Multiple-order diffractive engineered surface lenses.

A multiple-order diffractive engineered surface (MODE) lens is introduced, in which focal position change with wavelength exhibits both refractive and diffractive characteristics. Engineering calculations are provided that indicate Strehl ratio and encircled energy performance over a large range of focal length and aperture diameter design space. A prototype lens is designed and constructed for the astronomical R-band (589 nm to 727 nm) wavelength range. Test results show that measured full-width-at-half-maximum focal spot diameter is 2.1 times larger than the ideal Airy spot diameter, and focal position versus wavelength is nearly identical to the design. The 48 mm aperture diameter, /3.12 prototype telescope exhibits angularly resolved features in natural scenes at 0.006°, with subtense of the Airy spot diameter at 0.002°. Applications include eventual use in large aperture, ultralightweight space telescopes.