm-2 s-1 A-1).   Plot the spectrum in AB magnitudes
       IDL> plot, w, flux2mag(f,ABwave = w), /nozero

       Written    J. Hill        STX Co.       1988
       Converted to IDL V5.0   W. Landsman   September 1997
       Added ABwave keyword    W. Landsman   September 1998

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