Observations of the microwave background on a scale of 8 - I. The observing system

A sensitive observing system for observing the cosmic microwave background on an angular scale of 8° is described. This angular scale is appropriate for studies of intrinsic fluctuations in the CMB generated by gravitational potential fluctuations (the Sachs–Wolfe effect). The beam-switching system employs low-noise and broad-band cryogenically cooled receivers. A major improvement in the stability of the system has resulted from triple-beam operation which removes the effect of atmospheric water vapour and ambient temperature changes to a level of several millikelvin on a single scan. The evaluation and assessment of this system operating at an elevation of 2400 m on Tenerife is described. Results for an RA circle at Dec. = 40° are shown; these reach a sensitivity of better than 100 µK in an $8^\circ_. 4$ beam.