4,204 research outputs found
A new map-making algorithm for CMB polarisation experiments
With the temperature power spectrum of the cosmic microwave background (CMB)
at least four orders of magnitude larger than the B-mode polarisation power
spectrum, any instrumental imperfections that couple temperature to
polarisation must be carefully controlled and/or removed. Here we present two
new map-making algorithms that can create polarisation maps that are clean of
temperature-to-polarisation leakage systematics due to differential gain and
pointing between a detector pair. Where a half wave plate is used, we show that
the spin-2 systematic due to differential ellipticity can also by removed using
our algorithms. The algorithms require no prior knowledge of the imperfections
or temperature sky to remove the temperature leakage. Instead, they calculate
the systematic and polarisation maps in one step directly from the time ordered
data (TOD). The first algorithm is designed to work with scan strategies that
have a good range of crossing angles for each map pixel and the second for scan
strategies that have a limited range of crossing angles. The first algorithm
can also be used to identify if systematic errors that have a particular spin
are present in a TOD. We demonstrate the use of both algorithms and the ability
to identify systematics with simulations of TOD with realistic scan strategies
and instrumental noise.Comment: 11 pages, 6 figure
Optimal scan strategies for future CMB satellite experiments
The B-mode polarisation power spectrum in the Cosmic Microwave Background
(CMB) is about four orders of magnitude fainter than the CMB temperature power
spectrum. Any instrumental imperfections that couple temperature fluctuations
to B-mode polarisation must therefore be carefully controlled and/or removed.
We investigate the role that a scan strategy can have in mitigating certain
common systematics by averaging systematic errors down with many crossing
angles. We present approximate analytic forms for the error on the recovered
B-mode power spectrum that would result from differential gain, differential
pointing and differential ellipticity for the case where two detector pairs are
used in a polarisation experiment. We use these analytic predictions to search
the parameter space of common satellite scan strategies in order to identify
those features of a scan strategy that have most impact in mitigating
systematic effects. As an example we go on to identify a scan strategy suitable
for the CMB satellite proposed for the ESA M5 call. considering the practical
considerations of fuel requirement, data rate and the relative orientation of
the telescope to the earth. Having chosen a scan strategy we then go on to
investigate the suitability of the scan strategy.Comment: 21 pages, 11 figures, Comments welcom
Removing beam asymmetry bias in precision CMB temperature and polarisation experiments
Asymmetric beams can create significant bias in estimates of the power
spectra from CMB experiments. With the temperature power spectrum many orders
of magnitude stronger than the B-mode power spectrum any systematic error that
couples the two must be carefully controlled and/or removed. Here, we derive
unbiased estimators for the CMB temperature and polarisation power spectra
taking into account general beams and general scan strategies. A simple
consequence of asymmetric beams is that, even with an ideal scan strategy where
every sky pixel is seen at every orientation, there will be residual coupling
from temperature power to B-mode power if the orientation of the beam asymmetry
is not aligned with the orientation of the co-polarisation. We test our
correction algorithm on simulations of two temperature-only experiments and
demonstrate that it is unbiased. The simulated experiments use realistic scan
strategies, noise levels and highly asymmetric beams. We also develop a
map-making algorithm that is capable of removing beam asymmetry bias at the map
level. We demonstrate its implementation using simulations and show that it is
capable of accurately correcting both temperature and polarisation maps for all
of the effects of beam asymmetry including the effects of temperature to
polarisation leakage.Comment: 18 pages, 9 figure
Removing beam asymmetry bias in precision CMB temperature and polarisation experiments
Asymmetric beams can create significant bias in estimates of the power
spectra from CMB experiments. With the temperature power spectrum many orders
of magnitude stronger than the B-mode power spectrum any systematic error that
couples the two must be carefully controlled and/or removed. Here, we derive
unbiased estimators for the CMB temperature and polarisation power spectra
taking into account general beams and general scan strategies. A simple
consequence of asymmetric beams is that, even with an ideal scan strategy where
every sky pixel is seen at every orientation, there will be residual coupling
from temperature power to B-mode power if the orientation of the beam asymmetry
is not aligned with the orientation of the co-polarisation. We test our
correction algorithm on simulations of two temperature-only experiments and
demonstrate that it is unbiased. The simulated experiments use realistic scan
strategies, noise levels and highly asymmetric beams. We also develop a
map-making algorithm that is capable of removing beam asymmetry bias at the map
level. We demonstrate its implementation using simulations and show that it is
capable of accurately correcting both temperature and polarisation maps for all
of the effects of beam asymmetry including the effects of temperature to
polarisation leakage.Comment: 18 pages, 9 figure
The use of optical flow and splay angle in steering a central path
In the present experiment we investigated the importance of velocity information during a lane-centering task between the walls of a simulated tunnel. We varied both simulated velocity and the spatial frequency content of the walls' surfaces, in order to address the influence of each parameter on steering performance. Further, this performance was compared to the effectiveness of lateral control using lane border information. We found that drivers used both velocity and spatial frequency information to maintain a centered position on a path, and that the presence of lane borders improved accuracy. The results suggest that multiple sources of visual information, rather than mere demarcating lines, are used for lateral control on a straight path
Sparse Bayesian mass-mapping with uncertainties: hypothesis testing of structure
A crucial aspect of mass-mapping, via weak lensing, is quantification of the
uncertainty introduced during the reconstruction process. Properly accounting
for these errors has been largely ignored to date. We present results from a
new method that reconstructs maximum a posteriori (MAP) convergence maps by
formulating an unconstrained Bayesian inference problem with Laplace-type
-norm sparsity-promoting priors, which we solve via convex
optimization. Approaching mass-mapping in this manner allows us to exploit
recent developments in probability concentration theory to infer theoretically
conservative uncertainties for our MAP reconstructions, without relying on
assumptions of Gaussianity. For the first time these methods allow us to
perform hypothesis testing of structure, from which it is possible to
distinguish between physical objects and artifacts of the reconstruction. Here
we present this new formalism, demonstrate the method on illustrative examples,
before applying the developed formalism to two observational datasets of the
Abel-520 cluster. In our Bayesian framework it is found that neither Abel-520
dataset can conclusively determine the physicality of individual local massive
substructure at significant confidence. However, in both cases the recovered
MAP estimators are consistent with both sets of data
Customer mobility and congestion in supermarkets
The analysis and characterization of human mobility using population-level
mobility models is important for numerous applications, ranging from the
estimation of commuter flows in cities to modeling trade flows between
countries. However, almost all of these applications have focused on large
spatial scales, which typically range between intra-city scales to
inter-country scales. In this paper, we investigate population-level human
mobility models on a much smaller spatial scale by using them to estimate
customer mobility flow between supermarket zones. We use anonymized, ordered
customer-basket data to infer empirical mobility flow in supermarkets, and we
apply variants of the gravity and intervening-opportunities models to fit this
mobility flow and estimate the flow on unseen data. We find that a
doubly-constrained gravity model and an extended radiation model (which is a
type of intervening-opportunities model) can successfully estimate 65--70\% of
the flow inside supermarkets. Using a gravity model as a case study, we then
investigate how to reduce congestion in supermarkets using mobility models. We
model each supermarket zone as a queue, and we use a gravity model to identify
store layouts with low congestion, which we measure either by the maximum
number of visits to a zone or by the total mean queue size. We then use a
simulated-annealing algorithm to find store layouts with lower congestion than
a supermarket's original layout. In these optimized store layouts, we find that
popular zones are often in the perimeter of a store. Our research gives insight
both into how customers move in supermarkets and into how retailers can arrange
stores to reduce congestion. It also provides a case study of human mobility on
small spatial scales
Cohort profile: the avon longitudinal study of parents and children: ALSPAC mothers cohort
The Avon Longitudinal Study of Children and Parents (ALSPAC) was established to understand how genetic and environmental characteristics influence health and development in parents and children. All pregnant women resident in a defined area in the South West of England, with an expected date of delivery between 1st April 1991 and 31st December 1992, were eligible and 13 761 women (contributing 13 867 pregnancies) were recruited. These women have been followed over the last 19–22 years and have completed up to 20 questionnaires, have had detailed data abstracted from their medical records and have information on any cancer diagnoses and deaths through record linkage. A follow-up assessment was completed 17–18 years postnatal at which anthropometry, blood pressure, fat, lean and bone mass and carotid intima media thickness were assessed, and a fasting blood sample taken. The second follow-up clinic, which additionally measures cognitive function, physical capability, physical activity (with accelerometer) and wrist bone architecture, is underway and two further assessments with similar measurements will take place over the next 5 years. There is a detailed biobank that includes DNA, with genome-wide data available on >10 000, stored serum and plasma taken repeatedly since pregnancy and other samples; a wide range of data on completed biospecimen assays are available. Details of how to access these data are provided in this cohort profile
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