Results 1 - 10 of 1221
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[en] The Sloan Digital Sky Survey calibration is revisited to obtain the most accurate photometric calibration. A small but significant error is found in the flat-fielding of the Photometric telescope used for calibration. Two SDSS star catalogs are compared and the average difference in magnitude as a function of right ascension and declination exhibits small systematic errors in relative calibration. The photometric transformation from the SDSS Photometric Telescope to the 2.5 m telescope is recomputed and compared to synthetic magnitudes computed from measured filter bandpasses.
[en] In this Letter, we search for concentric circles with low variance in cosmic microwave background sky maps. The detection of such circles would hint at new physics beyond the current cosmological concordance model, which states that the universe is isotropic and homogeneous, and filled with Gaussian fluctuations. We first describe a set of methods designed to detect such circles, based on matched filters and χ2 statistics, and then apply these methods to the best current publicly available data, the 7 year Wilkinson Microwave Anisotropy Probe (WMAP) temperature sky maps. We compare the observations with an ensemble of 1000 Gaussian ΛCDM simulations. Based on these tests, we conclude that the WMAP sky maps are fully compatible with the Gaussian and isotropic hypothesis as measured by low-variance ring statistics.
[en] This work presents a characterization of the luminous efficacy of the direct radiation for Florianopolis, Brazil, for which several direct luminous efficacy models available in the scientific literature were evaluated. In the first part of the work, the evaluation was restricted to clear sky conditions. The models evaluated were used with the original coefficients given by their authors and also with local coefficients obtained for Florianopolis, showing their dependence on the local characteristics. None of the models presented significantly better statistics than the others, although one of them, developed by Robledo and Soler, seems to justify better the physical behavior of the direct luminous efficacy. In the second part of the paper, three models of direct luminous efficacy for all sky types have been evaluated, the first two having been specifically developed for the direct luminous efficacy, whereas and the third was developed for the global luminous efficacy and has been adapted in this paper to characterize the direct luminous efficacy. The statistical evaluation of the three models showed that the last general model has accuracy similar to the two other ones
[en] The breaking up of comets into separate pieces, each with its own tail, was seen many times by astronomers of the past. The phenomenon was in sharp contrast to the idea of the eternal and unchangeable celestial firmament and was commonly believed to be an omen of impending disaster, especially for comets with tails stretching across half the sky. It is only now that we have efficient enough space exploration tools to see comet nuclei and even - in the particular case of small comet Hartley-2 in 2010 - to watch their disintegration stage. There are also other suspected candidates for disintegration in the vast family of comet nuclei and other Solar System bodies. (physics of our days)
[en] The first release of the Chandra Source Catalog (CSC) contains ∼95,000 X-ray sources in a total area of 0.75% of the entire sky, using data from ∼3900 separate ACIS observations of a multitude of different types of X-ray sources. In order to maximize the scientific benefit of such a large, heterogeneous data set, careful characterization of the statistical properties of the catalog, i.e., completeness, sensitivity, false source rate, and accuracy of source properties, is required. Characterization efforts of other large Chandra catalogs, such as the ChaMP Point Source Catalog or the 2 Mega-second Deep Field Surveys, while informative, cannot serve this purpose, since the CSC analysis procedures are significantly different and the range of allowable data is much less restrictive. We describe here the characterization process for the CSC. This process includes both a comparison of real CSC results with those of other, deeper Chandra catalogs of the same targets and extensive simulations of blank-sky and point-source populations.
[en] We present an algorithm to photometrically calibrate widefield optical imaging surveys, that simultaneously solves for the calibration parameters and relative stellar fluxes using overlapping observations. The algorithm decouples the problem of 'relative' calibrations from that of 'absolute' calibrations; the absolute calibration is reduced to determining a few numbers for the entire survey. We pay special attention to the spatial structure of the calibration errors, allowing one to isolate particular error modes in downstream analyses. Applying this to the Sloan Digital Sky Survey imaging data, we achieve ∼1 percent relative calibration errors across 8500sq.deg/ in griz; the errors are ∼2 percent for the u band. These errors are dominated by unmodelled atmospheric variations at Apache Point Observatory. These calibrations, dubbed ubercalibration, are now public with SDSS Data Release 6, and will be a part of subsequent SDSS data releases
[en] In the hierarchical all-sky search method for periodic sources developed in the Virgo Collaboration, after the first incoherent step based on the Hough transform we have a number of candidates that must be analyzed using a much longer time baseline periodogram. In this step, we correct the signal power spread due to the Earth rotation, which becomes relevant for observation times greater than ∼1 sidereal day, using a bank of matched filters on the power spectrum. Describing the wave as the combination of a linearly polarized and a circularly polarized component, the spread depends on two parameters: the polarization angle of the linearly polarized wave component and the percentage of linear polarization. Here we discuss in detail the issue of how many filters must be built to properly cover the parameter space and give some implementation details of the procedure.
[en] An investigation of estimating daily clear sky insolation has been conducted. Some discrepancies have been found between the conventional method of estimation and the data. It has been noticed that the conventional method under estimates the insolation. A simpler, more elegant and more accurate 'universal' relationship has been derived for estimating the daily clear sky insolation as a function of the day of the year and the latitude and altitude of a given location