39,574 research outputs found
Towards More Precise Survey Photometry for PanSTARRS and LSST: Measuring Directly the Optical Transmission Spectrum of the Atmosphere
Motivated by the recognition that variation in the optical transmission of
the atmosphere is probably the main limitation to the precision of ground-based
CCD measurements of celestial fluxes, we review the physical processes that
attenuate the passage of light through the Earth's atmosphere. The next
generation of astronomical surveys, such as PanSTARRS and LSST, will greatly
benefit from dedicated apparatus to obtain atmospheric transmission data that
can be associated with each survey image. We review and compare various
approaches to this measurement problem, including photometry, spectroscopy, and
LIDAR. In conjunction with careful measurements of instrumental throughput,
atmospheric transmission measurements should allow next-generation imaging
surveys to produce photometry of unprecedented precision. Our primary concerns
are the real-time determination of aerosol scattering and absorption by water
along the line of sight, both of which can vary over the course of a night's
observations.Comment: 41 pages, 14 figures. Accepted PAS
Benchmark of machine learning methods for classification of a Sentinel-2 image
Thanks to mainly ESA and USGS, a large bulk of free images of the Earth is readily available nowadays. One of the main goals of
remote sensing is to label images according to a set of semantic categories, i.e. image classification. This is a very challenging issue
since land cover of a specific class may present a large spatial and spectral variability and objects may appear at different scales and
orientations.
In this study, we report the results of benchmarking 9 machine learning algorithms tested for accuracy and speed in training and
classification of land-cover classes in a Sentinel-2 dataset. The following machine learning methods (MLM) have been tested: linear
discriminant analysis, k-nearest neighbour, random forests, support vector machines, multi layered perceptron, multi layered
perceptron ensemble, ctree, boosting, logarithmic regression. The validation is carried out using a control dataset which consists of an
independent classification in 11 land-cover classes of an area about 60 km2, obtained by manual visual interpretation of high resolution
images (20 cm ground sampling distance) by experts. In this study five out of the eleven classes are used since the others have too few
samples (pixels) for testing and validating subsets. The classes used are the following: (i) urban (ii) sowable areas (iii) water (iv) tree
plantations (v) grasslands.
Validation is carried out using three different approaches: (i) using pixels from the training dataset (train), (ii) using pixels from the
training dataset and applying cross-validation with the k-fold method (kfold) and (iii) using all pixels from the control dataset. Five
accuracy indices are calculated for the comparison between the values predicted with each model and control values over three sets of
data: the training dataset (train), the whole control dataset (full) and with k-fold cross-validation (kfold) with ten folds. Results from
validation of predictions of the whole dataset (full) show the random forests method with the highest values; kappa index ranging from
0.55 to 0.42 respectively with the most and least number pixels for training. The two neural networks (multi layered perceptron and its
ensemble) and the support vector machines - with default radial basis function kernel - methods follow closely with comparable
performanc
Detection Techniques of Microsecond Gamma-Ray Bursts using Ground-Based Telescopes
Gamma-ray observations above 200 MeV are conventionally made by
satellite-based detectors. The EGRET detector on the Compton Gamma Ray
Observatory (CGRO) has provided good sensitivity for the detection of bursts
lasting for more than 200 ms. Theoretical predictions of high-energy gamma-ray
bursts produced by quantum-mechanical decay of primordial black holes (Hawking
1971) suggest the emission of bursts on shorter time scales. The final stage of
a primordial black hole results in a burst of gamma-rays, peaking around 250
MeV and lasting for a tenth of a microsecond or longer depending on particle
physics. In this work we show that there is an observational window using
ground-based imaging Cherenkov detectors to measure gamma-ray burst emission at
energies E greater than 200 MeV. This technique, with a sensitivity for bursts
lasting nanoseconds to several microseconds, is based on the detection of
multi-photon-initiated air showers.Comment: accepted for publication in the Astrophysical Journa
Breakup of diminutive Rayleigh jets
Discharging a liquid from a nozzle at sufficient large velocity leads to a
continuous jet that due to capillary forces breaks up into droplets. Here we
investigate the formation of microdroplets from the breakup of micron-sized
jets with ultra high-speed imaging. The diminutive size of the jet implies a
fast breakup time scale of the
order of 100\,ns{}, and requires imaging at 14 million frames per second. We
directly compare these experiments with a numerical lubrication approximation
model that incorporates inertia, surface tension, and viscosity [Eggers and
Dupont, J. Fluid Mech. 262, 205 (1994); Shi, Brenner, and Nagel, Science 265,
219 (1994)]. The lubrication model allows to efficiently explore the parameter
space to investigate the effect of jet velocity and liquid viscosity on the
formation of satellite droplets. In the phase diagram we identify regions where
the formation of satellite droplets is suppressed. We compare the shape of the
droplet at pinch-off between the lubrication approximation model and a boundary
integral (BI) calculation, showing deviations at the final moment of the
pinch-off. Inspite of this discrepancy, the results on pinch-off times and
droplet and satellite droplet velocity obtained from the lubrication
approximation agree with the high-speed imaging results
Dark Matter Structures in the Universe: Prospects for Optical Astronomy in the Next Decade
The Cold Dark Matter theory of gravitationally-driven hierarchical structure
formation has earned its status as a paradigm by explaining the distribution of
matter over large spans of cosmic distance and time. However, its central
tenet, that most of the matter in the universe is dark and exotic, is still
unproven; the dark matter hypothesis is sufficiently audacious as to continue
to warrant a diverse battery of tests. While local searches for dark matter
particles or their annihilation signals could prove the existence of the
substance itself, studies of cosmological dark matter in situ are vital to
fully understand its role in structure formation and evolution. We argue that
gravitational lensing provides the cleanest and farthest-reaching probe of dark
matter in the universe, which can be combined with other observational
techniques to answer the most challenging and exciting questions that will
drive the subject in the next decade: What is the distribution of mass on
sub-galactic scales? How do galaxy disks form and bulges grow in dark matter
halos? How accurate are CDM predictions of halo structure? Can we distinguish
between a need for a new substance (dark matter) and a need for new physics
(departures from General Relativity)? What is the dark matter made of anyway?
We propose that the central tool in this program should be a wide-field optical
imaging survey, whose true value is realized with support in the form of
high-resolution, cadenced optical/infra-red imaging, and massive-throughput
optical spectroscopy.Comment: White paper submitted to the 2010 Astronomy & Astrophysics Decadal
Surve
Max '91: Flare research at the next solar maximum
To address the central scientific questions surrounding solar flares, coordinated observations of electromagnetic radiation and energetic particles must be made from spacecraft, balloons, rockets, and ground-based observatories. A program to enhance capabilities in these areas in preparation for the next solar maximum in 1991 is recommended. The major scientific issues are described, and required observations and coordination of observations and analyses are detailed. A program plan and conceptual budgets are provided
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