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ELODIE's spectra

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 $1024 \times 1024$ 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.


next up previous
Next: Preparation of the spectra Up: On-line determination of stellar Previous: Introduction

9/11/1998