STARAST

       Compute astrometric solution using positions of 2 or 3 reference stars

       Computes an exact astrometric solution using the positions and
       coordinates from 2 or 3 reference stars and assuming a tangent
       (gnomonic) projection.   If 2 stars are used, then
       the X and Y plate scales are assumed to be identical, and the
       axis are assumed to be orthogonal.   Use of three stars will
       allow a unique determination of each element of the CD matrix.

       starast, ra, dec, x, y, cd, [/Righthanded, HDR = h, PROJECTION=]

       RA - 2 or 3 element vector containing the Right Ascension in DEGREES
       DEC- 2 or 3 element vector containing the Declination in DEGREES
       X -  2 or 3 element vector giving the X position of reference stars
       Y -  2 or 3 element vector giving the Y position of reference stars

       CD - CD (Coordinate Description) matrix (DEGREES/PIXEL) determined
               from stellar positions and coordinates.

       /RightHanded - If only 2 stars are supplied, then there is an ambiguity
               in the orientation of the coordinate system.   By default,
               STARAST assumes the astronomical standard left-handed system
               (R.A. increase to the left).   If /Right is set then a
               righthanded coordinate is assumed.  This keyword has no effect
               if 3 star positions are supplied.
        PROJECTION - Either a 3 letter scalar string giving the projection
               type (e.g. 'TAN' or 'SIN') or an integer 1 - 25 specifying the
               projection as given in the WCSSPH2XY procedure.   If not
               specified then a tangent projection is computed.
 OPTIONAL INPUT-OUTPUT KEYWORD:
        HDR - If a FITS header string array is supplied, then an astrometry
              solution is added to the header using the CD matrix and star 0
              as the reference pixel (see example).   Equinox 2000 is assumed.

        To use STARAST to add astrometry to a FITS header H;
        IDL> starast,ra,dec,x,y,cd       ;Determine CD matrix
        IDL> crval = [ra[0],dec[0]]      ;Use Star 0 as reference star
        IDL> crpix = [x[0],y[0]] +1      ;FITS is offset 1 pixel from IDL
        IDL> putast,H,cd,crpix,crval     ;Add parameters to header
        This is equivalent to the following command:
        IDL> STARAST,ra,dec,x,y,hdr=h

       The CD parameters are determined by solving the linear set of equations
       relating position to local coordinates (l,m)
       For highest accuracy the first star position should be the one closest
       to the reference pixel.

       Written, W. Landsman             January 1988
       Converted to IDL V5.0   W. Landsman   September 1997
       Added /RightHanded and HDR keywords   W. Landsman   September 2000
       Write CTYPE values into header   W. Landsman/A. Surkov  December 2002
       CD matrix was mistakenly transpose in 3 star solution
       Added projection keyword    W. Landsman   September 2003

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