FOC Instrument Science Report #079 "FOC Optical Distortion Including COSTAR" by P.E. Hodge Abstract excerpted from the report: This report describes the calculation of the optical distortion of the FOC including COSTAR, based on ray tracing. Observations of 47 Tuc at different roll angles demonstrate that the pipeline calibration file for the f/96 relay does in fact remove most of the distortion. A procedure for creating geometric correction reference files, combining optical and detector distortions, is also described in detail. Poly- nomial fits that model the optical distortion for each relay have been installed in the 'focgeom' package directory.