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