The effects of the mechanical performance and alignment of the Atacama Cosmology Telescope on the sensitivity of microwave observations

The Atacama Cosmology Telescope is a six meter, off-axis Gregorian telescope for measuring the cosmic microwave background at arcminute resolutions. The Millimeter Bolometer Array Camera (MBAC) is its current science instrument. Erected in the Atacama Desert of Chile in early 2007, it saw first light with the MBAC on 22 October 2007. In this paper we review its performance after one month of observing, focusing in particular on issues surrounding the alignment of the optical system that impact the sensitivity of the experiment. We discuss the telescope motion, pointing, and susceptibility to thermal distortions. We describe the mirror alignment procedure, which has yielded surface deviations of 31 mu m rms on the primary and 10 mu m rms on the secondary. Observations of planets show that the optical performance is consistent with the telescope design parameters. Preliminary analysis measures a, solid angle of about 215 nanosteradians with a full width at half maximum of 1.44 arcminutes at 145 GHz.