Anisotropy in the microwave sky at intermediate angular scales

Observations of a low-background region of sky at high Galactic latitude in a frequency band centered at ν = 90 GHz reveal highly significant detections of anisotropy on intermediate (∼1°) angular scales. The instrument is a 2 × 2 array of bolometric detectors operating at 50 mK, coupled to a 0.75 m off-axis parabolic telescope by single mode corrugated feedhorns. The FWHM beamwidth of the instrument is 0.°75, with a beamswitch of ±2.°75 on the sky. Interpreting the detected fluctuations as due to anisotropy in the cosmic microwave background, a likelihood analysis assuming a Gaussian correlation function in the temperature fluctuations yields most likely values of ΔT/T of 3.1_-0.9^+2.5 × 10^-5 and 3.8_-1.2^+2.9 × 10^-5 at a coherence angle of 1.°0, for the left- and right-hand detectors, respectively. The stated errors are 95% confidence limits. Confirmation at other frequencies is required to rule out the possibility of foreground contamination.