The EBEX Balloon-borne Experiment—Optics, Receiver, and Polarimetry

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https://doi.org/10.3847/1538-4365/aae434
 http://hdl.handle.net/11603/24146

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UMBC Center for Space Sciences and Technology (CSST) / Center for Research and Exploration in Space Sciences & Technology II (CRSST II)

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https://orcid.org/0000-0001-9238-4918

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2018-11-06

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journal articles
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Aboobaker, Asad M. et al. The EBEX Balloon-borne Experiment—Optics, Receiver, and Polarimetry. The Astrophysical Journal Supplement Series 239 (Nov. 6, 2018) 1. https://doi.org/10.3847/1538-4365/aae434.

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This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
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Abstract

The E and B Experiment (EBEX) was a long-duration balloon-borne cosmic microwave background (CMB) polarimeter that flew over Antarctica in 2012. We describe the experiment’s optical system, receiver, and polarimetric approach and report on their in-flight performance. EBEX had three frequency bands centered on 150, 250, and 410 GHz. To make efficient use of limited mass and space, we designed a 115 cm2 sr highthroughput optical system that had two ambient temperature mirrors and four antireflection-coated polyethylene lenses per focal plane. All frequency bands shared the same optical train. Polarimetry was achieved with a continuously rotating achromatic half-wave plate (AHWP) that was levitated with a superconducting magnetic bearing (SMB). This is the first use of an SMB in astrophysics. Rotation stability was 0.45% over a period of 10 hr, and angular position accuracy was 0°. 01. The measured modulation efficiency was above 90% for all bands. To our knowledge the 109% fractional bandwidth of the AHWP was the broadest implemented to date. The receiver, composed of one lens and the AHWP at a temperature of 4 K, the polarizing grid and other lenses at 1 K, and the two focal planes at 0.25 K, performed according to specifications, giving focal plane temperature stability with a fluctuation power spectrum that had a 1/f knee at 2 mHz. EBEX was the first balloon-borne instrument to implement technologies characteristic of modern CMB polarimeters, including high-throughput optical systems, and large arrays of transition edge sensor bolometric detectors with multiplexed readouts.