JG_DI - Publications of the Astronomical Society of the...

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1016 Publications of the Astronomical Society of the PaciFc , 114 :1016–1028, 2002 September ± 2002. The Astronomical Society of the Pacific. All rights reserved. Printed in U.S.A. An Externally Dispersed Interferometer for Sensitive Doppler Extrasolar Planet Searches Jian Ge, 1 David J. Erskine, and Mike Rushford Lawrence Livermore National Laboratory, Livermore, CA 94550; [email protected] Received 2000 November 1; accepted 2002 May 29 ABSTRACT. A new kind of instrument for sensitive Doppler extrasolar planet searches, called an externally dispersed interferometer, is described in this paper. It is a combination of an optical Michelson-type interferometer and an intermediate-resolution grating spectrometer. The interferometer measures Doppler radial velocity (RV) variations of starlight through the phase shifts of moire ´ fringes, created by multiplication of the interferometer fringes with stellar absorption lines. The intermediate-resolution spectrograph disperses the moire ´ fringes into thousands of parallel-wavelength channels. This increases the instrument bandwidth and fringe visibility by preventing fringe cross-talk between neighboring spectral lines. This results in a net increase in the signal-to- noise ratio over an interferometer used alone with broadband light. Compared to current echelle spectrometers for extrasolar planet searches, this instrument offers two unique instrument properties: a simple, stable, well-defined sinusoidal instrument response function (point-spread function) and magnification of Doppler motion through moire ´ fringe techniques. Since instrument noise is chiefly limited by the ability to characterize the instrument response, this new technique provides unprecedented low instrumental noise in an economical compact apparatus, enabling higher precision for Doppler RV measurements. In practice, the moire ´ magnification can be 5–10 times depending on the interferometer comb angle. This instrument has better sensitivity for smaller Doppler shifts than echelle spectrometers. The instrument can be designed with much lower spectral resolving power without losing Doppler sensitivity and optimized for higher throughput than echelle spectrometers to allow a potential survey for planets around fainter stars than current magnitude limits. Lab-based experiments with a prototype instrument with a spectral resolution of demonstrated R 20,000 0.7 m s ± 1 precision for short-term RV measurements. A fiber-fed version of the prototype with was R 5600 tested with starlight at the Lick 1 m telescope and demonstrated 7ms ± 1 RV precision at 340 A ˚ bandwidth. The increased velocity noise is attributed to the lower spectral resolution, lower fringe visibility, and uncontrolled instrument environment.
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This note was uploaded on 01/22/2012 for the course AST 1022l taught by Professor Colon during the Fall '07 term at University of Florida.

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JG_DI - Publications of the Astronomical Society of the...

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