The optical design of the Background Emission Anisotropy Scanning Telescope (BEAST)

We present the optical design of the Background Emission Anisotropy Scanning Telescope (BEAST), an off-axis Gregorian telescope designed to measure the angular distribution of the cosmic microwave background radiation (CMBR) at 30 and 41.5 GHz on angular scales ranging from 20' to 10 degrees. The aperture of the telescope is 1.9 m, and our design meets the strict requirements imposed by the scientific goals of the mission: the beam size is 200 at 41.5 GHz and 260 at 30 GHz, while the illumination at the edge of the mirrors is lower than -30 dB for the central horn. The primary mirror is an off-axis section of a paraboloid, and the secondary an off-axis section of an ellipsoid. A spinning flat mirror located between the sky and the primary provides a two-dimensional chop by rotating the beams around an ellipse on the sky. BEAST uses a receiver array of cryogenic low noise InP High Electron Mobility Transistor (HEMT) amplifiers. The baseline array has seven horns matched to one amplifier each and one horn matched to two amplifiers (two polarizations) for a total of nine amplifiers. Two horns operate around 30 GHz, and six operate around 41.5 GHz. Subsequent campaigns will include 90 GHz and higher frequency channels.