ESO has started since 2002 the construction of a complex
and high performance dual feed system that will allow Phase Referenced
Imaging and Micro Arcsecond Astrometry (PRIMA) on the Very Large Telescope
Interferemoter (VLTI) on Cerro Paranal
in Chile (cf the view of the VLTI site in the Atacama desert just below).
This facility will be composed of some subsystems, distributed on the
whole VLTI and would be available at the end of 2005.
Cerro Paranal, Atacama Desert
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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. Actually
until nowadays the interferometry is limited in the sensitivity
of observing faint objetcs by the atmospheric turbulences: the
integration time must be short to freeze the fringe jitter: the
compromise forced a sufficient observing time to be able to distinguish
the faint object fringe pattern. PRIMA will improve the sensitivity
by using two stars to bring some corrections to the atmospheric
turbulences: a bright star guide near the fainter object we want
to image. 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 to the atmospheric
disturbances allowing a longer integration.
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Depending on the mode of operation, 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 thus
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.
To introduce this project, we will describe in details
what are the main PRIMA objectives
of this facility and and the technical challenges that PRIMA has to
surmount, in particular the
atmospheric and physical constraints after having recall some
general principles in Interferometry.
