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ESPaDOnS is an ongoing project for a cross-dispersed echelle spectrograph/spectropolarimeter, for general community use at the Canada-France-Hawaii Telescope. This instrument will provide: (1) a complete optical spectrum from 370 to 1,000 nm in a single exposure, with a variable resolution between 50,000 and 75,000, (2) all polarization components of the stellar light, and (3) 20% peak total throughput. The wide spectral coverage will maximize the multiplex gain associated wtih multi-line techniques. The high resolution will give sufficient spatial resolution for Doppler and Zeeman-Doppler imaging of rapid rotators, increasing the sensitivity of the observations to small-scale structures. It will also allow accurate brightness, abundance and magnetic field mapping of moderate rotators, and help study stellar pulsations, abundances, and extrasolar planets. The possibility of recording two interleaved spectra will be very useful in polarimetric mode, and very convenient for faint object spectroscopy in non-polarimetric mode, where the spectrum of the adjacent sky will be recorded along with that of the target. The spectrographic is bench-mounted and fed by low-OH H-treated Ceram-Optec optical fibers from a Cassegran module containing all calibration and polarimetric facilities, making it possible to have extremely good wavelength stability and minimal instrumental polarization. The achromatic polarimeter includes one quarter-wave and two half-wave Fresnel rhombs coupled to a removable Wollaston prism. ESPaDOnS will be a unqiue instrument worldwide in polarimetric mode, competitive wiht similar instruments on 8-m class telescopes in non-polarimetric mode. Astronomers will be able to address with unprecedented details a broad range of important issues in stellar physics, circumstellar environments, and extrasolar planets.
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