The TopHat experiment was designed to measure the anisotropy in the cosmic microwave background radiation on angular scales from 0N3 to 30 and the thermal emission from both Galactic and extragalactic dust. The balloon-borne instrument had five spectral bands spanning frequencies from 175 to 630 GHz. The telescope was a compact, 1 m, onaxis Cassegrain telescope designed to scan the sky at a fixed elevation of 78 . The radiometer used cryogenic bolometers coupled to a single feed horn via a dichroic filter system. The observing strategy was intended to efficiently cover a region 48 in diameter centered on the south polar cap with a highly cross-linked and redundant pattern with nearly uniform sky coverage. The Long Duration Balloon flight over Antarctica in 2001 January surveyed about 6% of the sky. Here we describe the design of the instrument and the achieved in-flight performance and provide a brief discussion of the data analysis.
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