A Spin-Modulated Telescope to Make Two-Dimensional Cosmic Microwave Background Maps

We describe the HEMT Advanced Cosmic Microwave Explorer (HACME), a balloon-borne experiment designed to measure subdegree-scale cosmic microwave background anisotropy over hundreds of deg2, using a unique two-dimensional scanning strategy. A spinning flat mirror that is canted relative to its spin axis modulates the direction of beam response in a nearly elliptical path on the sky. The experiment was successfully flown in 1996 February, achieving near laboratory performance for several hours at float altitude. A map free of instrumental systematic effects is produced for a 3.5 hr observation of 630 deg2, resulting in a flat-band power upper limit of ⟨l(l + 1)Cl/(2π)⟩0.5 < 77 μK at l = 38 (95% confidence). The experiment design, flight operations, and data, including atmospheric effects and noise performance, are discussed.