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      Compute the coordinates of the native pole


      WCS_GETPOLE is used to determine the celestial position of the
      native pole. See section 2.4 of the paper
      "Representation of Celestial Coordinates in FITS" by Calabretta
      Greisen (2002, A&A, 395, 1077, also available at
      http://fits.gsfc.nasa.gov/fits_wcs.html Called by WCS_ROTATE

Calling Sequence

      WCS_GETPOLE, crval, lonpole, theta0, alpha_p, delta_p, [LATPOLE= AT_POLE=]

Input Parameters

      crval - 2 element vector containing standard system coordinates (the
              longitude and latitude) of the reference point in degrees
      lonpole - native longitude of the celestial North Pole (degrees)
                *unless* the fiducial point is at non-zero native longitude
                (phi_0 =/ 0), in which case phi_0 should have been subtracted,
                i.e. lonpole = phi_p - phi_0.
      theta0 - native latitude of the fiducial point (degrees)

Output Parameters

      alpha_p, delta_p - celestial longitude and latitude of the native pole

Optional Keyword Input Parameters

      LATPOLE - native latitude of the celestial North Pole (degrees)
                NB only used to resolve ambiguity. Final value is the one
                nearest to input value of LATPOLE. Can be set outside range
      AT_POLE (byte) true if delta_p = pi/2 (avoiding some round-off errors)

Revision History

      Written W. Landsman June, 2003
      Fix calculation when theta0 is not 0 or 90 February 2004
      E. Hivon: alpha_p, delta_p consistenly in Radians May 2010
      J. P. Leahy introduced AT_POLE, more traps for special cases to
      avoid rounding errors July 2013

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