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

• Main Authors: Aboobaker, AM, Ade, P, Araujo, D, Aubin, F, Baccigalupi, C, Bao, C, Chapman, D, Didier, J, Dobbs, M, Geach, C, Grainger, W, Hanany, S, Helson, K, Hillbrand, S, Hubmayr, J, Jaffe, A, Johnson, B, Jones, T, Klein, J, Korotkov, A, Lee, A, Levinson, L, Limon, M, MacDermid, K, Matsumura, T, Miller, AD, Milligan, M, Raach, K, Reichborn-Kjennerud, B, Sagiv, I, Savini, G, Spencer, L, Tucker, C, Tucker, GS, Westbrook, B, Young, K, Zilic, K
• Format: Article in Journal/Newspaper
• Language: English
• Published: IOP PUBLISHING LTD 2018
• Subjects: Antarc*; Antarctica
• Online Access: https://discovery.ucl.ac.uk/id/eprint/10064677/1/The_EBEX_Collaboration_2018_ApJS_239_7.pdf; https://discovery.ucl.ac.uk/id/eprint/10064677/

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.