Gaia features the Radial Velocity Spectrometer (RVS) instrument. The RVS provides the third component of the space velocity of each star down to about 17th magnitude. The instrument is a near-infrared (845–872 nm), medium-resolution (λ/Δλ ~ 11500), integral-field spectrograph dispersing all the light entering the field of view. The spectral dispersion of objects in the field of view is materialised by means of an optical module located between M6 and the focal plane. This module contains a grating plate and an afocal field corrector lens, composed of four fused-silica prisms.
The optical module of the radial-velocity spectrometer, containing a grating plate (middle), four fused-silica prismatic lenses, as well as a bandpass-filter plate (far right).
credit: EADS Astrium SAS, France