MAKE_ASTR

       Build an astrometry structure from input parameter values

       This structure can be subsequently placed in a FITS header with
       PUTAST

       MAKE_ASTR, astr, CD = , DELT =, CRPIX =, CRVAL =, CTYPE =,
               LATPOLE = , LONGPOLE =, PV2_1 = PV2_2 = , PV2_3 =

       ASTR - Anonymous structure containing astrometry info.  See the
              documentation for EXTAST for descriptions of the individual
              tags

       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

       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
              CDELT default = [1.0D, 1.0D].
       CTYPE - 2 element string vector giving projection types, default
              ['RA---TAN','DEC--TAN']
       LATPOLE - Scalar latitude of the north pole, default = 0
       LONGPOLE - scalar longitude of north pole, default = 180
                Note that the default value of 180 is valid only for zenithal
               projections; it should be set to PV2_1 for conic projections,
               and zero for other projections.
       PV2 - Vector of projection parameters.   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

       (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.

       Written by   W. Landsman              Mar. 1994
       Converted to IDL V5.0                 Jun  1998
       Added LATPOLE, all angles double precision  W. Landsman July 2003
       Use PV2 keyword rather than PROJP1, PROJP2 W. Landsman May 2004

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