Remove pincushion/barrel distortion from a distorted [x,y] to give tangent-plane [x,y].
disco double pincushion/barrel distortion coefficient x,y double distorted coordinates
*x,*y double tangent-plane 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 arguments, 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
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 slaPcd, obtained from two iterations of Newton's method. The mismatch between the slaPcd and slaUnpcd 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 15 July 1993