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      Build an astrometry structure from input parameter values


      This structure can be subsequently placed in a FITS header with

Calling Sequence

      MAKE_ASTR, astr, CRPIX =, CRVAL =, [CD = , DELT =, CTYPE =, $
              LATPOLE = , LONGPOLE =, PV2 =, NAXIS =, AXES =, PV1 =, $
              RADECSYS =, EQUINOX =, DATEOBS =, MJDOBS =]

Output Parameter

      ASTR - Anonymous structure containing astrometry info. See the
              documentation for EXTAST for descriptions of the individual
      CRPIX - 2 element vector giving X and Y coordinates of reference pixel
              (def = NAXIS/2). VALUES MUST BE IN FITS CONVENTION (first pixel
              is [1,1]) AND NOT IDL CONVENTION (first pixel is [0,0]).
      CRVAL - 2 element double precision vector giving R.A. and DEC of
              reference pixel in DEGREES

Optional Input Keywords

      CD - 2 x 2 array containing the astrometry parameters CD1_1 CD1_2
              in DEGREES/PIXEL CD2_1 CD2_2
      DELT - 2 element vector giving physical increment at reference pixel
              in DEGREES/PIXEL default = [-1.0D, 1.0D]/3600. (1 arcsec/pixel)
      CTYPE - 2 element string vector giving projection types, default
      LATPOLE - Scalar latitude of the north pole, default = +90
      LONGPOLE - scalar longitude of north pole
      PV2 - Vector of projection parameters associated with latitude axis.
            Not required for some projections (e.g. TAN) and optional for
            others (e.g. SIN).
            Usually a 2 element vector, but may contain up to 21 elements
            for the Zenithal Polynomial (ZPN) projection. Corresponds to
            the keywords PV2_1, PV2_2... Defaults to 0.0
      Added for version 2 astrometry structure:
      AXES - 2 element integer vector giving the FITS-convention axis
              numbers associated with astrometry, in ascending order.
              Default [1,2].
      NAXIS - 2 element integer vector giving number of pixels on each axis
      PV1 - Vector of projection parameters associated with longitude axis
            Elements 4 & 5 (if present) are equivalent to LONGPOLE & LATPOLE
            and take precedence if both are specified, i.e. LONGPOLE & LATPOLE
            in the structure are forced to agree with PV1.
      RADECSYS - String giving RA/Dec system e.g. 'FK4', 'ICRS' etc.
      EQUINOX - Double giving the epoch of the mean equator and equinox
      DATEOBS - Text string giving (start) date/time of observations
      MJDOBS - Modified julian date of start of observations.
              (specify one or other of DATEOBS or MJDOBS)


      (1) An anonymous structure is created to avoid structure definition
              conflicts. This is needed because some projection systems
              require additional dimensions (i.e. spherical cube
              projections require a specification of the cube face).
      (2) The name of the keyword for the CDELT parameter is DELT because
              the IDL keyword CDELT would conflict with the CD keyword
      (3) The astrometry structure definition was slightly modified in
              July 2003; all angles are now double precision, and the
              LATPOLE tag was added. In April 2007 the CRPIX tag was also
              changed to double precision.

Revision History

      Written by W. Landsman Mar. 1994
      Added LATPOLE, all angles double precision W. Landsman July 2003
      Use PV2 keyword rather than PROJP1, PROJP2 W. Landsman May 2004
      Make .CRPIX tag double precision, change CDELT default to 1"/pixel
                      W. Landsman April 2007
        Default plate scale is now 1"/pixel (not 1 deg/pix) WL Oct. 2010
        Oct 2010 change should only apply when CD matrix not given
                    M. Cushing/W.L. Aug 2011
        added v2 parameters; more filling out of defaults; default
        LATPOLE changed to 90 (FITS standard) J. P. Leahy Jul 2013

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