Apply pincushion/barrel distortion to a tangent-plane [x,y].
disco double pincushion/barrel distortion coefficient x,y double tangent-plane coordinates
*x,*y double distorted coordinates
1) The distortion is of the form rp = r*(1 + c*r2), 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
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, slaUnpcd, which performs an approximately inverse operation. P.T.Wallace Starlink 15 July 1993