The radial velocity precision of fiber-fed spectrographs

We have measured the radial velocities of five 51 Peg-type stars and one star known to be constant in velocity. Our measurements, on 20 Å centered at 3947 Å, were conventional, using Th/Ar comparison spectra taken every 20 or 40 minutes between the stellar exposures. Existing IRAF routines were used for the reduction. We find σ_RV ≤20 m s ^-1, provided that four measurements (out of 72) with residuals greater than 5 σ_RV are neglected. The observations were made on five nights with the CFHT Gecko spectrograph (R ∼ 110,000), fiber-fed by the CAFE system; σ_RV ≤ 10 m s^-1 seems possible with additional care. This study was incidental to the main observing program and is certainly not exhaustive, but the small value of σ_RV implies that the fiber feed/image slicer system on Gecko+CAFE essentially eliminates the long-standing problem of guiding errors in radial velocity measurements. We are not promoting this conventional approach for serious Doppler planet searches (especially with Gecko, which has such a small multiplex gain), but the precision is valuable for observations made in spectral regions remote from telluric lines or captive-gas fiducials. Instrument builders might consider the advantages of the CAFE optics, which incorporate agitation and invert the object and pupil to illuminate the slit and grating, respectively, in future spectrograph designs.