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An Externally Dispersed Interferometer for Sensitive Doppler Extrasolar Planet Searches

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 moiré fringes, created by multiplication of the interferometer fringes with stellar absorption lines. The intermediate‐resolution spectrograph disperses the moiré 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 moiré 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 moiré 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 R ≈ 20,000 demonstrated ∼0.7 m s−1 precision for short‐term RV measurements. A fiber‐fed version of the prototype with R ≈ 5600 was tested with starlight at the Lick 1 m telescope and demonstrated ∼7 m s−1 RV precision at 340 Å bandwidth. The increased velocity noise is attributed to the lower spectral resolution, lower fringe visibility, and uncontrolled instrument environment.