Examples are directed to optimal field mappings that provide the highest contrast images for back-scanned imaging methods. The mappings can be implemented for back-scanned imaging with afocal optics including an anamorphic field correcting assembly configured to implement a non-rotationally symmetric field mapping between object space and image space to adjust distortion characteristics of the afocal optics to control image wander on a focal plane array. The anamorphic field correcting assembly can include one or more mirrors or lenses having non-rotationally symmetric aspherical departures.