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    Extract ASTrometry parameters from a FITS image header.


    Extract World Coordinate System information
    ( http://fits.gsfc.nasa.gov/fits_wcs.html ) from a FITS header and
    place it into an IDL structure.

Calling Sequence

    EXTAST, hdr, astr, [ noparams, ALT= ]


    HDR - variable containing the FITS header (string array)


    In the following, index 1 & 2 refer to the first and second astrometry
    axes respectively. The actual axis numbers are stored in .AXIS
    ASTR - Anonymous structure containing astrometry info from the FITS
            header ASTR always contains the following tags (even though
            some projections do not require all the parameters)
      .NAXIS - 2 element array giving image size
      .CD - 2 x 2 array containing the astrometry parameters CD1_1 CD1_2
              in DEGREES/PIXEL CD2_1 CD2_2
      .CDELT - 2 element double vector giving physical increment at the
                reference pixel
      .CRPIX - 2 element double vector giving X and Y coordinates of reference
              pixel (def = NAXIS/2) in FITS convention (first pixel is 1,1)
      .CRVAL - 2 element double precision vector giving R.A. and DEC of
              reference pixel in DEGREES
      .CTYPE - 2 element string vector giving projection types, default
      .LONGPOLE - scalar giving native longitude of the celestial pole
            (default = 180 for zenithal projections)
      .LATPOLE - scalar giving native latitude of the celestial pole default=0)
      .PV2 - Vector of projection parameters associated with latitude axis
            PV2 will have up to 21 elements for the ZPN projection, up to 3
            for the SIN projection and no more than 2 for any other
    Fields added for version 2:
      .PV1 - Vector of projection parameters associated with longitude axis
      .AXES - 2 element integer vector giving the FITS-convention axis
              numbers associated with astrometry, in ascending order.
              Default [1,2].
      .REVERSE - byte, true if first astrometry axis is Dec/latitude
      .COORD_SYS - 1 or 2 character code giving coordinate system, including
                'C' = RA/Dec, 'G' = Galactic, 'E' = Ecliptic, 'X' = unknown.
      .PROJECTION - 3-letter WCS projection code
      .KNOWN - true if IDL WCS routines recognise this projection
      .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.
      .X0Y0 - Implied offset in intermediate world coordinates (x,y)
                  if a non-standard fiducial point is set via PV1 and also
                  PV1_0a =/ 0, indicating that an offset should be
                  applied to place CRVAL at the (x,y) origin.
                  Should be *added* to the IWC derived from application of
                  CRPIX, CDELT, CD to the pixel coordinates.
      .DISTORT - optional substructure specifying any distortion parameters
                currently implement only for "SIP" (Spitzer Imaging
                Polynomial), "TPV" (tangent PV* polynomial) distortion
                parameters, and "TNX" (tangent plus iraf distortion)
      NOPARAMS - Scalar indicating the results of EXTAST
            -1 = Failure - Header missing astrometry parameters
            1 = Success - Header contains CROTA + CDELT (AIPS-type) astrometry
            2 = Success - Header contains CDn_m astrometry, rec.
            3 = Success - Header contains PCn_m + CDELT astrometry.
            4 = Success - Header contains ST Guide Star Survey astrometry
                          (see gsssextast.pro )

Optional Input/output Keywords

      ALT - single character 'A' through 'Z' or ' ' specifying an alternate
              astrometry system present in the FITS header. The default is
              to use the primary astrometry or ALT = ' '. If /ALT is set,
              then this is equivalent to ALT = 'A'. See Section 3.3 of
              Greisen & Calabretta (2002, A&A, 395, 1061) for information about
              alternate astrometry keywords. If not set on input, then
              ALT is set to ' ' on output.


      EXTAST checks for astrometry parameters in the following order:
      (1) the CD matrix PC1_1,PC1_2...plus CDELT*, CRPIX and CRVAL
      (2) the CD matrix CD1_1,CD1_2... plus CRPIX and CRVAL.
      (3) CROTA2 (or CROTA1) and CDELT plus CRPIX and CRVAL.
      All three forms are valid FITS according to the paper "Representations
      of World Coordinates in FITS by Greisen and Calabretta (2002, A&A, 395,
      1061 http://fits.gsfc.nasa.gov/fits_wcs.html ) although form (1) is


      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, FITS headers created by SCAMP
      (http://www.astromatic.net/software/scamp) contain a tangent
      projection with distortion polynomial coefficients in the PV[1|2]_?
      keywords. These will be flagged as being a TPV projection
      (http://fits.gsfc.nasa.gov/registry/tpvwcs.html) in the
      astr.projection keyword.

Procedures Called


Revision History

      Written by B. Boothman 4/15/86
      Accept CD001001 keywords 1-3-88
      Accept CD1_1, CD2_1... keywords W. Landsman Nov. 92
      Recognize GSSS FITS header W. Landsman June 94
      Get correct sign, when converting CDELT* to CD matrix for right-handed
      coordinate system W. Landsman November 1998
      Consistent conversion between CROTA and CD matrix October 2000
      CTYPE = 'PIXEL' means no astrometry params W. Landsman January 2001
      Don't choke if only 1 CTYPE value given W. Landsman August 2001
      Recognize PC00n00m keywords again (sigh...) W. Landsman December 2001
      Recognize GSSS in ctype also D. Finkbeiner Jan 2002
      Introduce ALT keyword W. Landsman June 2003
      Fix error introduced June 2003 where free-format values would be
      truncated if more than 20 characters. W. Landsman Aug 2003
      Further fix to free-format values -- slash need not be present Sep 2003
      Default value of LATPOLE is 90.0 W. Landsman February 2004
      Allow for distortion substructure, currently implemented only for
          SIP (Spitzer Imaging Polynomial) W. Landsman February 2004
      Correct LONGPOLE computation if CTYPE = ['*DEC','*RA'] W. L. Feb. 2004
      Assume since V5.3 (vector STRMID) W. Landsman Feb 2004
      Yet another fix to free-format values W. Landsman April 2004
      Introduce PV2 tag to replace PROJP1, PROJP2.. etc. W. Landsman May 2004
      Convert NCP projection to generalized SIN W. Landsman Aug 2004
      Add NAXIS tag to output structure W. Landsman Jan 2007
      .CRPIX tag now Double instead of Float W. Landsman Apr 2007
      If duplicate keywords use the *last* value W. Landsman Aug 2008
      Fix typo for AZP projection, nonzero longpole N. Cunningham Feb 2009
      Give warning if reverse SIP coefficient not present W. Landsman Nov 2011
      Allow obsolete CD matrix representations W. Landsman May 2012
      Work for Paritel headers with extra quotes R. Gutermuth/WL April 2013
      Version 2: J. P. Leahy, July 2013
        - Support long & lat axes not being the first 2.
        - Implemented PV1 and hence non-default phi0 and theta0
          DATEOBS, MJDOBS, PV1, and X0Y0 tags to the structure.
        - More checks for inconsistencies in the keywords.
      v2.1 21/7/13 Missing mjdobs & equinox changed to NaN (was -1 & 0);
          Converts GLS to SFL if possible; added KNOWN tag.
      v2.2 21/9/13 GLS conversion fixed.
      v2.3 1 Dec 13 Add warning if distortions from SCAMP astrometry present
      v2.4. Extract SCAMP or TPV distortion astrometry, if present Jan 2014
      v2.5 Fix bug when SIP parameters not recognized when NAXIS=0 May 2014

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