Title: "Modern optical interferometry, a technological and scientific
testbed for ELTs" 

Jorg-Uwe Pott (UCLA)

Abstract: The Keck Interferometer (KI) concept will be presented, with a
focus on the deployed technologies, in regular use to observe K-band
fringes at lower spectral resolution (up to R~230) with the large
apertures of the Keck twins. While standard AO-systems on both apertures
are a prerequisite for precise and sensitive interferometric
measurements, several additional helper systems are used to stabilize
and characterize the instrumental environment in a controlled way, and
to enable efficient scientific use of the sky time with the most
sensitive NIR interferometer in the world. Currently, a major
sensitivity upgrade called ASTRA is on its way, which will equip the KI
with phase-referencing capabilities, the interferometric version of a
natural guide star AO system. The significantly increased complexity of
an interferometric observation with respect to single telescope
measurements is shown to be a natural testbed for developing control
loop technology and observing procedures of an extremely large
near-infrared telescope, currently in design. 
The last part of the talk is dedicated to present typical science cases
for this extreme angular resolution of 5 mas and the future astrometric
precision expected to be better than 50uas. The birth places of planets,
and their natal systems can be studied as well as the immediate
environement of the supermassive black hole at the Galactic center, and
the dusty obscuring material of powerful active galactic nuclei.
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