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SKY2PIX

SKY2PIX

Name


  SKY2PIX
 

Purpose


  This function translates between 'real' coordinates (eg, ra dec) to
  pixel coordinates in a fits file. Pixel co-ordinates are given as
  zero-indexed values (IDL convention) as opposed to 1-indexed (FITS
  convention). This was designed as a wrapper to / substute for
  commands like ad2xy, which demand that the input data be 2D. This
  program processes data cubes as well.

Calling Sequence


  result=sky2pix(header,coords,/backwards)

Inputs


  header: A string array containing a fits header (eg, from mrdfits)
  coords: An n by m array containing the n-dimensional coordinates of
          m datapoints. These are 'real sky' coordinates, unless the
          backwards keyword is set.

Keyword Parameters


  backwards: coords are given in pixel coordinates, and the output is
            given in sky coordinates

Outputs


  result: An array contining the pixel coordinates of the real
          coordinates given by coords. This array is the same size as
          coords. result [i,j] contains the coordinate component of the jth
          data point along the ith pixel dimension.

Examples


  find the pixel location of (ra,dec,vel)=(5,6,7) in header h
  IDL> result=sky2pix(h,[5,6,7])
 
  find the pixel location of (ra,dec)=(5,6) in a data cube with
  header h
  IDL> result=sky2pix(h,[5,6])

Procedure


  This program is a wrapper to the ad2xy, xy2ad, etc programs. Unlike
  those procedures, however, sky2pix can handle simple 3D cubes. For
  this to work, however, the cubes must be "aligned" along the third
  dimension. That is, the third "sky" axis (e.g. velocity) must
  correspond directly to the third "data" axis. This is equivalent
  to saying that, if the header uses the CDX_Y keywords, then CD3_X
  and CDX_3 must be zero, except for CD3_3. Note that this is almost
  always the case for normal data cubes.

Modification History


  Written by: Chris Beaumont. July 27 2008
  December 17 2008: Cosmetic changes. cnb.
  November 21 2009: Complete re-write by cnb. Uses the extast3
                    program, and more directly relies on ad2xy to
                    handle complicated wcs stuff.



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