GLACTC

        Convert between celestial and Galactic (or Supergalactic) coordinates.

       Program to convert right ascension (ra) and declination (dec) to
       Galactic longitude (gl) and latitude (gb) (j=1) or vice versa (j=2).

       GLACTC, ra, dec, year, gl, gb, j, [ /DEGREE, /FK4, /SuperGalactic ]

       year     equinox of ra and dec, scalar       (input)
       j        direction of conversion     (input)
               1:  ra,dec --> gl,gb
               2:  gl,gb  --> ra,dec
 INPUTS OR OUTPUT PARAMETERS: ( depending on argument J )
       ra       Right ascension, hours (or degrees if /DEGREES is set),
                         scalar or vector
       dec      Declination, degrees,scalar or vector
       gl       Galactic longitude, degrees, scalar or vector
       gb       Galactic latitude, degrees, scalar or vector
       All results forced double precision floating.

       /DEGREE - If set, then the RA parameter (both input and output) is
                given in degrees rather than hours.
       /FK4 - If set, then the celestial (RA, Dec) coordinates are assumed
              to be input/output in the FK4 system.    By default,  coordinates
              are assumed to be in the FK5 system.    For B1950 coordinates,
              set the /FK4 keyword *and* set the year to 1950.
       /SuperGalactic - If set, the GLACTC returns SuperGalactic coordinates
              as defined by deVaucouleurs et al. (1976) to account for the
              local supercluster. The North pole in SuperGalactic coordinates
              has Galactic coordinates l = 47.47, b = 6.32, and the origin is
              at Galactic coordinates l = 137.37, b= 0

       Find the Galactic coordinates of Altair (RA (J2000): 19 50 47
       Dec (J2000): 08 52 06)
       IDL> glactc, ten(19,50,47),ten(8,52,6),2000,gl,gb,1
       ==> gl = 47.74, gb = -8.91

       BPRECESS, JPRECESS, PRECESS

       FORTRAN subroutine by T. A. Nagy, 21-MAR-78.
       Conversion to IDL, R. S. Hill, STX, 19-OCT-87.
       Modified to handle vector input, E. P. Smith, GSFC, 14-OCT-94
       Converted to IDL V5.0   W. Landsman   September 1997
       Added DEGREE keyword, C. Markwardt, Nov 1999
       Major rewrite, default now FK5 coordinates, added /FK4 keyword
       use external precession routines    W. Landsman   April 2002
       Add /Supergalactic keyword W. Landsman  September 2002
       Fix major bug when year not 2000 and /FK4 not set W. Landsman July 2003

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