35 research outputs found
First Season QUIET Observations: Measurements of CMB Polarization Power Spectra at 43 GHz in the Multipole Range 25 <= ell <= 475
The Q/U Imaging ExperimenT (QUIET) employs coherent receivers at 43GHz and
95GHz, operating on the Chajnantor plateau in the Atacama Desert in Chile, to
measure the anisotropy in the polarization of the CMB. QUIET primarily targets
the B modes from primordial gravitational waves. The combination of these
frequencies gives sensitivity to foreground contributions from diffuse Galactic
synchrotron radiation. Between 2008 October and 2010 December, >10,000hours of
data were collected, first with the 19-element 43GHz array (3458hours) and then
with the 90-element 95GHz array. Each array observes the same four fields,
selected for low foregrounds, together covering ~1000deg^2. This paper reports
initial results from the 43GHz receiver which has an array sensitivity to CMB
fluctuations of 69uK sqrt(s). The data were extensively studied with a large
suite of null tests before the power spectra, determined with two independent
pipelines, were examined. Analysis choices, including data selection, were
modified until the null tests passed. Cross correlating maps with different
telescope pointings is used to eliminate a bias. This paper reports the EE, BB
and EB power spectra in the multipole range ell=25-475. With the exception of
the lowest multipole bin for one of the fields, where a polarized foreground,
consistent with Galactic synchrotron radiation, is detected with 3sigma
significance, the E-mode spectrum is consistent with the LCDM model, confirming
the only previous detection of the first acoustic peak. The B-mode spectrum is
consistent with zero, leading to a measurement of the tensor-to-scalar ratio of
r=0.35+1.06-0.87. The combination of a new time-stream double-demodulation
technique, Mizuguchi-Dragone optics, natural sky rotation, and frequent
boresight rotation leads to the lowest level of systematic contamination in the
B-mode power so far reported, below the level of r=0.1Comment: 19 pages, 14 figures, higher quality figures are available at
http://quiet.uchicago.edu/results/index.html; Fixed a typo and corrected
statistical error values used as a reference in Figure 14, showing our
systematic uncertainties (unchanged) vs. multipole; Revision to ApJ accepted
version, this paper should be cited as "QUIET Collaboration et al. (2011)
The QUIET Instrument
The Q/U Imaging ExperimenT (QUIET) is designed to measure polarization in the
Cosmic Microwave Background, targeting the imprint of inflationary
gravitational waves at large angular scales (~ 1 degree). Between 2008 October
and 2010 December, two independent receiver arrays were deployed sequentially
on a 1.4 m side-fed Dragonian telescope. The polarimeters which form the focal
planes use a highly compact design based on High Electron Mobility Transistors
(HEMTs) that provides simultaneous measurements of the Stokes parameters Q, U,
and I in a single module. The 17-element Q-band polarimeter array, with a
central frequency of 43.1 GHz, has the best sensitivity (69 uK sqrt(s)) and the
lowest instrumental systematic errors ever achieved in this band, contributing
to the tensor-to-scalar ratio at r < 0.1. The 84-element W-band polarimeter
array has a sensitivity of 87 uK sqrt(s) at a central frequency of 94.5 GHz. It
has the lowest systematic errors to date, contributing at r < 0.01. The two
arrays together cover multipoles in the range l= 25-975. These are the largest
HEMT-based arrays deployed to date. This article describes the design,
calibration, performance of, and sources of systematic error for the
instrument
Second Season QUIET Observations: Measurements of the CMB Polarization Power Spectrum at 95 GHz
The Q/U Imaging ExperimenT (QUIET) has observed the cosmic microwave
background (CMB) at 43 and 95GHz. The 43-GHz results have been published in
QUIET Collaboration et al. (2011), and here we report the measurement of CMB
polarization power spectra using the 95-GHz data. This data set comprises 5337
hours of observations recorded by an array of 84 polarized coherent receivers
with a total array sensitivity of 87 uK sqrt(s). Four low-foreground fields
were observed, covering a total of ~1000 square degrees with an effective
angular resolution of 12.8', allowing for constraints on primordial
gravitational waves and high-signal-to-noise measurements of the E-modes across
three acoustic peaks. The data reduction was performed using two independent
analysis pipelines, one based on a pseudo-Cl (PCL) cross-correlation approach,
and the other on a maximum-likelihood (ML) approach. All data selection
criteria and filters were modified until a predefined set of null tests had
been satisfied before inspecting any non-null power spectrum. The results
derived by the two pipelines are in good agreement. We characterize the EE, EB
and BB power spectra between l=25 and 975 and find that the EE spectrum is
consistent with LCDM, while the BB power spectrum is consistent with zero.
Based on these measurements, we constrain the tensor-to-scalar ratio to
r=1.1+0.9-0.8 (r<2.8 at 95% C.L.) as derived by the ML pipeline, and
r=1.2+0.9-0.8 (r<2.7 at 95% C.L.) as derived by the PCL pipeline. In one of the
fields, we find a correlation with the dust component of the Planck Sky Model,
though the corresponding excess power is small compared to statistical errors.
Finally, we derive limits on all known systematic errors, and demonstrate that
these correspond to a tensor-to-scalar ratio smaller than r=0.01, the lowest
level yet reported in the literature.Comment: 10 pages, 6 figures, 3 tables, submitted to ApJ, This paper should be
cited as "QUIET Collaboration (2012)." v2: updated to reflect published
versio
Recommended from our members
Novel calibration system with sparse wires for CMB polarization receivers
B-modes in the cosmic microwave background (CMB) polarization is a smoking gun signature of the inflationary universe. To achieve better sensitivity to this faint signal, CMB polarization experiments aim to maximize the number of detector elements, resulting in a large focal plane receiver. Detector calibration of the polarization response becomes essential. It is extremely useful to be able to calibrate 'simultaneously' all detectors on the large focal plane. We developed a novel calibration system that rotates a large 'sparse' grid of metal wires, in front of and fully covering the field of view of the focal plane receiver. Polarized radiation is created via the reflection of ambient temperature from the wire surface. Since the detector has a finite beam size, the observed signal is smeared according to the beam property. The resulting smeared polarized radiation has a reasonable intensity (a few Kelvin or less) compared to the sky temperature ({approx}10 K observing condition). The system played a successful role for receiver calibration of QUIET, a CMB polarization experiment located in the Atacama desert in Chile. The successful performance revealed that this system is applicable to other experiments based on different technologies, e.g. TES bolometers
A Strategy for Quadruped Walking on Uneven Terrain
We describe the design and construction of a quadruped robot which walks on uneven terrain. A control system which produces a statically stable gait has been implemented; results showing a straight and turning gait are presented. The control of quadruped robots poses interesting challenges due to a small stability margin (when compared to hexapods for example) . For this reason most implemented systems for outdoor walking on uneven terrain have been hexapods. The system described here has the added virtue of using very few inexpensive sensors and actuators. One of the aims of this work is to build a reduced complexity (low power, low mass and direct drive) walking robot for statically stable walking. The other aim is to compare the performance of this robot with a wheeled robot roughly the same size and weight. In this paper we report on progress towards the first of these two goals. 1 Introduction The advantages that walking robots possess have been extolled for many years. Chief amo..