Doppler high precision extra-solar planet surveys by a fixed delay interferometer

A fixed delay interferometer combined with a post-disperser is a new technique for high precision radial velocity (RV) measurements (Erskine & Ge 2000; Ge, Erskine & Rushford 2002; Ge 2002). The Doppler measurements are conducted by monitoring the stellar fringe phase shifts of the interferometer instead of absorption line centroid shifts as in the echelle. High Doppler sensitivity is achieved through optimizing the optical delay in the interferometer and measuring multiple fringes over a broadband. The broadband operation is achieved by using the post-disperser for dispersing fringes in different wavelengths. Comparing to the state-of-the-art cross-dispersed echelle spectroscopy, this interferometer technique provides almost identical RV precision based on photon statistics. However, the interferometer method has a potential for lower systematic noise due to its simpler instrument response than the echelle. The interferometer can be optimized for higher throughput than the echelle. The interferometer approach also allows fringes to be recorded in one dispersion order instead of many cross-dispersed echelle orders. Therefore, this instrument opens up a great opportunity for multi-object observations to allow all sky surveys for extra-solar planets at moderate sized wide field telescopes. Initial observations with a prototype at the Hobby-Eberly 9 m and Palomar 5 m telescopes demonstrate ∼9 m/s Doppler RV precision with stellar fringe data recorded on a 1kx1k CCD detector (or 140 Å wavelength coverage), a S/N ∼ 120 per pixel and a post-disperser spectral resolving power of R = 6,700. This precision is consistent with the photon noise limit. Future improvement in wavelength coverage and wavelength calibration can reduce the Doppler error to a few m/s or less.