Purpose

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  This procedure converts from (a,d,v) sky coordinates to (x,y,z)
  pixel coordinates in a data cube. It is a wrapper to ad2xy in the
  IDL Astronomy User's Library, which does not handle 3D data.

Inputs

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  head: A fits header. String array
  a: List of sky x coordinates (alpha, lon). Scalar or vector
  d: List of sky y coordinates (dec, lat). Scalar or vector
  v: List of sky z coordinates (velocity, freq). Scalar or vector.

Keyword Parameters

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  ortho: Set this keyword if the third axis of the input coordinates
        is orthogonal to the first two; that is, if adv describe a
        cube, where a and d are constant across the cube. When this
        is true, the computation can be performed more quickly.
 

Outputs

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  x: Pixel x coordinates. These are zero-indexed, IDL style.
  y: Pixel y coordinates
  z: Pizel z coordinates

Procedure

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  This function calls ad2xy, and hence can handle any wcs stuff that
  that procedure can. The only restriction is that the fits header
  have a simple description for transforming from v->z. That is,
  CDELT3/CD3_3, CRPIX3, CRVAL3, keywords should completely define the
  transformation. This is almost always the case.

Modification History

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  March 2010: Written by Chris Beaumont
  May 2010: Fixed a bug in call to ad2xy. cnb.
  Sep 2010: Fixed a bug when testing whether head is a string
            array. cnb.
  June 2011: Optimized for the case where a,d,v describe a cube with
            a,d constant along slices. Added ortho keyword. cnb.