CCM_UNRED

     Deredden a flux vector using the CCM 1989 parameterization

     The reddening curve is that of Cardelli, Clayton, and Mathis (1989 ApJ.
     345, 245), including the update for the near-UV given by O'Donnell
     (1994, ApJ, 422, 158).   Parameterization is valid from the IR to the
     far-UV (3.5 microns to 0.1 microns).
     Users might wish to consider using the alternate procedure FM_UNRED
     which uses the extinction curve of Fitzpatrick (1999).

     CCM_UNRED, wave, flux, ebv, funred, [ R_V = ]
             or
     CCM_UNRED, wave, flux, ebv, [ R_V = ]

     WAVE - wavelength vector (Angstroms)
     FLUX - calibrated flux vector, same number of elements as WAVE
             If only 3 parameters are supplied, then this vector will
             updated on output to contain the dereddened flux.
     EBV  - color excess E(B-V), scalar.  If a negative EBV is supplied,
             then fluxes will be reddened rather than deredenned.

     FUNRED - unreddened flux vector, same units and number of elements
             as FLUX

     R_V - scalar specifying the ratio of total selective extinction
             R(V) = A(V) / E(B - V).    If not specified, then R_V = 3.1
             Extreme values of R(V) range from 2.75 to 5.3

     Determine how a flat spectrum (in wavelength) between 1200 A and 3200 A
     is altered by a reddening of E(B-V) = 0.1.   Assume an "average"
     reddening for the diffuse interstellar medium (R(V) = 3.1)
       IDL> w = 1200 + findgen(40)*50      ;Create a wavelength vector
       IDL> f = w*0 + 1                    ;Create a "flat" flux vector
       IDL> ccm_unred, w, f, -0.1, fnew  ;Redden (negative E(B-V)) flux vector
       IDL> plot,w,fnew

     (1) The CCM curve shows good agreement with the Savage & Mathis (1979)
             ultraviolet curve shortward of 1400 A, but is probably
             preferable between 1200 and 1400 A.
     (2)  Many sightlines with peculiar ultraviolet interstellar extinction
             can be represented with a CCM curve, if the proper value of
             R(V) is supplied.
     (3)  Curve is extrapolated between 912 and 1000 A as suggested by
             Longo et al. (1989, ApJ, 339,474)
     (4) Use the 4 parameter calling sequence if you wish to save the
               original flux vector.
     (5) Valencic et al. (2004, ApJ, 616, 912) revise the ultraviolet CCM
             curve (3.3 -- 8.0 um-1).    But since their revised curve does
             not connect smoothly with longer and shorter wavelengths, it is
             not included here.

       Written   W. Landsman        Hughes/STX   January, 1992
       Extrapolate curve for wavelengths between 900 and 1000 A   Dec. 1993
       Use updated coefficients for near-UV from O'Donnell   Feb 1994
       Allow 3 parameter calling sequence      April 1998
       Converted to IDLV5.0                    April 1998

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