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+      SUBROUTINE slPCD (DISCO,X,Y)
+*+
+*     - - - -
+*      P C D
+*     - - - -
+*
+*  Apply pincushion/barrel distortion to a tangent-plane [x,y].
+*
+*  Given:
+*     DISCO    d      pincushion/barrel distortion coefficient
+*     X,Y      d      tangent-plane coordinates
+*
+*  Returned:
+*     X,Y      d      distorted coordinates
+*
+*  Notes:
+*
+*  1)  The distortion is of the form RP = R*(1 + C*R**2), where R is
+*      the radial distance from the tangent point, C is the DISCO
+*      argument, and RP is the radial distance in the presence of
+*      the distortion.
+*
+*  2)  For pincushion distortion, C is +ve;  for barrel distortion,
+*      C is -ve.
+*
+*  3)  For X,Y in units of one projection radius (in the case of
+*      a photographic plate, the focal length), the following
+*      DISCO values apply:
+*
+*          Geometry          DISCO
+*
+*          astrograph         0.0
+*          Schmidt           -0.3333
+*          AAT PF doublet  +147.069
+*          AAT PF triplet  +178.585
+*          AAT f/8          +21.20
+*          JKT f/8          +13.32
+*
+*  4)  There is a companion routine, slUPCD, which performs the
+*      inverse operation.
+*
+*  P.T.Wallace   Starlink   3 September 2000
+*
+*  Copyright (C) 2000 Rutherford Appleton Laboratory
+*
+*  License:
+*    This program is free software; you can redistribute it and/or modify
+*    it under the terms of the GNU General Public License as published by
+*    the Free Software Foundation; either version 2 of the License, or
+*    (at your option) any later version.
+*
+*    This program is distributed in the hope that it will be useful,
+*    but WITHOUT ANY WARRANTY; without even the implied warranty of
+*    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+*    GNU General Public License for more details.
+*
+*    You should have received a copy of the GNU General Public License
+*    along with this program (see SLA_CONDITIONS); if not, write to the
+*    Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+*    Boston, MA  02110-1301  USA
+*
+*  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.
+*-
+
+      IMPLICIT NONE
+
+      DOUBLE PRECISION DISCO,X,Y
+
+      DOUBLE PRECISION F
+
+
+
+      F=1D0+DISCO*(X*X+Y*Y)
+      X=X*F
+      Y=Y*F
+
+      END