The Background Emission Anisotropy Scanning Telescope (BEAST) Instrument Description and Performances

The Background Emission Anisotropy Scanning Telescope (BEAST) is a millimeter wavelength experiment designed to generate maps of fluctuations in the cosmic microwave background (CMB). The telescope is composed of an off-axis Gregorian optical system with a 2.2 m primary that focuses the collected microwave radiation onto an array of cryogenically cooled high electron mobility transistor (HEMT) receivers. This array is composed of six corrugated scalar feed horns in the Q band (38 to 45 GHz) and two more in the Ka band (26 to 36 GHz) with one of the six Q-band horns connected to an ortho-mode transducer for extraction of both polarizations incident on the single feed. The system has a minimum beam size of 20' with an average sensitivity of 900 μK per receiver. This paper describes the design and performance of the BEAST instrument and provides the details of subsystems developed and used toward the goal of generating a map of CMB fluctuations on 20' scales with sensitivity in l space between l ∼ 100 and l ∼ 500. A map of the CMB centered on the north celestial pole has been generated from the BEAST telescope in a 9° wide annulus at declination 37° with a typical pixel error of 57 ± 5 μK when smoothed to 30' resolution. A brief summary of the map and results generated by an observing campaign at the University of California White Mountain Research Station are also included.