The fibre-fed echelle spectrograph of Observatoire de Haute-Provence, ELODIE, is presented. This instrument has been in operation since the end of 1993 on the 1.93m telescope. ELODIE is designed as an updated version of the cross-correlation spectrometer CORAVEL, to perform very accurate radial velocity measurements such as needed in the search, by Doppler shift, for brown-dwarfs or giant planets orbiting around nearby stars. In one single exposure a spectrum at a resolution of 42000 (
/
)
ranging from 3906Å to 6811Å is recorded
on a 1024x1024 CCD. This performance is achieved by using a
tan 
=4 echelle grating and a
combination of a prism and a grism
as cross-disperser. An automatic on-line data treatment reduces all
the ELODIE echelle spectra and computes cross-correlation functions.
The instrument design and the data reduction algorithms are described
in this paper. The efficiency and accuracy of the instrument and its
long term instrumental stability allow us to measure radial velocities
with an accuracy better than 15ms for stars up to 9th magnitude in
less than 30 minutes exposure time. Observations of 16th magnitude
stars are also possible to measure velocities at about 1kms accuracy.
For classic spectroscopic studies (S/N>100) 9th magnitude stars can
be observed in one hour exposure time.