378 research outputs found
Image Ellipticity from Atmospheric Aberrations
We investigate the ellipticity of the point-spread function (PSF) produced by
imaging an unresolved source with a telescope, subject to the effects of
atmospheric turbulence. It is important to quantify these effects in order to
understand the errors in shape measurements of astronomical objects, such as
those used to study weak gravitational lensing of field galaxies. The PSF
modeling involves either a Fourier transform of the phase information in the
pupil plane or a ray-tracing approach, which has the advantage of requiring
fewer computations than the Fourier transform. Using a standard method,
involving the Gaussian weighted second moments of intensity, we then calculate
the ellipticity of the PSF patterns. We find significant ellipticity for the
instantaneous patterns (up to more than 10%). Longer exposures, which we
approximate by combining multiple (N) images from uncorrelated atmospheric
realizations, yield progressively lower ellipticity (as 1 / sqrt(N)). We also
verify that the measured ellipticity does not depend on the sampling interval
in the pupil plane using the Fourier method. However, we find that the results
using the ray-tracing technique do depend on the pupil sampling interval,
representing a gradual breakdown of the geometric approximation at high spatial
frequencies. Therefore, ray tracing is generally not an accurate method of
modeling PSF ellipticity induced by atmospheric turbulence unless some
additional procedure is implemented to correctly account for the effects of
high spatial frequency aberrations. The Fourier method, however, can be used
directly to accurately model PSF ellipticity, which can give insights into
errors in the statistics of field galaxy shapes used in studies of weak
gravitational lensing.Comment: 9 pages, 5 color figures (some reduced in size). Accepted for
publication in the Astrophysical Journa
A Search for Scalar Chameleons with ADMX
Scalar fields with a "chameleon" property, in which the effective particle
mass is a function of its local environment, are common to many theories beyond
the standard model and could be responsible for dark energy. If these fields
couple weakly to the photon, they could be detectable through the "afterglow"
effect of photon-chameleon-photon transitions. The ADMX experiment was used in
the first chameleon search with a microwave cavity to set a new limit on scalar
chameleon-photon coupling excluding values between 2*10^9 and 5*10^14 for
effective chameleon masses between 1.9510 and 1.9525 micro-eV.Comment: 4 pages, 3 figure
Dilaton as a Dark Matter Candidate and its Detection
Assuming that the dilaton is the dark matter of the universe, we propose an
experiment to detect the relic dilaton using the electromagnetic resonant
cavity, based on the dilaton-photon conversion in strong electromagnetic
background. We calculate the density of the relic dilaton, and estimate the
dilaton mass for which the dilaton becomes the dark matter of the universe.
With this we calculate the dilaton detection power in the resonant cavity, and
compare it with the axion detection power in similar resonant cavity
experiment.Comment: 23 pages, 2 figure
A SQUID-based microwave cavity search for dark-matter axions
Axions in the micro eV mass range are a plausible cold dark matter candidate
and may be detected by their conversion into microwave photons in a resonant
cavity immersed in a static magnetic field. The first result from such an axion
search using a superconducting first-stage amplifier (SQUID) is reported. The
SQUID amplifier, replacing a conventional GaAs field-effect transistor
amplifier, successfully reached axion-photon coupling sensitivity in the band
set by present axion models and sets the stage for a definitive axion search
utilizing near quantum-limited SQUID amplifiers.Comment: 4 pages, 5 figures, submitted to PR
Move of a large but delicate apparatus on a trailer with air-ride suspension
When valuable delicate goods are shipped by truck, attention must be paid to
vibrations that may cause damage. We present a case study of moving an
extremely delicate 6230-kg superconducting magnet, immersed in liquid nitrogen,
from Livermore, CA to Seattle, WA showing the steps of fatigue analysis of the
load, a test move, and acceleration monitoring of the final move to ensure a
successful damage-free transport
Superdeformation in Po
The Yb(Si,5n) reaction at 148 MeV with thin targets was used
to populate high-angular momentum states in Po. Resulting rays
were observed with Gammasphere. A weakly-populated superdeformed band of 10
-ray transitions was found and has been assigned to Po. This is
the first observation of a SD band in the region in a nucleus
with . The of the new band is very similar to those of
the yrast SD bands in Hg and Pb. The intensity profile suggests
that this band is populated through states close to where the SD band crosses
the yrast line and the angular momentum at which the fission process dominates.Comment: 10 pages, revtex, 2 figs. available on request, submitted to Phys.
Rev. C. (Rapid Communications
Properties of Ellipticity Correlation with Atmospheric Structure from Gemini South
Cosmic shear holds great promise for a precision independent measurement of
, the mass density of the universe relative to the critical
density. The signal is expected to be weak, so a thorough understanding of
systematic effects is crucial. An important systematic effect is the
atmosphere: shear power introduced by the atmosphere is larger than the
expected signal. Algorithms exist to extract the cosmic shear from the
atmospheric component, though a measure of their success applied to a range of
seeing conditions is lacking.
To gain insight into atmospheric shear, Gemini South imaging in conjunction
with ground condition and satellite wind data were obtained. We find that under
good seeing conditions Point-Spread-Function (PSF) correlations persist well
beyond the separation typical of high-latitude stars. Under these conditions,
ellipticity residuals based on a simple PSF interpolation can be reduced to
within a factor of a few of the shot-noise induced ellipticity floor. We also
find that the ellipticity residuals are highly correlated with wind direction.
Finally, we correct stellar shapes using a more sophisticated procedure and
generate shear statistics from stars. Under all seeing conditions in our data
set the residual correlations lie everywhere below the target signal level. For
good seeing we find that the systematic error attributable to atmospheric
turbulence is comparable in magnitude to the statistical error (shape noise)
over angular scales relevant to present lensing surveys.Comment: To appear in ApJ April 10, 2007, 659
Axion-Higgs Unification
In theories with no fundamental scalars, one gauge group can become strong at
a large scale Lambda and spontaneously break a global symmetry, producing the
Higgs and the axion as composite pseudo-Nambu-Goldstone bosons. We show how
KSVZ and DFSZ axion models can be naturally realised. The assumption Lambda
around 10^{11} GeV is phenomenologically favoured because: a) The axion solves
the QCD theta problem and provides the observed DM abundance; b) The observed
Higgs mass is generated via RGE effects from a small Higgs quartic coupling at
the compositeness scale, provided that the Higgs mass term is fine-tuned to be
of electroweak size; c) Lepton, quark as well as neutrino masses can be
obtained from four-fermion operators at the compositeness scale. d) The extra
fermions can unify the gauge couplings.Comment: 19 pages. Refs. added and eq. 3.6 fixe
- …