301 research outputs found
Speckle Statistics in Adaptively Corrected Images
(abridged) Imaging observations are generally affected by a fluctuating
background of speckles, a particular problem when detecting faint stellar
companions at small angular separations. Knowing the distribution of the
speckle intensities at a given location in the image plane is important for
understanding the noise limits of companion detection. The speckle noise limit
in a long-exposure image is characterized by the intensity variance and the
speckle lifetime. In this paper we address the former quantity through the
distribution function of speckle intensity. Previous theoretical work has
predicted a form for this distribution function at a single location in the
image plane. We developed a fast readout mode to take short exposures of
stellar images corrected by adaptive optics at the ground-based UCO/Lick
Observatory, with integration times of 5 ms and a time between successive
frames of 14.5 ms ( m). These observations temporally
oversample and spatially Nyquist sample the observed speckle patterns. We show,
for various locations in the image plane, the observed distribution of speckle
intensities is consistent with the predicted form. Additionally, we demonstrate
a method by which and can be mapped over the image plane. As the
quantity is proportional to the PSF of the telescope free of random
atmospheric aberrations, this method can be used for PSF calibration and
reconstruction.Comment: 7 pages, 4 figures, ApJ accepte
Lucky Imaging: High Angular Resolution Imaging in the Visible from the Ground
We use a Lucky Imaging system to obtain I-band images with much improved
angular resolution on a ground-based 2.5m telescope. We present results from a
10-night assessment campaign on the 2.56m Nordic Optical Telescope and quantify
the performance of our system in seeings better than 1.0''. In good seeing we
have acquired near diffraction-limited images; in poorer seeing the angular
resolution has been routinely improved by factors of 2.5-4. The system can use
guide stars as faint as I=16 with full performance and its useful field of view
is consistently larger than 40" diameter. The technique shows promise for a
number of science programmes, both galactic (eg. binary candidates, brown
dwarfs, globular cluster cores) and extragalactic (eg. quasar host galaxies,
damped Lyman-alpha absorbers).Comment: 7 pages, 10 figures, submitted to A&A. For further information, see
http://www.ast.cam.ac.uk/~optics/Lucky_Web_Site
A New Strategy for Deep Wide-Field High Resolution Optical Imaging
We propose a new strategy for obtaining enhanced resolution (FWHM = 0.12
arcsec) deep optical images over a wide field of view. As is well known, this
type of image quality can be obtained in principle simply by fast guiding on a
small (D = 1.5m) telescope at a good site, but only for target objects which
lie within a limited angular distance of a suitably bright guide star. For high
altitude turbulence this 'isokinetic angle' is approximately 1 arcminute. With
a 1 degree field say one would need to track and correct the motions of
thousands of isokinetic patches, yet there are typically too few sufficiently
bright guide stars to provide the necessary guiding information. Our proposed
solution to these problems has two novel features. The first is to use
orthogonal transfer charge-coupled device (OTCCD) technology to effectively
implement a wide field 'rubber focal plane' detector composed of an array of
cells which can be guided independently. The second is to combine measured
motions of a set of guide stars made with an array of telescopes to provide the
extra information needed to fully determine the deflection field. We discuss
the performance, feasibility and design constraints on a system which would
provide the collecting area equivalent to a single 9m telescope, a 1 degree
square field and 0.12 arcsec FWHM image quality.Comment: 46 pages, 22 figures, submitted to PASP, a version with higher
resolution images and other supplementary material can be found at
http://www.ifa.hawaii.edu/~kaiser/wfhr
The effects of disk and dust structure on observed polarimetric images of protoplanetary disks
Imaging polarimetry is a powerful tool for imaging faint circumstellar
material. For a correct analysis of observations we need to fully understand
the effects of dust particle parameters, as well as the effects of the
telescope, atmospheric seeing, and assumptions about the data reduction and
processing of the observed signal. Here we study the major effects of dust
particle structure, size-dependent grain settling, and instrumental properties.
We performed radiative transfer modeling using different dust particle models
and disk structures. To study the influence of seeing and telescope diffraction
we ran the models through an instrument simulator for the ExPo dual-beam
imaging polarimeter mounted at the 4.2m William Herschel Telescope (WHT).
Particle shape and size have a strong influence on the brightness and
detectability of the disks. In the simulated observations, the central
resolution element also contains contributions from the inner regions of the
protoplanetary disk besides the unpolarized central star. This causes the
central resolution element to be polarized, making simple corrections for
instrumental polarization difficult. This effect strongly depends on the
spatial resolution, so adaptive optics systems are needed for proper
polarization calibration. We find that the commonly employed homogeneous sphere
model gives results that differ significantly from more realistic models. For a
proper analysis of the wealth of data available now or in the near future, one
must properly take the effects of particle types and disk structure into
account. The observed signal depends strongly on the properties of these more
realistic models, thus providing a potentially powerful diagnostic. We conclude
that it is important to correctly understand telescope depolarization and
calibration effects for a correct interpretation of the degree of polarization.Comment: Accepted for publication in A&
Astronomical site selection: On the use of satellite data for aerosol content monitoring
The main goal of this work is the analysis of new approaches to the study of
the properties of astronomical sites. In particular, satellite data measuring
aerosols have recently been proposed as a useful technique for site
characterization and searching for new sites to host future very large
telescopes. Nevertheless, these data need to be critically considered and
interpreted in accordance with the spatial resolution and spectroscopic
channels used. In this paper we have explored and retrieved measurements from
satellites with high spatial and temporal resolutions and concentrated on
channels of astronomical interest. The selected datasets are OMI on board the
NASA Aura satellite and MODIS on board the NASA Terra and Aqua satellites. A
comparison of remote sensing and in situ techniques is discussed. As a result,
we find that aerosol data provided by satellites up to now are not reliable
enough for aerosol site characterization, and in situ data are required.Comment: LaTeX2e MN v2.2, 16 pages with 13 figures. Accepted for Mon. Not. R.
Astron. Soc.(2008
Fréquence du syndrome d'hyperthermie maligne dans des populations porcines françaises; relation avec le développement musculaire
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