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        Information from the European Southern Observatory

ESO Press Release 06/03

27 March 2003

For immediate release
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"First Light" for HARPS at La Silla

Part 2 of 2

The goal of measuring velocities of stars with an accuracy comparable
to that of a pedestrian has required extraordinary efforts for the
design and construction of this instrument. Indeed, HARPS is the most
stable spectrograph ever built for astronomical applications. A
crucial measure in this respect is the location of the HARPS
spectrograph in a climatized room in the telescope building. The
starlight captured by the 3.6-m telescope is guided to the instrument
through a very efficient optical fibre from the telescope's
Cassegrain focus. 

Moreover, the spectrograph is placed inside a vacuum tank to reduce
to a minimum any movement of the sensitive optical elements because
of changes in pressure and temperature. The temperature of the
critical components of HARPS itself is kept very stable, with less
than 0.005 degree variation and the spectrum therefore drifts by less
than 2 m/s per night. This is a very small value - 1 m/s corresponds
to a displacement of the stellar spectrum on the CCD detector by
about 1/1000 the size of one CCD pixel, which is equivalent to 15 nm
or only about 150 silicon atoms! This drift is continuously measured
by means of a Thorium spectrum which is simultaneously recorded on
the detector with an accuracy of only 20 cm/s. 

PR Photo 08e/03 illustrates two fundamental issues: HARPS performs
with an overall stability never before reached by any other
astronomical spectrograph, and it is possible to measure any nightly
drift with an accuracy never achieved before [1].

During this first commissioning period in February 2003, all
instrument functions were tested, as well as the complete data flow
system hard- and software. Already during the second test night, the
data-reduction pipeline was used to obtain the extracted and
wavelength-calibrated spectra in a completely automatic way. The
first spectra obtained with HARPS will now allow the construction of
templates needed to compute the radial velocities of different types
of stars with the best efficiency. 

The second commissioning period in June will then be used to achieve
the optimal performance of this new, very powerful instrument.
Astronomers in the ESO community will have the opportunity to observe
with HARPS from October 1, 2003.

Other research opportunities opening

This superb radial velocity machine will also play an important role
for the study of stellar interiors by asteroseismology. Oscillation
modes were recently discovered in the nearby solar-type star Alpha
Centauri A from precise radial velocity measurements carried out with
CORALIE (see ESO PR 15/01). HARPS is able to carry out similar
measurements on fainter stars, thus reaching a much wider range of
masses, spectral characteristics and ages.

Michel Mayor, Director of the Geneva Observatory and co-discoverer of
the first known exoplanet, is confident: "With HARPS operating so
well already during the first test nights, there is every reason to
believe that we shall soon see some breakthroughs in this field
also".

The HARPS Consortium

HARPS has been designed and built by an international consortium of
research institutes, led by the Observatoire de Geneve (Switzerland)
and including Observatoire de Haute-Provence (France), Physikalisches
Institut der Universitaet Bern (Switzerland), the Service d'
Aeronomie (CNRS, France), as well as ESO La Silla and ESO Garching.

The HARPS consortium has been granted 100 observing nights per year
during a 5-year period at the ESO 3.6-m telescope to perform what
promises to be the most ambitious systematic search for exoplanets so
far implemented worldwide.

The project team is directed by Michel Mayor (Principal
Investigator), Didier Queloz (Mission Scientist), Francesco Pepe
(Project Managers Consortium) and Gero Rupprecht (ESO
representative).

More information

For more details about the HARPS Spectrograph, please consult the
HARPS websites at the Geneva Observatory and at ESO, or the articles
about this new instrument in the ESO Messenger Nos. 105 and 110.

Note

[1] The high level of stability achieved by the thermostatically
controlled vacuum-spectrograph HARPS has never before been reached by
any other astronomical spectrograph. In the case of more conventional
instruments, drifts of several hundreds of m/s may occur during one
observing night due to the variation of atmospheric pressure (at the
rate of about 90 m/s for 1 mbar variation) or the ambient air
temperature (300 m/s for 1=B0 variation). It is expected that there
will be a smooth drift of perhaps a few tens of m/s during one year,
due to thermo-mechanical flexure and relaxation of the HARPS
instrument. However, since such a long-term drift can be measured
regularly and very accurately by means of the Thorium calibration
spectrum, this effect can be fully compensated for and thus poses no
problem. In fact, the measured nightly drift of HARPS is so small and
so smooth that it is possible to compute an average value of this
drift with an accuracy of only a few cm/s.

Contacts

Didier Queloz
Observatoire de Geneve
Sauverny, Switzerland
Phone: +41 22 755 26 11
email: didier.queloz{at}obs.unige.ch

Gero Rupprecht
European Southern Observatory
Garching, Germany
Phone: +49-89-3200-6355
email: grupprec{at}eso.org

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