THE TOPHAT EXPERIMENT: A BALLOON-BORNE INSTRUMENT FOR MAPPING MILLIMETER AND SUBMILLIMETER EMISSION

The TopHat experiment was designed to measure the anisotropy in the cosmic microwave background radiation on angular scales from 03 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, on-axis 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.