Far sidelobes measurement of the Atacama Cosmology Telescope

摘要

The Atacama Cosmology Telescope (ACT) is a 6m telescope designed to map the Cosmic Microwave Background(CMB) simultaneously at 145GHz, 220 GHz and 280 GHz. Its off-axis Gregorian design is intended to minimizeand control the off-axis sidelobe response, which is critical for scientific purposes. The expected sidelobe level forthis kind of design is less than -50 dB and can be challenging to measure. Here we present a measurement of the145GHz far sidelobes of ACT done on the near-field of the telescope. We used a 1mW microwave source placed13 meters away from the telescope and a chopper wheel to produce a varying signal that could be detected by thecamera for different orientations of the telescope. The source feed was designed to produce a wide beam profile.Given that the coupling is expected to be dominated by diffraction over the telescope shielding structure, whencombined with a measurements of the main beam far field response, these measurement can be used to validateelements of optical design and constrain the level of spurious coupling at large angles. Our results show that thediffractive coupling beyond the ground screen is consistently below -75 dB, satisfying the design expectations.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.