A MEDIUM-SCALE MEASUREMENT OF THE COSMIC MICROWAVE BACKGROUND AT 3.3 MILLIMETERS

We have developed a system for making measurements of spatial fluctuations in the cosmic microwave background radiation, on an angular scale of 5' to a few degrees. The system consists of an off-axis Gregorian telescope with a nearly Gaussian response with full width at half-maximum (FWHM) adjustable from 20' to 50', a superconductor-insulator-superconductor (SIS) coherent receiver operating at 3.3 mm, and a pointing system capable of better than 1' RMS stabilization. We report on results from the system's first balloon flight in 1988 August, and ground-based measurements made from the South Pole in 1988 December. We use a portion of the South Pole data to place a 95% confidence level upper limit of DELTA-T/T < 3.5 x 10(-5) for Gaussian sky fluctuations in the background radiation at 20' angular scale and a limit of DELTA-T/T < 3.3 x 10(-5) on overall excess intrinsic sky noise. We also estimate dust contamination in our cosmic background radiation data using measurements of the Galaxy from this flight and a previous one, along with the IRAS 100-mu-m map. These anisotropy results give the most stringent limits on cold dark matter theories to date.