Phase-Referenced Imaging and Micro-arcsecond Astrometry

The PRIMA facility is a system designed to enable simultaneous interferometric observations of two objects - each with size of at most 2 arcsec - that are separated by up to 1 arcmin, without requiring a large continuous field of view. currently interferometry is still limited in sensitivity by the atmospheric turbulences: the integration time has to be short to freeze the fringe jitter.

PRIMA will improve the sensitivity by using two objects (a bright star guide near a fainter object) to bring some corrections to the atmospheric turbulences. The brighter of the two objects is used to increase the sensitivity on the fainter one, in fact it is used as a reference for stabilizing the fringes and for the correction of the atmospheric disturbances allowing thus a longer integration.

Depending on the operation mode, PRIMA can be used either to measure the angular separation between the two objects (astrometry mode) or to produce images of the fainter of the two objects using a phase reference technique (imaging mode).

These capabilities will enable the VLTI to address directly a number of extremely important scientific issues that are currently at the top of the list of challenges for future astronomical high resolution instrumentation.