High Accuracy Pointing for Quasi-optical THz Mixer Arrays

We report a high accuracy pointing technique for quasi-optical hot electron bolometer (HEB) mixers in focal-plane arrays designed to operate at 1.4, 1.9 and 4.7 THz. The high accuracy pointing is achieved by pre-alignment of a HEB chip to a lens, measuring the angular error of each mixer in an array assembly, and then re-alignment of the chip to the same lens to correct the error. The re-aligned mixers, using 5 mm diameter Si elliptical lenses designed for operation at 4.7 THz, show a final pointing error distribution with an average () = 0.13 deg and standard deviation () = 0.06 deg, with respect to the normal direction of the respective array plane. Those using 10 mm diameter lenses designed for operation either at 1.4 or 1.9 THz, show = 0.08 deg and = 0.03 deg. We demonstrated our pointing technique in five 42 HEB focal plane arrays developed for NASAs balloon borne GUSTO THz observatory. Our results corroborate the simulated beam steering factors used to calculate the re-alignment corrections. With the unprecedented pointing accuracy at the high frequencies, our technique can significantly facilitate the use of lens-antenna, quasi optical mixers for future focal-plane arrays, which is able to compete with traditional feedhorn-waveguide mixer arrays, operated typically below 1 THz, for astronomical instrumentation.