The EBEX balloon-borne experiment-optics, receiver, and polarimetry

• Published in: The Astrophysical Journal Supplement Series
• Main Authors: Aboobaker, Asad M., Ade, Peter, Araujo, Derek, Aubin, François, Baccigalupi, Carlo, Bao, Chaoyun, Chapman, Daniel, Didier, Joy, Dobbs, Matt, Geach, Christopher, Grainger, Will, Hanany, Shaul, Helson, Kyle, Hillbrand, Seth, Hubmayr, Johannes, Jaffe, Andrew, Johnson, Bradley, Jones, Terry, Klein, Jeff, Korotkov, Andrei, Lee, Adrian, Levinson, Lorne, Limon, Michele, MacDermid, Kevin, Matsumura, Tomotake, Miller, Amber D., Milligan, Michael, Raach, Kate, Reichborn-Kjennerud, Britt, Sagiv, Ilan, Savini, Giorgio, Spencer, Locke, Tucker, Carole, Tucker, Gregory S., Westbrook, Benjamin, Young, Karl, Zilic, Kyle
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Astronomical Society 2018
• Subjects: Antarc*; Antarctica
• Online Access: https://orca.cardiff.ac.uk/id/eprint/117847/; https://doi.org/10.3847/1538-4365/aae434; https://orca.cardiff.ac.uk/id/eprint/117847/1/The_EBEX_Collaboration_2018_ApJS_239_7.pdf

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 high-throughput 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 0fdg01. 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.