Name

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      MAKE_ASTR

Purpose

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      Build an astrometry structure from input parameter values

Explanation

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

Calling Sequence

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

Output Parameter

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

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      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
              ['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

Notes

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      (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

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      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