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We will present preliminary results of our experiment to measure the anisotropy of the microwave background on medium (2.75 degree) angular scales. Observations of 14 separate regions (two rows of seven regions) on the sky were made during the 15 day period of 1 Jan to 15 Jan 1993. The observing site, the geographic South Pole, is an ideal location for microwave background anisotropy studies where long integration times, low sky background (10K at 90GHz), and stable atmospheric conditions are required. The geometric advantage of the Pole is that a patch of sky never rises or sets, allowing 24 hour observations of the patch with no corrections required for changes in elevation. Approximately 100 hours of usable data were obtained during this time.
The instrument is a five channel bolometer array, with the detectors operating at 50 mK, cooled by a hybrid $^3$He-Adiabatic demagnetization refrigerator. There are four pixels in the focal plane arranged in a square grid so that the angular separation between pixels on the sky is 2.75 degrees. The fifth channel is a dark channel acting as a monitor for extraneous noise or pickup. Individual pixels consist of corrugated feedhorns coupled to waveguide waffle filters combined with glass and plastics to define the high frequency cutoff (110 GHz); the low frequency cutoff is determined by the cutoff of the waveguide (75 GHz). The band was selected for minimal expected contributions from foreground sources such as free-free, synchrotron and cool dust.
This detector system is coupled to the sky via a 0.75m off axis parabolic primary. A gradient free 3-beam switching pattern on the sky is obtained by moving a low moment of inertia flat plate quickly (15 msec transition time, with 85 msec spent integrating on the point) between 3 points on the sky. The plate is oriented vertically so the switching is at constant elevation.
This work was supported by The Center for Astrophysics in Antarctica, PYI grant NSF AST-9057089 and the James S. McDonnell Foundation.
