233,069 research outputs found
Full-sky maps for gravitational lensing of the CMB
We use the large cosmological Millennium Simulation (MS) to construct the
first all-sky maps of the lensing potential and the deflection angle, aiming at
gravitational lensing of the CMB, with the goal of properly including
small-scale non-linearities and non-Gaussianity. Exploiting the Born
approximation, we implement a map-making procedure based on direct ray-tracing
through the gravitational potential of the MS. We stack the simulation box in
redshift shells up to , producing continuous all-sky maps with
arcminute angular resolution. A randomization scheme avoids repetition of
structures along the line of sight and structures larger than the MS box size
are added to supply the missing contribution of large-scale (LS) structures to
the lensing signal. The angular power spectra of the projected lensing
potential and the deflection-angle modulus agree quite well with semi-analytic
estimates on scales down to a few arcminutes, while we find a slight excess of
power on small scales, which we interpret as being due to non-linear clustering
in the MS. Our map-making procedure, combined with the LS adding technique, is
ideally suited for studying lensing of CMB anisotropies, for analyzing
cross-correlations with foreground structures, or other secondary CMB
anisotropies such as the Rees-Sciama effect.Comment: LaTeX file, 10 pages, MNRAS in press, scales larger than the
Millennium Simulation box size semi-analytically added, maps changed,
references added, typos correcte
Single-shot layered reflectance separation using a polarized light field camera
We present a novel computational photography technique for single shot separation of diffuse/specular reflectance as well as novel angular domain separation of layered reflectance. Our solution consists of a two-way polarized light field (TPLF) camera which simultaneously captures two orthogonal states of polarization. A single photograph of a subject acquired with the TPLF camera under polarized illumination then enables standard separation of diffuse (depolarizing) and polarization preserving specular reflectance using light field sampling. We further demonstrate that the acquired data also enables novel angular separation of layered reflectance including separation of specular reflectance and single scattering in the polarization preserving component, and separation of shallow scattering from deep scattering in the depolarizing component. We apply our approach for efficient acquisition of facial reflectance including diffuse and specular normal maps, and novel separation of photometric normals into layered reflectance normals for layered facial renderings. We demonstrate our proposed single shot layered reflectance separation to be comparable to an existing multi-shot technique that relies on structured lighting while achieving separation results under a variety of illumination conditions
Small-scale chromospheric jets above a sunspot light bridge
High-resolution broadband filtergrams of active region NOAA 11271 in Ca ii H
and G band were obtained with the Solar Optical Telescope on board Hinode to
identify the physical driver responsible for the dynamic and small-scale
chromospheric jets above a sunspot light bridge. We identified the jets in the
Ca images using a semi-automatic routine. The chromospheric jets consist of a
bright, triangular-shaped blob that lies on the light bridge, while the apex of
this blob extends into a spike-like structure that is bright against the dark
umbral background. Most of the jets have apparent lengths of less than 1000 km
and about 30% of them have lengths between 1000-1600 km. They are oriented
within +/-35 deg. to the normal of the light bridge axis. Many of them are
clustered near the central part within a 2 arcsec area. The jets are seen to
move rapidly along the light bridge and most of them cannot be identified in
successive images taken with a 2 min cadence. The jets are primarily located on
one side of the light bridge and are directed into the umbral core. The Stokes
profiles at or close to the location of the blobs on the LB exhibit both a
significant net circular polarization and multiple components, including
opposite-polarity lobes. The magnetic field diverges from the light bridge
towards the umbral cores that it separates. In the photosphere there is a
predominantly uni-directional flow with speeds of 100-150 m/s along the light
bridge which is interrupted by a patch of weak motions that also moves along
the light bridge. The dynamic short-lived jets above the LB seem to be guided
by the magnetic field lines. Reconnection events are a likely trigger for such
phenomenon since they occur at locations where the magnetic field changes
orientation sharply. We find no clear relation between the jets and the
photospheric flow pattern.Comment: Accepted for publication in A&A, 9 pages, 7 figure
A qualitative enquiry into OpenStreetMap making
Based on a case study on the OpenStreetMap community, this paper provides a contextual and embodied understanding of the user-led, user-participatory and user-generated produsage phenomenon. It employs Grounded Theory, Social Worlds Theory, and qualitative methods to illuminate and explores the produsage processes of OpenStreetMap making, and how knowledge artefacts such as maps can be collectively and collaboratively produced by a community of people, who are situated in different places around the world but engaged with the same repertoire of mapping practices. The empirical data illustrate that OpenStreetMap itself acts as a boundary object that enables actors from different social worlds to co-produce the Map through interacting with each other and negotiating the meanings of mapping, the mapping data and the Map itself. The discourses also show that unlike traditional maps that black-box cartographic knowledge and offer a single dominant perspective of cities or places, OpenStreetMap is an embodied epistemic object that embraces different world views. The paper also explores how contributors build their identities as an OpenStreetMaper alongside some other identities they have. Understanding the identity-building process helps to understand mapping as an embodied activity with emotional, cognitive and social repertoires
Wavelets: a powerful tool for studying rotation, activity, and pulsation in Kepler and CoRoT stellar light curves
Aims. The wavelet transform has been used as a powerful tool for treating
several problems in astrophysics. In this work, we show that the time-frequency
analysis of stellar light curves using the wavelet transform is a practical
tool for identifying rotation, magnetic activity, and pulsation signatures. We
present the wavelet spectral composition and multiscale variations of the time
series for four classes of stars: targets dominated by magnetic activity, stars
with transiting planets, those with binary transits, and pulsating stars.
Methods. We applied the Morlet wavelet (6th order), which offers high time and
frequency resolution. By applying the wavelet transform to the signal, we
obtain the wavelet local and global power spectra. The first is interpreted as
energy distribution of the signal in time-frequency space, and the second is
obtained by time integration of the local map. Results. Since the wavelet
transform is a useful mathematical tool for nonstationary signals, this
technique applied to Kepler and CoRoT light curves allows us to clearly
identify particular signatures for different phenomena. In particular, patterns
were identified for the temporal evolution of the rotation period and other
periodicity due to active regions affecting these light curves. In addition, a
beat-pattern signature in the local wavelet map of pulsating stars over the
entire time span was also detected.Comment: Accepted for publication on A&
Real-time on-board obstacle avoidance for UAVs based on embedded stereo vision
In order to improve usability and safety, modern unmanned aerial vehicles
(UAVs) are equipped with sensors to monitor the environment, such as
laser-scanners and cameras. One important aspect in this monitoring process is
to detect obstacles in the flight path in order to avoid collisions. Since a
large number of consumer UAVs suffer from tight weight and power constraints,
our work focuses on obstacle avoidance based on a lightweight stereo camera
setup. We use disparity maps, which are computed from the camera images, to
locate obstacles and to automatically steer the UAV around them. For disparity
map computation we optimize the well-known semi-global matching (SGM) approach
for the deployment on an embedded FPGA. The disparity maps are then converted
into simpler representations, the so called U-/V-Maps, which are used for
obstacle detection. Obstacle avoidance is based on a reactive approach which
finds the shortest path around the obstacles as soon as they have a critical
distance to the UAV. One of the fundamental goals of our work was the reduction
of development costs by closing the gap between application development and
hardware optimization. Hence, we aimed at using high-level synthesis (HLS) for
porting our algorithms, which are written in C/C++, to the embedded FPGA. We
evaluated our implementation of the disparity estimation on the KITTI Stereo
2015 benchmark. The integrity of the overall realtime reactive obstacle
avoidance algorithm has been evaluated by using Hardware-in-the-Loop testing in
conjunction with two flight simulators.Comment: Accepted in the International Archives of the Photogrammetry, Remote
Sensing and Spatial Information Scienc
Chromospheric heating by acoustic waves compared to radiative cooling
Acoustic and magnetoacoustic waves are among the possible candidate
mechanisms that heat the upper layers of solar atmosphere. A weak chromospheric
plage near a large solar pore NOAA 11005 was observed on October 15, 2008 in
the lines Fe I 617.3 nm and Ca II 853.2 nm with the Interferometric
Bidimemsional Spectrometer (IBIS) attached to the Dunn Solar Telescope.
Analyzing the Ca II observations with spatial and temporal resolutions of 0.4"
and 52 s, the energy deposited by acoustic waves is compared with that released
by radiative losses. The deposited acoustic flux is estimated from power
spectra of Doppler oscillations measured in the Ca II line core. The radiative
losses are calculated using a grid of seven 1D hydrostatic semi-empirical model
atmospheres. The comparison shows that the spatial correlation of maps of
radiative losses and acoustic flux is 72 %. In quiet chromosphere, the
contribution of acoustic energy flux to radiative losses is small, only of
about 15 %. In active areas with photospheric magnetic field strength between
300 G and 1300 G and inclination of 20-60 degrees, the contribution increases
from 23 % (chromospheric network) to 54 % (a plage). However, these values have
to be considered as lower limits and it might be possible that the acoustic
energy flux is the main contributor to the heating of bright chromospheric
network and plages.Comment: 9 pages, 10 figures. Accepted for publication in The Astrophysical
Journa
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