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 backg round anisotropy over hundreds of deg(2), using a unique two-dimensional sc anning 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, achievi ng near laboratory performance for several hours at float altitude. A map f ree of instrumental systematic effects is produced for a 3.5 hr observation of 630 deg(2), resulting in a flat-band power upper limit of [l(l + 1)C-I/ (2 pi)](0.5) < 77 mu K at l = 38(-20)(+25) (95% confidence). The experiment design, flight operations, and data, including atmospheric effects and noi se performance, are discussed.