330,697 research outputs found
Cosmic-Ray Rejection by Laplacian Edge Detection
Conventional algorithms for rejecting cosmic-rays in single CCD exposures
rely on the contrast between cosmic-rays and their surroundings, and may
produce erroneous results if the Point Spread Function (PSF) is smaller than
the largest cosmic-rays. This paper describes a robust algorithm for cosmic-ray
rejection, based on a variation of Laplacian edge detection. The algorithm
identifies cosmic-rays of arbitrary shapes and sizes by the sharpness of their
edges, and reliably discriminates between poorly sampled point sources and
cosmic-rays. Examples of its performance are given for spectroscopic and
imaging data, including HST WFPC2 images.Comment: Accepted for publication in the PASP (November 2001 issue). The
algorithm is implemented in the program L.A.Cosmic, which can be obtained
from http://www.astro.caltech.edu/~pgd/lacosmic
Optimization problems involving the first Dirichlet eigenvalue and the torsional rigidity
We present some open problems and obtain some partial results for spectral
optimization problems involving measure, torsional rigidity and first Dirichlet
eigenvalue.Comment: 18 pages, 4 figure
A new modelling framework for statistical cumulus dynamics
We propose a new modelling framework suitable for the description of atmospheric convective systems as a collection of distinct plumes. The literature contains many examples of models for collections of plumes in which strong simplifying assumptions are made, a diagnostic dependence of convection on the large-scale environment and the limit of many plumes often being imposed from the outset. Some recent studies have sought to remove one or the other of those assumptions. The proposed framework removes both, and is explicitly time-dependent and stochastic in its basic character. The statistical dynamics of the plume collection are defined through simple probabilistic rules applied at the level of individual plumes, and van Kampen's system size expansion is then used to construct the macroscopic limit of the microscopic model. Through suitable choices of the microscopic rules, the model is shown to encompass previous studies in the appropriate limits, and to allow their natural extensions beyond those limits
Slow relaxation, dynamic transitions and extreme value statistics in disordered systems
We show that the dynamics of simple disordered models, like the directed Trap
Model and the Random Energy Model, takes place at a coexistence point between
active and inactive dynamical phases. We relate the presence of a dynamic phase
transition in these models to the extreme value statistics of the associated
random energy landscape
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