SETI public: Two Papers on the Hipparcos Data

Title: Rights and wrongs of the Hipparcos data: A critical assessment of the
Hipaprcos catalogue

Authors: Floor van Leeuwen
Comments: Accepted for publication by A&A, 18 pages, 23 figures
\\
A critical assessment of the quality of the Hipparcos data, partly supported
by a completely new analysis of the raw data, is presented with the aim of
clarifying reliability issues that have surfaced since the publication of the
Hipparcos catalogue in 1997. A number of defects in the data are identified,
such as scan-phase discontinuities and effects of external hits. These defects
can be repaired when re-reducing the raw data. Instabilities in the
great-circle reduction process are recognised and identified in a number of
data sets. These resulted mainly from the difficult observing conditions
imposed by the anomalous orbit of the satellite. The stability of the basic
angle over the mission is confirmed, but the connectivity between the two
fields of view has been less than optimal for some parts of the sky. Both are
fundamental conditions for producing absolute parallaxes. Although there is
clear room for improvement of the Hipparcos data, the catalogue as published
remains generally reliable within the quoted accuracies. Some of the findings
presented here are also relevant for the forthcoming Gaia mission.

\\ ( http://arXiv.org/abs/astro-ph/0505431 , 402kb)
------------------------------------------------------------------------------

\\
Paper: astro-ph/0505432
Date: Fri, 20 May 2005 09:19:26 GMT (399kb)

Title: A new reduction of the raw Hipparcos data

Authors: Floor van Leeuwen, Elena Fantino
Comments: Accepted for publication by A&A, 13 pages, 18 figures
\\
We present an outline of a new reduction of the Hipparcos astrometric data,
the justifications of which are described in the accompanying paper. The
emphasis is on those aspects of the data analysis that are fundamentally
different from the ones used for the catalogue published in 1997. The new
reduction uses a dynamical modelling of the satellite's attitude. It
incorporates provisions for scan-phase discontinuities and hits, most of which
have now been identified. Solutions for the final along-scan attitude (the
reconstruction of the satellite's scan phase), the abscissa corrections and the
instrument model, originally solved simultaneously in the great-circle
solution, are now de-coupled. This is made possible by starting the solution
iterations with the astrometric data from the published catalogue. The
de-coupling removes instabilities that affected great-circle solutions for
short data sets in the published data. The modelling-noise reduction implies
smaller systematic errors, which is reflected in a reduction of the
abscissa-error correlations by about a factor 40. Special care is taken to
ensure that measurements from both fields of view contribute significantly to
the along-scan attitude solution. This improves the overall connectivity of the
data and rigidity of the reconstructed sky, which is of critical importance to
the reliability of the astrometric data. The changes in the reduction process
and the improved understanding of the dynamics of the satellite result in
considerable formal-error reductions for stars brighter than 8th magnitude.

\\ ( http://arXiv.org/abs/astro-ph/0505432 , 399kb)