NAME:
  astinvrt
 PURPOSE:
  Invert an astrometric solution to get x,y given ($\xi$,$\eta$).
 DESCRIPTION:

 CATEGORY:
  Astrometry
 CALLING SEQUENCE:
  astinvrt,xi,cxi,xiterms,eta,ceta,etaterms,x,y
 INPUTS:
  xi       - Coordinate in tangent plane.
  cxi      - Coefficients of xi astrometric fit, this must be a vector that has
              the terms to be used, the length must be equal to total(xiterms)
  xiterms  - Flag array, which terms to use, see ASTEVAL.PRO for description.
  eta      - Coordinate in tangent plane.
  ceta     - Coefficients of eta astrometric fit, this must be a vector that has
              the terms to be used, the length must be equal to total(etaterms)
  etaterms - Flag array, which terms to use, see ASTEVAL.PRO for description.
  x        - Initial guess of x coordinate
  y        - Initial guess of y coordinate
  
 OPTIONAL INPUT PARAMETERS:

 KEYWORD INPUT PARAMETERS:

 OUTPUTS:
  x        - x coordinate
  y        - y coordinate

 KEYWORD OUTPUT PARAMETERS:

 COMMON BLOCKS:

 SIDE EFFECTS:

 RESTRICTIONS:

 PROCEDURE:

 MODIFICATION HISTORY:
  2000/9/5, Written by Marc W. Buie, Lowell Observatory