401 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
Optical Seeing at Sierra Negra
Optical seeing measurements carried out at Sierra Negra, the site of the
Large Millimeter Telescope, are reported. The site, one of the highest peaks of
Central Mexico, offers good coverage of Northern and Southern hemispheres and
we have undertaken several campaigns to investigate the astronomical potential
of the site in the optical. Here we report on our campaign aimed at
establishing the seeing quality of the site. We present data of the first three
campaigns of optical seeing monitoring covering from February 2000 to May 2002,
carried out with a Differential Image Motion Monitor. The results clearly
indicate a sub-arcsec seeing, better statistics during the dry season and no
dependence with the time of night. We find no dependence of our results with
the integration time used.Comment: Accepted for publication in the July 2003 issue of the PAS
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
The best skies for astronomy
Seminario impartido por Casiana Muñoz-Tuñón (IAC) el 15 de noviembre de 2016 en las Oficinas del CIAI en Santa Cruz de Tenerife
Dome C site testing: surface layer, free atmosphere seeing and isoplanatic angle statistics
This paper analyses 3.5 years of site testing data obtained at Dome C,
Antarctica, based on measurements obtained with three DIMMs located at three
different elevations. Basic statistics of the seeing and the isoplanatic angle
are given, as well as the characteristic time of temporal fluctuations of these
two parameters, which we found to around 30 minutes at 8 m. The 3 DIMMs are
exploited as a profiler of the surface layer, and provide a robust estimation
of its statistical properties. It appears to have a very sharp upper limit
(less than 1 m). The fraction of time spent by each telescope above the top of
the surface layer permits us to deduce a median height of between 23 m and 27
m. The comparison of the different data sets led us to infer the statistical
properties of the free atmosphere seeing, with a median value of 0.36 arcsec.
The C_n^2 profile inside the surface layer is also deduced from the seeing data
obtained during the fraction of time spent by the 3 telescopes inside this
turbulence. Statistically, the surface layer, except during the 3-month summer
season, contributes to 95 percent of the total turbulence from the surface
level, thus confirming the exceptional quality of the site above it
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
European Extremely Large Telescope Site Characterization II: High angular resolution parameters
This is the second article of a series devoted to European Extremely Large
Telescope (E-ELT) site characterization. In this article we present the main
properties of the parameters involved in high angular resolution observations
from the data collected in the site testing campaign of the E-ELT during the
Design Study (DS) phase. Observations were made in 2008 and 2009, in the four
sites selected to shelter the future E-ELT (characterized under the ELT-DS
contract): Aklim mountain in Morocco, Observatorio del Roque de los Muchachos
(ORM) in Spain, Mac\'on range in Argentina, and Cerro Ventarrones in Chile. The
same techniques, instruments and acquisition procedures were taken on each
site. A Multiple Aperture Scintillation Sensor (MASS) and a Differential Image
Motion Monitor (DIMM) were installed at each site. Global statistics of the
integrated seeing, the free atmosphere seeing, the boundary layer seeing and
the isoplanatic angle were studied for each site, and the results are presented
here. In order to estimate other important parameters such as the coherence
time of the wavefront and the overall parameter "coherence \'etendue"
additional information of vertical profiles of the wind speed was needed. Data
were retrieved from the National Oceanic and Atmospheric Administration (NOAA)
archive. Ground wind speed was measured by Automatic Weather Stations (AWS).
More aspects of the turbulence parameters such as their seasonal trend, their
nightly evolution and their temporal stability were also obtained and analyzed.Comment: 46 pages and 17 figures. Accepted to be published in PAS
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