Performance Verification of the EXtreme PREcision Spectrograph
Ong, J. M. Joel; Fischer, Debra A.; Buchhave, Lars A.; Shao, Michael; Brewer, John M.; Cabot, Samuel H. C.; Davis, Allen B.; Tronsgaard, René; Llama, Joe; Szymkowiak, Andrew E.; Trahan, Russell; Genoni, Matteo; Pariani, Giorgio; Riva, Marco; Petersburg, Ryan R.; Zhao, Lily L.; Blackman, Ryan T.; Jurgenson, Colby A.; Leet, Christopher; McCracken, Tyler M.; Sawyer, David; Nemati, Bijan; Sawyer, Travis; Fournier, Paul; Pawluczyk, Rafal
United States, Denmark, Italy, Canada
Abstract
The EXtreme PREcision Spectrograph (EXPRES) is a new Doppler spectrograph designed to reach a radial-velocity measurement precision sufficient to detect Earth-like exoplanets orbiting nearby, bright stars. We report on extensive laboratory testing and on-sky observations to quantitatively assess the instrumental radial-velocity measurement precision of EXPRES, with a focused discussion of individual terms in the instrument error budget. We find that EXPRES can reach a single-measurement instrument calibration precision better than 10 cm s-1, not including photon noise from stellar observations. We also report on the performance of the various environmental, mechanical, and optical subsystems of EXPRES, assessing any contributions to radial-velocity error. For atmospheric and telescope related effects, this includes the fast tip-tilt guiding system, atmospheric dispersion compensation, and the chromatic exposure meter. For instrument calibration, this includes the laser fRequency comb (LFC), flat-field light source, CCD detector, and effects in the optical fibers. Modal noise is mitigated to a negligible level via a chaotic fiber agitator, which is especially important for wavelength calibration with the LFC. Regarding detector effects, we empirically assess the impact on the radial-velocity precision due to pixel-position nonuniformities and charge transfer inefficiency (CTI). EXPRES has begun its science survey to discover exoplanets orbiting G-dwarf and K-dwarf stars, in addition to transit spectroscopy and measurements of the Rossiter-McLaughlin effect.