ELODIE is a fibre-fed spectrograph devoted to the measurements of accurate radial
velocities (Baranne et al. 1996). It has been in operation since 1993 on the 193cm
telescope of Observatoire de Haute-Provence. The most striking result of this
instrument is the discovery of the first jovian planet orbiting the solar-type star
51 Peg (Mayor & Queloz 1995). Its resolution power is 42000, the spectra range from
390 nm to 680 nm and are recorded as 67 orders (numbered from 91 to 157)
on a Techtronic's CCD. The
reduction of the spectra is automatically performed on-line, as well as the computation
of radial velocities by cross-correlation
thanks to the ELODIE reduction software developed by D. Queloz (1996). The precision of radial
velocities is about 100 m.s-1 for F, G and K stars. A typical S/N of 100 is achieved with
one hour exposure on a 8.5th magnitude star or a S/N of 10 on a 12.75th
magnitude star, taking into account a read noise of 8 e- per CCD pixel
or 15 e- per spectral resolution element. The different configurations of ELODIE
are very stable, allowing to compare easily spectra observed at different epochs.
To develop this method, we have observed, between 1994 and 1997, 250 reference
stars at a mean S/N of 100, 211 of which
remained in the final library. We have also observed at low S/N (4 to 30)
30 well-known stars with
various atmospheric
parameters to test the software. In parallel, we have achieved our
primary objective which was to
probe the Galactic stellar populations in two directions,
where we observed 132 stars of our astrometric program (Soubiran 1993, Ojha et al. 1994).
Stars exhibiting a double correlation peak (identifying themselves as spectroscopic
binaries) during the radial velocity determination, were removed from the set of observations.
We have worked on already extracted spectra,
i.e. on one-dimensional spectra, of 1024 spectral elements per order. We will call pixel
any of these spectral elements along a given order, although it is not a CCD pixel anymore.