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 v = 90 GHz reveal highly significant detections of anisotropy on intermediate (~1^deg^) angular scales. The instrument is a 2 x 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^deg^, with a beamswitch of +/- 2.75^deg^ 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 {DELTA}T/T of 3.1_- 0.9_^+2.5^ x 10^-5^ and 3.8_-1.2_^+2.9 x 10^-5^ at a coherence angle of 1.0^deg^, 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.