ASTRA
The Keck Interferometer (KI) combines light from the two 10-m Keck apertures
to synthesize an aperture of 85-m (the baseline separation of the two
Kecks). The ASTRA project is an NSF-funded series of hardware
upgrades with the ultimate goal of enabling precise astrometry
(tens of micro-arcseconds) of stars orbiting the Galactic center. This
requires "phase referencing" from a bright guide star to fainter science
targets (analogous to natural guide star AO), and accurate measurement
and monitoring of the baseline. Along the way to the ultimate goal, several
other scientific programs will be enabled. We have recently implemented
a "spectral phase referencing" mode that allows broadband light to be used
as a guide source for a second, highly-dispersed channel. We have already made
observations of young and evolved stars with a spectral resolution of 1800
using this mode. The phase referencing and astrometry modes will also soon
enable observations of the central regions of AGN and measurements of
the astrometric orbits of exo-planets.
Josh Eisner; July 7, 2008