**ACTION:**- Remove pincushion/barrel distortion from a distorted
to give tangent-plane .
**CALL:**`CALL sla_UNPCD (DISCO,X,Y)`

**GIVEN:**-

*DISCO***D**pincushion/barrel distortion coefficient *X,Y***D**distorted

**RETURNED:**-

*X,Y***D**tangent-plane

**NOTES:**- 1.
- The distortion is of the form , where
*r*is the radial distance from the tangent point,*c*is the DISCO argument, and 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 +14.6

- 4.
- The present routine is an approximate inverse to the companion routine sla_PCD, obtained from two iterations of Newton's method. The mismatch between the sla_PCD and sla_UNPCD is negligible for astrometric applications; to reach 1 milliarcsec at the edge of the AAT triplet or Schmidt field would require field diameters of and respectively.

Starlink User Note 67

P. T. Wallace

12 October 1999

E-mail:ptw@star.rl.ac.uk