Design aspects of large-aperture MODE lenses

Design aspects of multiple-order diffraction engineered surface (MODE) lenses are discussed that result in significant improvement of geometrical off-axis performance. A new type of aberration that is characteristic of this type of segmented lens, which is called zonal field shift, is minimized by curving front intercepts of zone transitions. Three MODE designs are compared, based on a 240 mm aperture, 1 m focal length system with a 0.125° half field angle over the astronomical R wavelength band (589 nm to 727 nm). Optimized curved-front designs indicate diffraction-limited monochromatic geometrical performance over the full field of view. A technique is implemented with a combination of a non-sequential ray-trace model and a diffraction code to model physical optical effects, which indicates that the modulation transfer function (MTF) of MODE lenses are significantly improved compared to an equivalent refractive achromat.