10,620 research outputs found
Searchable Sky Coverage of Astronomical Observations: Footprints and Exposures
Sky coverage is one of the most important pieces of information about
astronomical observations. We discuss possible representations, and present
algorithms to create and manipulate shapes consisting of generalized spherical
polygons with arbitrary complexity and size on the celestial sphere. This shape
specification integrates well with our Hierarchical Triangular Mesh indexing
toolbox, whose performance and capabilities are enhanced by the advanced
features presented here. Our portable implementation of the relevant spherical
geometry routines comes with wrapper functions for database queries, which are
currently being used within several scientific catalog archives including the
Sloan Digital Sky Survey, the Galaxy Evolution Explorer and the Hubble Legacy
Archive projects as well as the Footprint Service of the Virtual Observatory.Comment: 11 pages, 7 figures, submitted to PAS
Flip Distance Between Triangulations of a Planar Point Set is APX-Hard
In this work we consider triangulations of point sets in the Euclidean plane,
i.e., maximal straight-line crossing-free graphs on a finite set of points.
Given a triangulation of a point set, an edge flip is the operation of removing
one edge and adding another one, such that the resulting graph is again a
triangulation. Flips are a major way of locally transforming triangular meshes.
We show that, given a point set in the Euclidean plane and two
triangulations and of , it is an APX-hard problem to minimize
the number of edge flips to transform to .Comment: A previous version only showed NP-completeness of the corresponding
decision problem. The current version is the one of the accepted manuscrip
A Generalization of the Convex Kakeya Problem
Given a set of line segments in the plane, not necessarily finite, what is a
convex region of smallest area that contains a translate of each input segment?
This question can be seen as a generalization of Kakeya's problem of finding a
convex region of smallest area such that a needle can be rotated through 360
degrees within this region. We show that there is always an optimal region that
is a triangle, and we give an optimal \Theta(n log n)-time algorithm to compute
such a triangle for a given set of n segments. We also show that, if the goal
is to minimize the perimeter of the region instead of its area, then placing
the segments with their midpoint at the origin and taking their convex hull
results in an optimal solution. Finally, we show that for any compact convex
figure G, the smallest enclosing disk of G is a smallest-perimeter region
containing a translate of every rotated copy of G.Comment: 14 pages, 9 figure
The Footprint Database and Web Services of the Herschel Space Observatory
Data from the Herschel Space Observatory is freely available to the public
but no uniformly processed catalogue of the observations has been published so
far. To date, the Herschel Science Archive does not contain the exact sky
coverage (footprint) of individual observations and supports search for
measurements based on bounding circles only. Drawing on previous experience in
implementing footprint databases, we built the Herschel Footprint Database and
Web Services for the Herschel Space Observatory to provide efficient search
capabilities for typical astronomical queries. The database was designed with
the following main goals in mind: (a) provide a unified data model for
meta-data of all instruments and observational modes, (b) quickly find
observations covering a selected object and its neighbourhood, (c) quickly find
every observation in a larger area of the sky, (d) allow for finding solar
system objects crossing observation fields. As a first step, we developed a
unified data model of observations of all three Herschel instruments for all
pointing and instrument modes. Then, using telescope pointing information and
observational meta-data, we compiled a database of footprints. As opposed to
methods using pixellation of the sphere, we represent sky coverage in an exact
geometric form allowing for precise area calculations. For easier handling of
Herschel observation footprints with rather complex shapes, two algorithms were
implemented to reduce the outline. Furthermore, a new visualisation tool to
plot footprints with various spherical projections was developed. Indexing of
the footprints using Hierarchical Triangular Mesh makes it possible to quickly
find observations based on sky coverage, time and meta-data. The database is
accessible via a web site (http://herschel.vo.elte.hu) and also as a set of
REST web service functions.Comment: Accepted for publication in Experimental Astronom
Quasi-Fuchsian manifolds with particles
We consider 3-dimensional hyperbolic cone-manifolds, singular along infinite
lines, which are ``convex co-compact'' in a natural sense. We prove an
infinitesimal rigidity statement when the angle around the singular lines is
less than : any first-order deformation changes either one of those angles
or the conformal structure at infinity, with marked points corresponding to the
endpoints of the singular lines. Moreover, any small variation of the conformal
structure at infinity and of the singular angles can be achieved by a unique
small deformation of the cone-manifold structure.Comment: Now 48 pages, no figure. v2: new title, various corrections, results
extended to include graph singularities ("interacting particles"). v3:
various corrections/improvements, in particular thanks to comments by an
anonymous refere
- …