The Radial Velocity Precision of Fiber‐fed Spectrographs

We have measured the radial velocities of five 51 Peg-type stars and one starwith constant velocity. Our measurements, on 20 \AA centered at 3947 \AA, wereconventional using Th/Ar comparison spectra taken every 20 or 40 minutesbetween the stellar exposures. Existing IRAF routines were used for thereduction. We find $\sigma_{RV}$ $\leq$ 20 m s$^{-1}$, provided 4 measurements(out of 72) with residuals $>5\sigma_{RV}$ are neglected. The observations weremade with the CFHT Gecko spectrograph, fiber-fed with the CAFE system(R$\sim$110,000). $\sigma_{RV}$ $\leq$10 m s$^{-1}$ seems possible withadditional care. This study was incidental to the main program and so notexhaustive but the small value of $\sigma_{RV}$ implies that the fiberfeed/image slicer system on Gecko + CAFE, essentially eliminates the longstanding problem of guiding errors in radial velocity measurements. We are notsuggesting this conventional approach for serious Doppler planet searches(especially with Gecko which has such a small multiplex gain), but theprecision is valuable for observations made in spectral regions remote fromtelluric lines or captive-gas fiducials. Instrument builders might consider theadvantages of the CAFE optics which incorporate agitation and invert the objectand pupil for slit and grating illumination in future spectrograph designs.Comment: 12 pages, 3 figure