Distortion calculation and removal for an off-axis and wide angle camera

For off-axis and wide angle systems, the calculation, calibration and removal of distortion effects from the images are often challenging tasks. Specific procedures have been implemented to assess and remove the distortion from the images acquired by the OSIRIS imaging instrument on-board the Rosetta ESA mission. OSIRIS consisted in a narrow and a wide angle camera. The Wide Angle Camera (WAC) is an off-axis, unobstructed and wide FoV (i.e. about 12°x12°) optical system. It has a peculiar optical configuration, and due to the off-axis design the camera presents a high level of intrinsic distortion, with the major component being anamorphism. The distortion has been estimated theoretically via raytracing during the design phase, then measured on-ground and inflight during the calibration campaigns. To obtain correct undistorted images, a distortion removal procedure has been implemented. The first step of the process has been to remove from the images the theoretical distortion. Then the distortion correction procedure has been refined using on-ground and in-flight calibration measurements. This work describes in detail the development of the procedure adopted to define, calculate and remove the distortion from the WAC images.