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  This procedure is an extenion of (and a wrapper for) the xyad
  program in the astronomy user's library. It converts (x,y,z)
  pixel coordinates to (a,d,v) sky / velocity coordinates for a 3D
  data cube, based on the fits header.


  head: A fits header. String array
    x: x pixel coordinates. scalar or vector
    y: y pixel coordinates. scalar or vector
    z: z pixel coordinates. scalar or vector

Keyword Parameters

  ortho: Set this keyword if the third axis of the input coordinates
        is orthogonal to the first two; that is, if xyz describe a
        cube, and x and y are the same for each slice. When this
        is true, the computation can be performed more quickly.


  a: x sky coordinate (e.g. ra or lon).
  d: y sky coordinate (e.g. dec or lat).
  v: z sky coordinate (e.g. vel or freq).


  This procedure calls extast3 from the Beaumont IDL library and xyad
  from the IDL Astronomy User's Library.

Modification History

  March 2010: Written by Chris Beaumont
  May 2010: Fixed a bug in the call to xy2ad. cnb.
  June 2011: Optimized for case when x,y,z describe a cube, with
            (x,y) constant along z slices.

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