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 2000
using this mode. The phase referencing and astrometry modes will also soon
enable observations of the central regions of AGN, exo-planets, and
the Galactic Center.
The Keck Interferometer, and a sketch of the principle behind
dual-star phase referencing and astrometry. A bright star is used to
measure atmospheric phase motions, and this information is used to
stabilize a second beam-train where a fainter source can be observed
with longer integration times. The measured differential delay
between the two stars, when combined with precise knowlegde of the
baseline separating the two telescopes, allows very accurate
astrometry.
Josh Eisner; May 25, 2010