Fig. 19. (a) histogram of the velocity shifts between the two way (sky fibre - star fibre). The line is a Gaussian with =2.3 m s-1. Note that a correction is done to suppress a slight mismatch between the two optical paths: Vr(sky) = Vr(star)-.04\Vr(star)+0.6 (Vr is the velocity variation of the spectrograph in units of m s-1). (b) histogram of the variation of the spectrograph monitor by the star-way during the measurements.