Further designs of optomechanical scanners for use in passive millimeter-wave imaging

Mm-wave imaging has high potential for all weather performance but requires large apertures to acheive acceptable spatial resolution. These large apertures need to collect radiation from a range of angles in the field of view and form a two dimensional image. The ideal method for achieving such an image would be to have an aperture completely filled with receivers and to electronically beam-form. Unfortunately this technology is not sufficiently developed at present to make this a practical propostion. Receivers are far too expensive to form a completely filled aperture and the technology of beam-forming is still under development. The alternative and most cost effective solution at present is to have a comparatively small number of receivers and scan them across the scene using an opto-mechanical scanner. In scanned thermal imaging systems it is usual to employ high speed rotating polygons to perform the line scan and a flapping mirror for the framing motion. The pupil size is typically 10mm in thermal imaging and the polygons are 40mm in diamenter. If such an arrangement were used at mm-waves where the pupil size is of the order of 1m, the rotating polygon would be 4m in diameter. The paper describes new compact opto-mechanical systems based on rotating discs, prisms and using frequency and polarization selective surfaces.