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+.help unpcd Jun99 "Slalib Package"
+.nf
+
+      SUBROUTINE slUPCD (DISCO,X,Y)
+
+     - - - - - -
+      U P C D
+     - - - - - -
+
+  Remove pincushion/barrel distortion from a distorted [x,y]
+  to give tangent-plane [x,y].
+
+  Given:
+     DISCO    d      pincushion/barrel distortion coefficient
+     X,Y      d      distorted coordinates
+
+  Returned:
+     X,Y      d      tangent-plane 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 "radians" - units of one projection radius,
+      which in the case of a photograph is the focal length of
+      the camera - 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)  The present routine is an approximate inverse to the
+      companion routine slPCD, obtained from two iterations
+      of Newton's method.  The mismatch between the slPCD and
+      slUPCD routines is negligible for astrometric applications;
+      to reach 1 milliarcsec at the edge of the AAT triplet or
+      Schmidt field would require field diameters of 2.4 degrees
+      and 42 degrees respectively.
+
+  P.T.Wallace   Starlink   1 August 1994
+
+  Copyright (C) 1995 Rutherford Appleton Laboratory
+  Copyright (C) 1995 Association of Universities for Research in Astronomy Inc.
+
+.fi
+.endhelp