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Mirror Illumination and Spillover Measurements of the Atacama Cosmology TelescopeThe Atacama Cosmology Telescope (ACT) is a 6 m telescope designed to map the Cosmic Microwave Background (CMB) simultaneously at 145 GHz, 220GHz 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 spmover 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, 1 m W source and a chopper wheel to produce a time-varying signal with a broad heam proflle, 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 plus or minus 0,17m in diameter (95 plus or minus 3% of its geometrical size), while the aperture of the secondary yielded 2 plus or minus 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 plus or minus 4,8%. We found that the attenuation outside the primary aperture was -16 plus or minus 2dB, which is below the theoretical expectations, and -22 plus or minus 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 2,24 pW to 188pW.
Document ID
20120015623
Acquisition Source
Goddard Space Flight Center
Document Type
Conference Paper
Authors
Gallardo, Patricio
(Pontificia Univ. Catolica de Chile Santiago, Chile)
Dunner, Rolando
(Pontificia Univ. Catolica de Chile Santiago, Chile)
Wollack, Ed
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Jerez-Hanckes, Carlos
(Pontificia Univ. Catolica de Chile Santiago, Chile)
Date Acquired
August 26, 2013
Publication Date
July 1, 2012
Subject Category
Astronomy
Report/Patent Number
GSFC.CP.7100.2012
Meeting Information
Meeting: Society of Photo-Optical Instrumentation Engineers conference
Location: Amsterdam
Country: Netherlands
Start Date: July 1, 2012
End Date: July 7, 2012
Sponsors: International Society for Optical Engineering
Distribution Limits
Public
Copyright
Public Use Permitted.
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