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 offaxis Gregorian telescope designed to measure the angular distribution of the cosmic microwave background radiation (CMBR)at30and 41.5 GHzonangularscalesrangingfrom 20 0 to10 � .Theapertureof thetelescope is1.9m, and our design meets the strict requirements imposed by the scientific goals of the mission: the beam size is 20 0 at 41.5 GHz and 26 0 at 30 GHz, while the illumination at the edge of the mirrors is lower than � 30 dB for the central horn.Theprimarymirror isanoff-axissectionofaparaboloid,andthesecondaryanoff-axissectionofanellipsoid.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 matchedtotwoamplifiers(twopolarizations)foratotalofnineamplifiers.Twohornsoperatearound30GHz,andsix operate around 41.5 GHz. Subsequent campaigns will include 90 GHz and higher frequency channels. Subject heading gs: cosmic microwave background — cosmology: observations — telescopes