Opto-mechanical design and performance of a compact three-frequency camera for the Millimeter Bolometer Array Camera on the Atacama Cosmology Telescope

The 6-meter Atacama Cosmology Telescope will map the cosmic microwave background at millimeter wavelengths.The commissioning instrument for the telescope, the Millimeter Bolometer Array Camera, is based on arefractive optical system which simultaneously images three separate fields of view at three different frequencies:145, 220, and 280 GHz. Each frequency band contains around twelve individual optical elements at five differenttemperature stages ranging from 300 K to 300 mK and a 32 x 32 array of Transition Edge Sensor bolometers at300 mK. We discuss the design of the close-packed on-axis optical design of the three frequencies. The thermaldesign and performance of the system are presented in the context of the scientific requirements and observingschedule. A major part of the design was the incorporation of multiple layers of magnetic shielding. We discussthe performance of the 145 GHz optical system in 2007 and the implementation of the additional two frequencychannels in 2008