708 research outputs found
The usability of the optical parametric amplification of light for high-angular-resolution imaging and fast astrometry
High-angular-resolution imaging is crucial for many applications in modern
astronomy and astrophysics. The fundamental diffraction limit constrains the
resolving power of both ground-based and spaceborne telescopes. The recent idea
of a quantum telescope based on the optical parametric amplification (OPA) of
light aims to bypass this limit for the imaging of extended sources by an order
of magnitude or more. We present an updated scheme of an OPA-based device and a
more accurate model of the signal amplification by such a device. The
semiclassical model that we present predicts that the noise in such a system
will form so-called light speckles as a result of light interference in the
optical path. Based on this model, we analysed the efficiency of OPA in
increasing the angular resolution of the imaging of extended targets and the
precise localization of a distant point source. According to our new model, OPA
offers a gain in resolved imaging in comparison to classical optics. For a
given time-span, we found that OPA can be more efficient in localizing a single
distant point source than classical telescopes.Comment: Received: 11 November 2017, revision received: 31 January 2018,
accepted: 31 January 201
Beyond the current noise limit in imaging through turbulent medium
Shift-and-add is an approach employed to mitigate the phenomenon of
resolution degradation in images acquired through a turbulent medium. Using
this technique, a large number of consecutive short exposures is registered
below the coherence time of the atmosphere or other blurring medium. The
acquired images are shifted to the position of the brightest speckle and
stacked together to obtain high-resolution and high signal-to-noise frame. In
this paper we present a highly efficient method for determination of frames
shifts, even if in a single frame the object cannot be distinguished from the
background noise. The technique utilizes our custom genetic algorithm, which
iteratively evolves a set of image shifts. We used the maximal energy of
stacked images as an objective function for shifts estimation and validate the
efficiency of the method on simulated and real images of simple and complex
sources. Obtained results confirmed, that our proposed method allows for the
recovery of spatial distribution of objects even only 2% brighter than their
background. The presented approach extends significantly current limits of
image reconstruction with the use of shift-and-add method. The applications of
our algorithm include both the optical and the infrared imaging. Our method may
be also employed as a digital image stabilizer in extremely low light level
conditions in professional and consumer applications.Comment: 8 pages, 4 figure
Quantum Telescopes: feasibility and constrains
Quantum Telescope is a recent idea aimed at beating the diffraction limit of
spaceborne telescopes and possibly also other distant target imaging systems.
There is no agreement yet on the best setup of such devices, but some
configurations have been already proposed. In this Letter we characterize the
predicted performance of Quantum Telescopes and their possible limitations. Our
extensive simulations confirm that the presented model of such instruments is
feasible and the device can provide considerable gains in the angular
resolution of imaging in the UV, optical and infrared bands. We argue that it
is generally possible to construct and manufacture such instruments using the
latest or soon to be available technology. We refer to the latest literature to
discuss the feasibility of the proposed QT system design.Comment: Optics Letters - published after major revisio
Spontaneous bilateral subdural haematomas in the posterior cranial fossa revealed by MRI
A 52-year-old woman treated for acute myeloproliferative disease developed progressive stupor. CT showed obstructive hydrocephalus resulting from unexplained mass effect on the fourth ventricle. MRI revealed bilateral extra-axial collections in the posterior cranial fossa, giving high signal on T1- and T2-weighted images, suggesting subacute subdural haematomas. Subdural haematomas can be suspected on CT when there is unexplained mass effect. MRI may be essential to confirm the diagnosis and plan appropriate treatmen
Catalog of quasars from the Kilo-Degree Survey Data Release 3
We present a catalog of quasars selected from broad-band photometric ugri
data of the Kilo-Degree Survey Data Release 3 (KiDS DR3). The QSOs are
identified by the random forest (RF) supervised machine learning model, trained
on SDSS DR14 spectroscopic data. We first cleaned the input KiDS data from
entries with excessively noisy, missing or otherwise problematic measurements.
Applying a feature importance analysis, we then tune the algorithm and identify
in the KiDS multiband catalog the 17 most useful features for the
classification, namely magnitudes, colors, magnitude ratios, and the stellarity
index. We used the t-SNE algorithm to map the multi-dimensional photometric
data onto 2D planes and compare the coverage of the training and inference
sets. We limited the inference set to r<22 to avoid extrapolation beyond the
feature space covered by training, as the SDSS spectroscopic sample is
considerably shallower than KiDS. This gives 3.4 million objects in the final
inference sample, from which the random forest identified 190,000 quasar
candidates. Accuracy of 97%, purity of 91%, and completeness of 87%, as derived
from a test set extracted from SDSS and not used in the training, are confirmed
by comparison with external spectroscopic and photometric QSO catalogs
overlapping with the KiDS footprint. The robustness of our results is
strengthened by number counts of the quasar candidates in the r band, as well
as by their mid-infrared colors available from WISE. An analysis of parallaxes
and proper motions of our QSO candidates found also in Gaia DR2 suggests that a
probability cut of p(QSO)>0.8 is optimal for purity, whereas p(QSO)>0.7 is
preferable for better completeness. Our study presents the first comprehensive
quasar selection from deep high-quality KiDS data and will serve as the basis
for versatile studies of the QSO population detected by this survey.Comment: Data available from the KiDS website at
http://kids.strw.leidenuniv.nl/DR3/quasarcatalog.php and the source code from
https://github.com/snakoneczny/kids-quasar
Thalamic Nuclei Clustering on High Angular Resolution Diffusion Images.
Thalamic nuclei can be distinguished by their characteristic fiber orientations, which influence the diffusion. Fiber orientations are relatively aligned within a nucleus due to the fact that the cerebrocortical striations within a nucleus all target the same region of cortex. The number of thalamic nuclei reported with histological methods varies with the method employed, although most cyto/myeloarchitec stains identify 14 major nuclei. We present a new approach for thalamic nuclei segmentation on High Angular Diffusion Resolution Images (HARDI), performed with a constrained k-means clustering. As described by John D.Carew[1], it is possible to classify HARDI data based on the shape of the diffusion, thanks to the complex information coming from them. Mette R. Wiegell [2] proposed a thalamic nuclei clustering with k- means on diffusion tensor images, using a combination of a voxel distance and a diffusion tensor distance. In the same way, we use the k-mean algorithm with a weighted sum of two distances to cluster the thalamic nuclei on HARDI data
Observational Evidence for the Co-evolution of Galaxy Mergers, Quasars, and the Blue/Red Galaxy Transition
We compile a number of observations to estimate the time-averaged rate of
formation or buildup of red sequence galaxies, as a function of mass and
redshift. Comparing this with the mass functions of mergers and quasar hosts,
and independently comparing their clustering properties as a function of
redshift, we find that these populations trace the same mass distribution, with
similar evolution, at redshifts 0<z<~1.5. Knowing one of the quasar, merger, or
elliptical mass/luminosity functions, it is possible to predict the others.
Allowing for greater model dependence, we compare the rate of early-type
buildup with the implied merger and quasar triggering rates as a function of
mass and redshift and find agreement. Over this redshift range, observed merger
fractions can account for the entire bright quasar luminosity function and
buildup of the red sequence at all but the highest masses at low redshift
(>~10^11 M_solar at z<~0.3) where 'dry' mergers appear to dominate. This
supports a necessary prediction of theories where mergers between gas-rich
galaxies produce ellipticals with an associated phase of quasar activity, after
which the remnant becomes red. These populations trace a similar characteristic
transition mass, possibly reflecting the mass above which the elliptical
population is mostly (>~50%) assembled at a given redshift, which increases
with redshift over the observed range in a manner consistent with suggestions
that cosmic downsizing may apply to red galaxy assembly as well as star
formation. These mass distributions as a function of redshift do not uniformly
trace the all/red/blue galaxy population, ruling out models in which quasar
activity is generically associated with star formation or is long lived in
'old' systems.Comment: 24 pages, 17 figures. Accepted to ApJ. Substantially revised and
expanded to match published versio
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