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GETROT

GETROT

Name


    GETROT

Purpose


    Return the rotation and plate scale of an image from its FITS header

Explanation


    Derive the counterclockwise rotation angle, and the X and Y scale
    factors of an image, from a FITS image header. The input parameter
    may be either a FITS image header or an astrometry structure (as
    obtained by extast.pro)

Calling Sequence


    GETROT, Hdr, [ Rot, CDelt, /SILENT, DEBUG = ]
            or
    GETROT, Astr, Rot, CDelt, /SILENT, DEBUG = ]

Input Parameters


    HDR - FITS Image header (string array). Program will extract the
            astrometry structure
              or
    ASTR - ASTROMETRY structure, of the type returned by EXTAST.
            See the documentation for EXTAST.PRO for details.

Optional Output Parameters


      ROT - Scalar giving the counterclockwise rotation of NORTH in DEGREES
              from the +Y axis of the image.
      CDELT- 2 element vector giving the scale factors in DEGREES/PIXEL in
              the X and Y directions. CDELT[1] is always positive, whereas
              CDELT[0] is negative for a normal left-handed coordinate system,
              and positive for a right-handed system.
      If no output variables are supplied (or /DEBUG is set), then GETROT
      will display the rotation and plate scale at the terminal.

Optional Input Keyword


 
      ALT - single character 'A' through 'Z' or ' ' specifying an alternate
            astrometry system present in the FITS header. See extast.pro
            for more information on the ALT keyword. Ignored if an
            astrometry structure rather than FITS header is supplied.
      DEBUG - if DEBUG is set, GETROT will print the rotation for both the
          X and Y axis when these values are unequal. If DEBUG is set to 2,
          then the output parameter ROT will contain both X and Y rotations.
      /SILENT - if set, then do not provide a warning about a right-handed
          coordinate system

Procedure


      If the FITS header already contains CDELT (and CD or CROTA) keyword,
      (as suggested by the Calabretta & Greisen (2002, A&A, 395, 1077) FITS
      standard) then this is used for the scale factor.
     
      If the header contains CD keywords but no CDELT keywords (as in IRAF
      headers) then the scale factor is derived from the CD matrix.
      In case of skew (different rotations of the X and Y axes), the rotations
      are averaged together if they are less than 2 degrees. Otherwise,
      a warning is given and the X rotation is used.

Procedures Used


      EXTAST, GSSS_EXTAST

Revision History


      Written W. Landsman STX January 1987
      Option to return both rotations added. J. D. Offenberg, STX, Aug 1991
      Use new astrometry structure W. Landsman Mar 1994
      Recognize a GSSS header W. Landsman June 1994
      Correct rotation determination with unequal CDELT values WL October 1998
      Consistent conversion between CROTA and CD matrix WL October 2000
      Correct CDELT computations for rotations near 90 deg WL November 2002
      Preserve sign in the CDELT output WL June 2003
      Check if latitude/longitude reversed in CTYPE WL February 2004
      Fix problem in latitude check M.Lombardi/W.Landsman Sep 2004
      Added ALT keyword W. Landsman May 2005
      Account for any rotation of the native system by examining the value
        of LONGPOLE H. Taylor/W. Landsman
      Account for case where X,Y rotations differ by 2*!pi WL. Aug 2011



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