The PRIMA (Phase Reference Imaging and Microarcsecond Astrometry) instrument on the VLTI is the last VLTI first-generation instrument (along with VINCI, AMBER, and MIDI). It is due to be commissioned in 2008. The acronym actually describes an instrument with two very different functions:

• PRI (Phase Reference Imaging): this is, in effect, the recombiner plus two telescopes, transforming the VLTI into an imaging instrument;

• MA (Microarcsecond Astrometry): the astrometric instrument proper.

PRIMA has been greatly delayed, because its commissioning depends on the availability of a fast fringe sensor unit.

Table 8.2 Principal spectrographs in service or being developed for velocimetry

Instrument /


Spectral range




Reference /

commission date

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