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

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cm 2 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.

A cryogenic continuously rotating half-wave plate mechanism for the POLARBEAR-2b cosmic microwave background receiver

“…The CHWP motor needs to drive stable 2 Hz rotation while being low-dissipation, low-magnetic-interference, and mechanically robust at cryogenic temperatures. While a belt drive with an external motor has been successfully operated during weeks of CMB observation, 24,70 the PB-2b CHWP must run for years, motivating a drive system free of mechanical fatigue. Therefore, we utilize a brushless, three-phase, synchronous electromagnetic motor driven by custom electronics.…”

Trajectories of long duration balloons launched from McMurdo Station in Antarctica

“…Long duration balloon (LDB) flights have been launched by NASA from McMurdo Station in Antarctica since the early 1990s. With the logistical support of NSF's Office of Polar Programs, more than 50 large scientific payloads have flown to date, addressing a wide array of science topics encompassing the structure and evolution of the universe, 1 the nature of high energy particles, 2,3 the origins of solar activity, 4,5 the precipitation of magnetospheric electrons, 6 and the processes controlling the formation of stars in the Milky Way. 7 In the Antarctic summer, the weakened polar vortex carries stratospheric balloons westward on a generally circumpolar trajectory.…”

Efficient ILC analysis on polarization maps after EB leakage correction

“…For now, the joint analysis of BICEP2/Keck Array, Planck and WMAP yields the constraint on tensor-scalar ratio r is r 0.05 < 0.07 at 95% confidence level [2]. Also, in near future, several large and small aperture ground-based microwave telescopes, as well as stratospheric balloons [3][4][5][6], are known to be deployed or planned for the detection in South pole [7][8][9][10], Atacama [11][12][13][14][15][16], and Tibet [17].…”