An Instrument for Investigation of the Cosmic Microwave Background Radiation at Intermediate Angular Scales

We describe an off-axis microwave telescope for observations of the anisotropy in the cosmic microwave background (CMB) radiation on angular scales between 0.5d and 3°. The receiver utilizes cryogenic high electron mobility transistor (HEMT) amplifiers and detects the total power in multiple 3 GHz wide channels. Both frequency and polarization information are recorded allowing discrimination between CMB radiation and potential foreground sources and allowing checks for systematic effects. The instrumental radiometric offset is small (~1 mK). Data are taken by rapidly sampling while sweeping the beam many beamwidths across the sky. After detection, a spatio-temporal filter is formed in software that optimizes the sensitivity in a multipole band in the presence of atmospheric fluctuations. Observations were made from Saskatoon, Saskatchewan (SK), Canada, during the winter of 1993 with six channels between 27.6 and 34.0 GHz, in 1994 with 12 channels between 27.6 and 44.1 GHz, and in 1995 with six channels between 38.2 and 44.1 GHz. The performance of the instrument and assessment of the atmospheric noise at this site are discussed.