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Proceedings Paper

Mirror illumination and spillover measurements of the Atacama Cosmology Telescope
Author(s): Patricio Gallardo; Rolando Dünner; Edward Wollack; Fernando Henriquez; Carlos Jerez-Hanckes
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Paper Abstract

The Atacama Cosmology Telescope (ACT) is a 6 m telescope designed to map the Cosmic Microwave Background (CMB) simultaneously at 145 GHz, 220 GHz and 280GHz. The receiver in ACT, the Millimeter Bolometer Array Camera, features 1000 TES bolometers in each band. The detector performance depends critically on the total optical loading, requiring the spillover contributions from the optics to be minimal. This inspired the use of a cold Lyot stop to limit the illumination of the primary and the use of guard rings surrounding the primary and secondary reflectors. Here, we present a direct measurement of the illumination aperture for both reflectors and of the attenuation level outside the main optical path. We used a 145 GHz, 1mW source and a chopper wheel to produce a time-varying signal with a broad beam profile. We sampled the response of the camera for different locations of the source, placed in front and beside the primary and secondary mirrors. The aperture of the primary was measured to be 5.72 ± 0.17m in diameter (95 ± 3% of its geometrical size), while the aperture of the secondary yielded 2 ± 0.12m in diameter. Both apertures are consistent with the optical design. Comparing to previous measurements of the beam solid angle from planet observations, we estimate an optical efficiency of 72.3 ± 4.8%. We found that the attenuation outside the primary aperture was −16 ± 2 dB, which is below the theoretical expectations, and −22 ± 1 dB outside the secondary aperture, which is consistent with simulations. These results motivated the extension of the baffles surrounding the secondary mirror, with the following reduction in detector optical loading from 2.24pW to 1.88 pW.

Paper Details

Date Published: 24 September 2012
PDF: 11 pages
Proc. SPIE 8452, Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VI, 845224 (24 September 2012); doi: 10.1117/12.926585
Show Author Affiliations
Patricio Gallardo, Pontificia Univ. Católica de Chile (Chile)
Rolando Dünner, Pontificia Univ. Católica de Chile (Chile)
Edward Wollack, NASA Goddard Space Flight Ctr. (United States)
Fernando Henriquez, Pontificia Univ. Católica de Chile (Chile)
Carlos Jerez-Hanckes, Pontificia Univ. Católica de Chile (Chile)


Published in SPIE Proceedings Vol. 8452:
Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VI
Wayne S. Holland, Editor(s)

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