Name

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XYXY

Purpose

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To use a pair of headers to convert X/Y positions from one frame
to another.

Calling Sequence

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XYXY, hdra, hdrb, xa, ya, [ xb, yb ]

Inputs

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hdra - The header containing the plate solution describing the
frame of reference being converted FROM.
hdra - The header containing the plate solution describing the
frame of reference being converted TO.
xa - A scalar or vector containing the x coordinate(s) to convert.
ya - A scalar or vector containing the y coordinate(s) to convert.
Must have the same number of elements as 'xa'.

Outputs

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xb - The converted x coordinate(s). If this parameter is not
specified, it is returned through 'xa'.
yb - The converted y coordinate(s). If this parameter is not
specified, it is returned through 'ya'.

Procedure

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The procedures 'xyad' and 'adxy' are used to perform the
      conversion. The equinoxes of each header are checked with
      "get_equinox" to make sure that they are identical, and "precess"
      is used if they are not. HEULER used if the headers have a different
      coordinate system (e.g. Celestial, Galactic, Ecliptic)
      Note that all X,Y coordinates are in the IDL convention (starting with
      0,0) and not the FITS convention (first pixel is 1,1)

Procedures Used

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GET_EQUINOX(), EXTAST, XYAD, ADXY, PRECESS, HEULER

Modification History

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Written by Michael R. Greason, Hughes-STX, 13 April 1992.
Updated to use ASTROMETRY structures. J.D.Offenberg, HSTX, Jan 1993
Converted to IDL V5.0 W. Landsman September 1997
      Check coordinate system J. Ballet/ W. Landsman April 2004