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 ($\lambda=2.2$ $\mu$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 $I_c$ and $I_s$ can be mapped over the image plane. As the quantity $I_c$ 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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