Mapping the southern polar cap with a balloon-borne millimeter-wave telescope

TopHat is a balloon-borne millimeter-wave telescope designed to make a map of a 48°-diameter region centered on the South Celestial Pole. The instrument consists of telescope optics, radiometer, rotational drive system, sun/earth shield, attitude and thermal sensors, and support electronics mounted on top of a 28-million cubic foot balloon, with a support gondola hanging below. The five-channel, single-pixel radiometer sits at the focus of an on-axis Cassegrain telescope with a 1-meter primary aperture. The detectors are monolithic, ion-implanted silicon bolometers, cooled to 265 mK by a sorption-pumped helium-3 fridge. The five frequency bands have effective centers of 175, 245, 400, 460, and 630 GHz. The two lowest-frequency bands are designed to be sensitive to the 2.7 K Cosmic Microwave Background (CMB), while the three highest bands are designed to monitor thermal emission from interstellar dust grains. Together with a modified Winston cone at the Cassegrain focus, the telescope optics define a beam designed to be steeper than gaussian with a full-width at half-maximum of 20′, rendering TopHat in principle sensitive to fluctuations in the CMB from scales of less than a degree up to the diameter of the map (6 ≤ ℓ ≤ 600). TopHat was launched on 4 January 2001 from McMurdo Station, Antarctica as part of the NASA National Scientific Balloon Facility (NSBF) Polar Long-Duration Balloon program and observed for four sidereal days until cryogens were exhausted. An unexpected ∼5° tilt in the mounting platform at the top of the balloon resulted in large scan-synchronous instrumental signals which were not removable at the level necessary to make an internally consistent measurement of the CMB power spectrum. Minimum-variance maps of the data in all five channels have been made and used to measure the integrated flux from three regions in the Magellanic Clouds, using a flux analysis technique that minimizes the aforementioned instrumental contamination. When combined with data from the COBE/DIRBE instrument, these measurements provide a first look at the integrated emission from extragalactic environments with nearly uniform frequency coverage over the range of 245 to 3000 GHz.