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

• Published in: The Astrophysical Journal Supplement Series
• Main Authors: Aboobaker, A. M., 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, A. D., Milligan, M., Raach, K., Reichborn-Kjennerud, B., Sagiv, I., Savini, G., Spencer, L., Tucker, C., Tucker, G. S., Westbrook, B., Young, K., Zilic, K.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2018
• Subjects: balloon; cosmic background radiation; cosmology: observation; instrumentation: polarimeter; polarization; Settore FIS/05 - Astronomia e Astrofisica; Antarc*; Antarctica
• Online Access: https://hdl.handle.net/20.500.11767/111372; https://doi.org/10.3847/1538-4365/aae434; https://arxiv.org/abs/1703.03847

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 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.