The Hipparcos Transit Data: What, why and how?

The Hipparcos Transit Data are a collection of partially reduced, fully calibrated observations of (mostly) double and multiple stars obtained with the ESA Hipparcos astrometry satellite. The data are publicly available, as part of the CD-ROM set distributed with the Hipparcos and Tycho Catalogues (ESA SP--1200, 1997), for about a third of the Hipparcos Catalogue entries including all confirmed or suspected non-single stars. The Transit Data consist of signal modulation parameters derived from the individual transits of the targets across the Hipparcos focal grid. The Transit Data permit re-reduction of the satellite data for individual objects, using arbitrarily complex object models in which time-variable photometric as well as geometric characteristics may be taken into account. We describe the structure and contents of the Transit Data files and give examples of how the data can be used. Some of the applications use standard astronomical software: Difmap or AIPS for aperture synthesis imaging, and GaussFit for detailed model fitting. Fortran code converting the data into formats suitable for these application programs has been made public in order to encourage and facilitate the use of Hipparcos Transit Data.Comment: A&AS, accepted for publication, 17 pages, 9 figures, 1 Table, Software available via http://www.astro.lu.se/~lennart/TD/index.html, Figures 4, 5, 6 and 7 need to copied separately, A complete postscript file can be found at http://www.astro.lu.se/~lennart/TD/ds1699.ps.g

Semantics and synthesis of signals in behavioral VHDL

Signals are a fundamental part of VHDL behavioral descriptions. There are many kinds of VHDL signals, each possesing complex and hence often misunderstood semantics. The result is that synthesis tools often inadequately address synthesis of signals. In this report, we first make clear the semantics of the various signal kinds shared by multiple processes through the use of conceptual hardware, rather than just text. Second, with the semantics firmly understood, we discuss techniques and issues in synthesizing actual hardware for shared signals. This information can be used to take a step towards synthesizing correct hardware from VHDL descriptions while greatly reducing current restrictions imposed by synthesis tools on allowable VHDL behavior

The non-coplanar baselines effect in radio interferometry: The W-Projection algorithm

We consider a troublesome form of non-isoplanatism in synthesis radio telescopes: non-coplanar baselines. We present a novel interpretation of the non-coplanar baselines effect as being due to differential Fresnel diffraction in the neighborhood of the array antennas. We have developed a new algorithm to deal with this effect. Our new algorithm, which we call "W-projection", has markedly superior performance compared to existing algorithms. At roughly equivalent levels of accuracy, W-projection can be up to an order of magnitude faster than the corresponding facet-based algorithms. Furthermore, the precision of result is not tightly coupled to computing time. W-projection has important consequences for the design and operation of the new generation of radio telescopes operating at centimeter and longer wavelengths.Comment: Accepted for publication in "IEEE Journal of Selected Topics in Signal Processing

Bioinformatics: A challenge for statisticians

Bioinformatics is a subject that requires the skills of biologists, computer scientists, mathematicians and staisticians. This paper introduces the reader to one small aspect of the subject: the study of microarrays. It describes some of the complexities of the enormous amounts of data that are available and shows how simple statistical techniques can be used to highlight deficiencies in that data