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 (µ) = 0.13 deg and standard deviation (σ) = 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 µ = 0.08 deg and σ = 0.03 deg. We demonstrated our pointing technique in five 4×2 HEB focal plane arrays developed for NASA's 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.
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