The RVS Instrument is a spectrometer. It will in particular measure the radial velocities of about 250 million stars in the Galaxy. The velocity is measured by the Doppler shift of the spectral lines, mainly relying on the Calcium lines in the faintest stars.
In order to obtain accurate measurements of the star spectra, there are four prisms and a diffraction grating in RVS. Their function is to disperse the spectrum on the Gaia CCD detectors in the focal plane.
The RVS prisms are Fery prisms. These are particular prisms because of their spherical shapes on the wedged faces. These are needed to focus the optical light rays of the large Gaia telescope onto the focal plane.
Bread-board prisms have been manufactured. This bread-board is used to assess the optical quality reachable for such a rarely manufactured component. The technological challenges were:
- the dimension (almost half A4 sheet)
- the accuracy of the sphere on accurately wedged faces of the prism
In the pictures, two full scale Breadboards are shown just before the final cut into the rectangular-shape phase.
Bread-boards have been realised by Selex Galileo, in the plant of Florence, under a contract with the Gaia prime contractor (EADS/Astrium, France).
Images courtesy of Luigi Mazza, Selex Galileo.
[A larger version of each picture is available: left image; right image]