1. Apparatus for imaging an area of a surface along a viewing angle that is oblique to the surface, the optics comprising: an afocal optical relay, which is adapted to form a tilted initial image of the area by collecting optical radiation from the area along an optical axis oriented at the viewing angle; a tilt correction unit, which is coupled to correct a tilt of the initial image so as to form a substantially undistorted intermediate image; and a magnification module, which is coupled to focus the intermediate image onto an image detector.