ATTAINING DOPPLER PRECISION OF 3 M S(-1)

Current spectroscopic techniques yield Doppler-shift errors of 10 to 5 0 m s(-1), barely adequate to detect reflex velocities caused by Jupit er-like and lower-mass planets. We describe a technique which yields r elative radial-velocity errors of 3 m s(-1). This technique makes use of a fast echelle spectrograph at resolution of R=62,000 and a large-f ormat CCD which acquires the entire visible and near-IR spectrum in ea ch exposure. Starlight is sent through an iodine absorption cell place d at the spectrometer entrance slit. The resulting superimposed iodine lines provide a fiducial wavelength scale against which to measure ra dial-velocity shifts, The shapes of iodine lines convey the PSF of the spectrometer to account for changes in spectrometer optics and illumi nation on all time scales. We construct a model of each observed spect rum by multiplying a stellar spectrum with an iodine spectrum and conv olving the result with the spectrometer PSF. The free parameters of th e model include the wavelength scale, spectrometer PSF, and stellar Do ppler shift. All model parameters are derived anew for each exposure a nd the synthesis is done on a grid of CCD sub-pixels, using spline fun ctions as interpolation predictors. We present Doppler rests of the Su n, tau Ceti, and 107 Psc, observed with the Lick and Keck echelles. Al l exhibit apparent errors of about 3 m s(-1), maintained on time scale s of minutes to a year. This precision agrees with the theoretically p redicted errors that stem primarily from photon statistics.