7,570 research outputs found
Observing the Sun with Atacama Large Millimeter/submillimeter Array (ALMA): High Resolution Interferometric Imaging
Observations of the Sun at millimeter and submillimeter wavelengths offer a
unique probe into the structure, dynamics, and heating of the chromosphere; the
structure of sunspots; the formation and eruption of prominences and filaments;
and energetic phenomena such as jets and flares. High-resolution observations
of the Sun at millimeter and submillimeter wavelengths are challenging due to
the intense, extended, low- contrast, and dynamic nature of emission from the
quiet Sun, and the extremely intense and variable nature of emissions
associated with energetic phenomena. The Atacama Large Millimeter/submillimeter
Array (ALMA) was designed with solar observations in mind. The requirements for
solar observations are significantly different from observations of sidereal
sources and special measures are necessary to successfully carry out this type
of observations. We describe the commissioning efforts that enable the use of
two frequency bands, the 3 mm band (Band 3) and the 1.25 mm band (Band 6), for
continuum interferometric-imaging observations of the Sun with ALMA. Examples
of high-resolution synthesized images obtained using the newly commissioned
modes during the solar commissioning campaign held in December 2015 are
presented. Although only 30 of the eventual 66 ALMA antennas were used for the
campaign, the solar images synthesized from the ALMA commissioning data reveal
new features of the solar atmosphere that demonstrate the potential power of
ALMA solar observations. The ongoing expansion of ALMA and solar-commissioning
efforts will continue to enable new and unique solar observing capabilities.Comment: 22 pages, 12 figures, accepted for publication in Solar Physic
Adaptive Optics Scanning Ophthalmoscopy with Annular Pupils
Annular apodization of the illumination and/or imaging pupils of an adaptive optics scanning light ophthalmoscope (AOSLO) for improving transverse resolution was evaluated using three different normalized inner radii (0.26, 0.39 and 0.52). In vivo imaging of the human photoreceptor mosaic at 0.5 and 10° from fixation indicates that the use of an annular illumination pupil and a circular imaging pupil provides the most benefit of all configurations when using a one Airy disk diameter pinhole, in agreement with the paraxial confocal microscopy theory. Annular illumination pupils with 0.26 and 0.39 normalized inner radii performed best in terms of the narrowing of the autocorrelation central lobe (between 7 and 12%), and the increase in manual and automated photoreceptor counts (8 to 20% more cones and 11 to 29% more rods). It was observed that the use of annular pupils with large inner radii can result in multi-modal cone photoreceptor intensity profiles. The effect of the annular masks on the average photoreceptor intensity is consistent with the Stiles-Crawford effect (SCE). This indicates that combinations of images of the same photoreceptors with different apodization configurations and/or annular masks can be used to distinguish cones from rods, even when the former have complex multi-modal intensity profiles. In addition to narrowing the point spread function transversally, the use of annular apodizing masks also elongates it axially, a fact that can be used for extending the depth of focus of techniques such as adaptive optics optical coherence tomography (AOOCT). Finally, the positive results from this work suggest that annular pupil apodization could be used in refractive or catadioptric adaptive optics ophthalmoscopes to mitigate undesired back-reflections
Geologic analysis of ERTS-1 imagery for the State of New Mexico
There are no author-identified significant results in this report
Olympic Coast National Marine Sanctuary Habitat Mapping: Survey report and classification of side scan sonar data from surveys HMPR-114-2004-02 and HMPR-116-2005-01.
The Olympic Coast National Marine Sanctuary (OCNMS) continues to invest significant resources into seafloor mapping activities along Washington’s outer coast (Intelmann and Cochrane 2006; Intelmann et al. 2006; Intelmann 2006). Results from these annual mapping efforts offer a snapshot of current ground conditions, help to guide research and management activities, and provide a baseline for assessing the impacts of various threats
to important habitat. During the months of August 2004 and May and July 2005, we used side scan sonar to image several regions of the sea floor in the northern OCNMS, and the
data were mosaicked at 1-meter pixel resolution. Video from a towed camera sled, bathymetry data, sedimentary samples and side scan sonar mapping were integrated to describe geological and biological aspects of habitat. Polygon features were created and attributed with a hierarchical deep-water marine benthic classification scheme (Greene et
al. 1999). For three small areas that were mapped with both side scan sonar and multibeam echosounder, we made a comparison of output from the classified images indicating little difference in results between the two methods. With these considerations, backscatter derived from multibeam bathymetry is currently a costefficient and safe method for seabed imaging in the shallow (<30 meters) rocky waters of
OCNMS. The image quality is sufficient for classification purposes, the associated depths provide further descriptive value and risks to gear are minimized. In shallow waters (<30 meters) which do not have a high incidence of dangerous rock pinnacles, a towed multi-beam side scan sonar could provide a better option for obtaining seafloor
imagery due to the high rate of acquisition speed and high image quality, however the high probability of losing or damaging such a costly system when deployed as a towed
configuration in the extremely rugose nearshore zones within OCNMS is a financially risky proposition. The development of newer technologies such as intereferometric
multibeam systems and bathymetric side scan systems could also provide great potential for mapping these nearshore rocky areas as they allow for high speed data acquisition,
produce precisely geo-referenced side scan imagery to bathymetry, and do not experience the angular depth dependency associated with multibeam echosounders allowing larger range scales to be used in shallower water. As such, further investigation of these systems is needed to assess their efficiency and utility in these environments compared to traditional side scan sonar and multibeam bathymetry. (PDF contains 43 pages.
FIREX mission requirements document for nonrenewable resources
The proposed mission requirements and a proposed experimental program for satellite synthetic aperture radar (SAR) system named FIREX (Free-Flying Imaging Radar Experiment) for nonrenewable resources is described. The recommended spacecraft minimum SAR system is a C-band imager operating in four modes: (1) low look angle HH-polarized; (2) intermediate look angle, HH-polarized; (3) intermediate look angle, IIV-polarized; and (4) high look angle HH-polarized. This SAR system is complementary to other future spaceborne imagers such as the Thematic Mapper on LANDSAT-D. A near term aircraft SAR based research program is outlined which addresses specific mission design issues such as preferred incidence angles or polarizations for geologic targets of interest
P algorithm, a dramatic enhancement of the waterfall transformation
This document has been extended by "Towards a unification of waterfalls, standard and P algorithms", see http://hal-ensmp.archives-ouvertes.fr/hal-00835016.This document describes an efficient enhancement of the waterfall algorithm, a hierarchical segmentation algorithm defined from the watershed transformation. The first part of the document recalls the definition of the waterfall algorithm, its various avatars as well as its links with the geodesic reconstruction. The second part starts by analyzing the different shortcomings of the algorithm and introduces several strategies to palliate them. Two enhancements are presented, the first one named standard algorithm and the second one, P algorithm. The different properties of P algorithm are analyzed. This analysis is detailed in the last part of the document. The performances of the two algorithms, in particular, are addressed and their analogies with perception mechanisms linked to the brightness constancy phenomenon are discussed
ATLBS: the Australia Telescope Low-brightness Survey
We present a radio survey carried out with the Australia Telescope Compact
Array. A motivation for the survey was to make a complete inventory of the
diffuse emission components as a step towards a study of the cosmic evolution
in radio source structure and the contribution from radio-mode feedback on
galaxy evolution. The Australia Telescope low-brightness survey (ATLBS) at 1388
MHz covers 8.42 sq deg of the sky in an observing mode designed to yield images
with exceptional surface brightness sensitivity and low confusion. The ATLBS
radio images, made with 0.08 mJy/beam rms noise and 50" beam, detect a total of
1094 sources with peak flux exceeding 0.4 mJy/beam. The ATLBS source counts
were corrected for blending, noise bias, resolution, and primary beam
attenuation; the normalized differential source counts are consistent with no
upturn down to 0.6 mJy. The percentage integrated polarization Pi_0 was
computed after corrections for the polarization bias in integrated polarized
intensity; Pi_0 shows an increasing trend with decreasing flux density.
Simultaneous visibility measurements made with longer baselines yielded images,
with 5" beam, of compact components in sources detected in the survey. The
observations provide a measurement of the complexity and diffuse emission
associated with mJy and sub-mJy radio sources. 10% of the ATLBS sources have
more than half of their flux density in extended emission and the fractional
flux in diffuse components does not appear to vary with flux density, although
the percentage of sources that have complex structure increases with flux
density. The observations are consistent with a transition in the nature of
extended radio sources from FR-II radio source morphology, which dominates the
mJy population, to FR-I structure at sub-mJy flux density. (Abridged)Comment: 18 pages, 8 figues, 6 tables, accepted for publication in MNRA
The Local Radio-IR Relation in M51
We observed M51 at three frequencies, 1.4 GHz (20 cm), 4.9 GHz (6 cm), and 8.4 GHz (3.6 cm), with the Very Large Array and the Effelsberg 100 m telescope to obtain the highest quality radio continuum images of a nearby spiral galaxy. These radio data were combined with deconvolved Spitzer IRAC 8 μm and MIPS 24 μm images to search for and investigate local changes in the radio-IR correlation. Utilizing wavelet decomposition, we compare the distribution of the radio and IR emission on spatial scales between 200 pc and 30 kpc. We show that the radio-IR correlation is not uniform across the galactic disk. It presents a complex behavior with local extrema corresponding to various galactic structures, such as complexes of H II regions, spiral arms, and interarm filaments, indicating that the contribution of the thermal and non-thermal radio emission is a strong function of environment. In particular, the relation of the 24 μm and 20 cm emission presents a linear relation within the spiral arms and globally over the galaxy, while it deviates from linearity in the interarm and outer regions as well in the inner region, with two different behaviors: it is sublinear in the interarm and outer region and overlinear in the central 3.5 kpc. Our analysis suggests that the changes in the radio/IR correlation reflect variations of interstellar medium properties between spiral arms and interarm region. The good correlation in the spiral arms implies that 24 μm and 20 cm are tracing recent star formation, while a change in the dust opacity, "Cirrus" contribution to the IR emission and/or the relation between the magnetic field strength and the gas density can explain the different relations found in the interarm, outer, and inner regions
The circumstellar envelope around the S-type AGB star W Aql Effects of an eccentric binary orbit
The CO(J=3-2) emission from the CSE of the binary S-type AGB star W Aql has
been observed at subarcsecond resolution using ALMA. The aim of this paper is
to investigate the wind properties of the AGB star and to analyse how the known
companion has shaped the CSE. The average mass-loss rate during the creation of
the detected CSE is estimated through modelling, using the ALMA brightness
distribution and previously published single-dish measurements as observational
constraints. The ALMA observations are presented and compared to the results
from a 3D smoothed particle hydrodynamics (SPH) binary interaction model with
the same properties as the W Aql system and with two different orbital
eccentricities. Three-dimensional radiative transfer modelling is performed and
the response of the interferometer is modelled and discussed. The estimated
average mass-loss rate of W~Aql agrees with previous results. The size of the
emitting region is consistent with photodissociation models. The CO(J=3-2)
emission is dominated by a smooth component overlayed with two weak arc
patterns with different separations. The larger pattern is predicted by the
binary interaction model with separations of 10" and therefore likely due to
the known companion. It is consistent with a binary orbit with low
eccentricity. The smaller separation pattern is asymmetric and coincides with
the dust distribution, but the separation timescale (200 yrs) is not consistent
with any known process of the system. The separation of the known companions of
the system is large enough to not have a very strong effect on the
circumstellar morphology. The density contrast across the envelope of a binary
with an even larger separation will not be easily detectable, even with ALMA,
unless the orbit is strongly asymmetric or the AGB star has a much larger
mass-loss rate.Comment: 10 pages, 8 figure
A Constraint on the Organization of the Galactic Center Magnetic Field Using Faraday Rotation
We present new 6 and 20 cm Very Large Array (VLA) observations of polarized
continuum emission of roughly 0.5 square degrees of the Galactic center (GC)
region. The 6 cm observations detect diffuse linearly-polarized emission
throughout the region with a brightness of roughly 1 mJy per 15"x10" beam. The
Faraday rotation measure (RM) toward this polarized emission has structure on
degree size scales and ranges from roughly +330 rad/m2 east of the dynamical
center (Sgr A) to -880 rad/m2 west of the dynamical center. This RM structure
is also seen toward several nonthermal radio filaments, which implies that they
have a similar magnetic field orientation and constrains models for their
origin. Modeling shows that the RM and its change with Galactic longitude are
best explained by the high electron density and strong magnetic field of the GC
region. Considering the emissivity of the GC plasma shows that while the
absolute RM values are indirect measures of the GC magnetic field, the RM
longitude structure directly traces the magnetic field in the central
kiloparsec of the Galaxy. Combining this result with previous work reveals a
larger RM structure covering the central ~2 degrees of the Galaxy. This RM
structure is similar to that proposed by Novak and coworkers, but is shifted
roughly 50 pc west of the dynamical center of the Galaxy. If this RM structure
originates in the GC region, it shows that the GC magnetic field is organized
on ~300 pc size scales. The pattern is consistent with a predominantly poloidal
field geometry, pointing from south to north, that is perturbed by the motion
of gas in the Galactic disk.Comment: Accepted to ApJ. emulateapj style, 14 pages, 15 figure
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