4,099 research outputs found
POLOCALC: a Novel Method to Measure the Absolute Polarization Orientation of the Cosmic Microwave Background
We describe a novel method to measure the absolute orientation of the
polarization plane of the CMB with arcsecond accuracy, enabling unprecedented
measurements for cosmology and fundamental physics. Existing and planned CMB
polarization instruments looking for primordial B-mode signals need an
independent, experimental method for systematics control on the absolute
polarization orientation. The lack of such a method limits the accuracy of the
detection of inflationary gravitational waves, the constraining power on the
neutrino sector through measurements of gravitational lensing of the CMB, the
possibility of detecting Cosmic Birefringence, and the ability to measure
primordial magnetic fields. Sky signals used for calibration and direct
measurements of the detector orientation cannot provide an accuracy better than
1 deg. Self-calibration methods provide better accuracy, but may be affected by
foreground signals and rely heavily on model assumptions. The POLarization
Orientation CALibrator for Cosmology, POLOCALC, will dramatically improve
instrumental accuracy by means of an artificial calibration source flying on
balloons and aerial drones. A balloon-borne calibrator will provide far-field
source for larger telescopes, while a drone will be used for tests and smaller
polarimeters. POLOCALC will also allow a unique method to measure the
telescopes' polarized beam. It will use microwave emitters between 40 and 150
GHz coupled to precise polarizing filters. The orientation of the source
polarization plane will be registered to sky coordinates by star cameras and
gyroscopes with arcsecond accuracy. This project can become a rung in the
calibration ladder for the field: any existing or future CMB polarization
experiment observing our polarization calibrator will enable measurements of
the polarization angle for each detector with respect to absolute sky
coordinates.Comment: 15 pages, 5 figures, Accepted by Journal of Astronomical
Instrumentatio
EBEX: A balloon-borne CMB polarization experiment
EBEX is a NASA-funded balloon-borne experiment designed to measure the
polarization of the cosmic microwave background (CMB). Observations will be
made using 1432 transition edge sensor (TES) bolometric detectors read out with
frequency multiplexed SQuIDs. EBEX will observe in three frequency bands
centered at 150, 250, and 410 GHz, with 768, 384, and 280 detectors in each
band, respectively. This broad frequency coverage is designed to provide
valuable information about polarized foreground signals from dust. The
polarized sky signals will be modulated with an achromatic half wave plate
(AHWP) rotating on a superconducting magnetic bearing (SMB) and analyzed with a
fixed wire grid polarizer. EBEX will observe a patch covering ~1% of the sky
with 8' resolution, allowing for observation of the angular power spectrum from
\ell = 20 to 1000. This will allow EBEX to search for both the primordial
B-mode signal predicted by inflation and the anticipated lensing B-mode signal.
Calculations to predict EBEX constraints on r using expected noise levels show
that, for a likelihood centered around zero and with negligible foregrounds,
99% of the area falls below r = 0.035. This value increases by a factor of 1.6
after a process of foreground subtraction. This estimate does not include
systematic uncertainties. An engineering flight was launched in June, 2009,
from Ft. Sumner, NM, and the long duration science flight in Antarctica is
planned for 2011. These proceedings describe the EBEX instrument and the North
American engineering flight.Comment: 12 pages, 9 figures, Conference proceedings for SPIE Millimeter,
Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy V
(2010
Pointing control for the SPIDER balloon-borne telescope
We present the technology and control methods developed for the pointing
system of the SPIDER experiment. SPIDER is a balloon-borne polarimeter designed
to detect the imprint of primordial gravitational waves in the polarization of
the Cosmic Microwave Background radiation. We describe the two main components
of the telescope's azimuth drive: the reaction wheel and the motorized pivot. A
13 kHz PI control loop runs on a digital signal processor, with feedback from
fibre optic rate gyroscopes. This system can control azimuthal speed with <
0.02 deg/s RMS error. To control elevation, SPIDER uses stepper-motor-driven
linear actuators to rotate the cryostat, which houses the optical instruments,
relative to the outer frame. With the velocity in each axis controlled in this
way, higher-level control loops on the onboard flight computers can implement
the pointing and scanning observation modes required for the experiment. We
have accomplished the non-trivial task of scanning a 5000 lb payload
sinusoidally in azimuth at a peak acceleration of 0.8 deg/s, and a peak
speed of 6 deg/s. We can do so while reliably achieving sub-arcminute pointing
control accuracy.Comment: 20 pages, 12 figures, Presented at SPIE Ground-based and Airborne
Telescopes V, June 23, 2014. To be published in Proceedings of SPIE Volume
914
A high-resolution pointing system for fast scanning platforms: The EBEX example
The E and B experiment (EBEX) is a balloon-borne telescope designed to
measure the polarization of the cosmic microwave background with 8' resolution
employing a gondola scanning with speeds of order degree per second. In January
2013, EBEX completed 11 days of observations in a flight over Antarctica
covering 6000 square degrees of the sky. The payload is equipped with
two redundant star cameras and two sets of three orthogonal gyroscopes to
reconstruct the telescope attitude. The EBEX science goals require the pointing
to be reconstructed to approximately 10" in the map domain, and in-flight
attitude control requires the real time pointing to be accurate to
0.5 . The high velocity scan strategy of EBEX coupled to its float
altitude only permits the star cameras to take images at scan turnarounds,
every 40 seconds, and thus requires the development of a pointing system
with low noise gyroscopes and carefully controlled systematic errors. Here we
report on the design of the pointing system and on a simulation pipeline
developed to understand and minimize the effects of systematic errors. The
performance of the system is evaluated using the 2012/2013 flight data, and we
show that we achieve a pointing error with RMS=25" on 40 seconds azimuth
throws, corresponding to an error of 4.6" in the map domain.Comment: 14 pages, Proceedings of the 2015 IEEE Aerospace Conferenc
The balloon-borne large-aperture submillimeter telescope for polarimetry: BLAST-Pol
The Balloon-borne Large Aperture Submillimeter Telescope for Polarimetry
(BLAST-Pol) is a suborbital mapping experiment designed to study the role
played by magnetic fields in the star formation process. BLAST-Pol is the
reconstructed BLAST telescope, with the addition of linear polarization
capability. Using a 1.8 m Cassegrain telescope, BLAST-Pol images the sky onto a
focal plane that consists of 280 bolometric detectors in three arrays,
observing simultaneously at 250, 350, and 500 um. The diffraction-limited
optical system provides a resolution of 30'' at 250 um. The polarimeter
consists of photolithographic polarizing grids mounted in front of each
bolometer/detector array. A rotating 4 K achromatic half-wave plate provides
additional polarization modulation. With its unprecedented mapping speed and
resolution, BLAST-Pol will produce three-color polarization maps for a large
number of molecular clouds. The instrument provides a much needed bridge in
spatial coverage between larger-scale, coarse resolution surveys and narrow
field of view, and high resolution observations of substructure within
molecular cloud cores. The first science flight will be from McMurdo Station,
Antarctica in December 2010.Comment: 14 pages, 9 figures Submitted to SPIE Astronomical Telescopes and
Instrumentation Conference 201
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