We present a method of cross-calibrating the polarization angle of a polarimeter using Bicep Galactic observations.
Bicep was a ground based experiment using an array of 49 pairs of polarization sensitive bolometers
observing from the geographic South Pole at 100 and 150 GHz. The Bicep polarimeter is calibrated to ±0.01
in cross-polarization and less than ±0.7° in absolute polarization orientation. Bicep observed the temperature
and polarization of the Galactic plane (R.A = 100° ~ 270° and Dec. = -67° ~ -48°). We show that the
statistical error in the 100 GHz Bicep Galaxy map can constrain the polarization angle offset of Wmap W band
to 0.6° ± 1.4°. The expected 1σ errors on the polarization angle cross-calibration for Planck or EPIC are 1.3°
and 0.3° at 100 and 150 GHz, respectively. We also discuss the expected improvement of the Bicep Galactic
field observations with forthcoming Bicep2 and Keck observations.
Bicep is a ground-based millimeter-wave bolometric array designed to target the primordial gravity wave signature
on the B-mode polarization of the cosmic microwave background (CMB) at degree angular scales. Currently
in its third year of operation at the South Pole, Bicep is measuring the CMB polarization with unprecedented
sensitivity at 100 and 150 GHz in the cleanest available 2% of the sky, as well as deriving independent constraints
on the diffuse polarized foregrounds with select observations on and off the Galactic plane. Instrument
calibrations are discussed in the context of rigorous control of systematic errors, and the performance during the
first two years of the experiment is reviewed.
We report on the design and tests of a prototype of the Millimeter-wave Bolometric Interferometer (MBI). MBI
is designed to make sensitive measurements of the polarization of the cosmic microwave background (CMB). It
combines the differencing capabilities of an interferometer with the high sensitivity of bolometers at millimeter
wavelengths. The prototype, which we call MBI-4, views the sky directly through four corrugated horn antennas.
MBI ultimately will have ~ 1000 antennas. These antennas have low sidelobes and nearly symmetric beam
patterns, so spurious instrumental polarization from reflective optics is avoided. The MBI-4 optical band is
defined by filters with a central frequency of 90 GHz. The set of baselines, determined by placement of the four
antennas, results in sensitivity to CMB polarization fluctuations over the multipole range ℓ = 150 - 270. The
signals are combined with a Fizeau beam combiner and interference fringes are detected by an array of spider-web
bolometers. In order to separate the visibility signals from the total power detected by each bolometer, the
phase of the signal from each antenna is modulated by a ferrite-based waveguide phase shifter. Initial tests and
observations have been made at Pine Bluff Observatory (PBO) outside Madison, WI.
The Millimeter-Wave Bolometric Interferometer (MBI) is designed for sensitive measurements of the polarization of the cosmic microwave background (CMB). MBI combines the differencing capabilities of an interferometer with the high sensitivity of bolometers at millimeter wavelengths. It views the sky directly through corrugated horn antennas with low sidelobes and nearly symmetric beam patterns to avoid spurious instrumental polarization from reflective optics. The design of the first version of the instrument with four 7-degree-FOV corrugated horns (MBI-4) is discussed. The MBI-4 optical band is defined by filters with a central frequency of 90 GHz. The set of baselines determined by the antenna separation makes the instrument sensitive to CMB polarization fluctuations over the multipole range <i>l</i>=150-270. In MBI-4, the signals from antennas are combined with a Fizeau beam combiner and interference fringes are detected by an array of spider-web bolometers with NTD germanium thermistors. In order to separate the visibility signals from the total power detected by each bolometer, the phase of the signal from each antenna is modulated by a ferrite-based waveguide phase shifter. Observations are planned from the Pine Bluff Observatory outside Madison, WI.
The Robinson Telescope (BICEP) is a ground-based millimeter-wave bolometric array designed to study the polarization of the cosmic microwave background radiation (CMB) and galactic foreground emission. Such measurements probe the energy scale of the inflationary epoch, tighten constraints on cosmological parameters, and verify our current understanding of CMB physics. Robinson consists of a 250-mm aperture refractive telescope that provides an instantaneous field-of-view of 17° with angular resolution of 55' and 37' at 100 GHz and 150 GHz, respectively. Forty-nine pair of polarization-sensitive bolometers are cooled to 250 mK using a <sup>4</sup>He/<sup>3</sup>He/<sup>3</sup>He sorption fridge system, and coupled to incoming radiation via corrugated feed horns. The all-refractive optics is cooled to 4 K to minimize polarization systematics and instrument loading. The fully steerable 3-axis mount is capable of continuous boresight rotation or azimuth scanning at speeds up to 5 deg/s. Robinson has begun its first season of observation at the South Pole. Given the measured performance of the instrument along with the excellent observing environment, Robinson will measure the E-mode polarization with high sensitivity, and probe for the B-modes to unprecedented depths. In this paper we discuss aspects of the instrument design and their scientific motivations, scanning and operational strategies, and the results of initial testing and observations.