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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
      an approximately inverse operation.

  P.T.Wallace   Starlink   31 December 1992

  Copyright (C) 1995 Rutherford Appleton Laboratory
  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.

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