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      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.

Calling Sequence

      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.

Optional Input Keyword

      /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.
        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.

Revision History

      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
      Test for singular matrix W. Landsman August 2011

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