Design and Performance of a New Generation, Compact, Low Cost, Very High Doppler Precision and Resolution Optical Spectrograph

This paper is to report the design and performance of a very high Doppler precision cross-dispersed echelle spectrograph, EXtremely high Precision ExtrasolaR planet Tracker III (EXPERT-III), as part of a global Exoplanet Tracker (ET) network. The ET network is designed to hunt low mass planets, especially habitable rocky planets, around GKM dwarfs. It has an extremely high spectral resolution (EHR) mode of R=110,000 and a high resolution (HR) mode of R=56,000 and can simultaneously cover 0.38-0.9 mu m with a 4kx4k back-illuminated Fairchild CCD detector with a single exposure. EXPERT-III is optimized for high throughput by using two-prisms cross-disperser and a large core diameter fiber (2 arcsec on sky, or 80 mu m at f/4) to collect photons from the Kitt Peak National Observatory (KPNO) 2.1m telescope. The average overall detection efficiency is similar to 6% from above the atmosphere to the detector for the EHR Mode and about 11% for the HR mode. The extremely high spectral resolution in a compact design (the spectrograph dimension, 1.34x0.8x0.48 m) is realized by coupling the single input 80 mu m telescope fiber into four 40 mu m fibers and re-arranging the four small core diameter fibers into a linear fiber slit array (a one-to-four fiber image slicer). EXPERT-III is operated in a vacuum chamber with temperature controlled to similar to 2 milli-Kelvin rms for an extended period of time. The radial velocity (RV) drift is controlled to within 10 meters/second (m/s) over a month. EXPERT-III can reach a photon noise limited RV measurement precision of similar to 0.3 m/s for a V=8 mag GKM type dwarf with small rotation (vsini = 2 km/s) in a 15 min exposure. EXPERT-III's RV measurement uncertainties for bright stars are primarily limited by the Thorium-Argon (ThAr) calibration source (similar to 0.5 m/s). EXPERT-III will serve as an excellent public accessible high resolution optical spectroscope facility at the KPNO 2.1m telescope. This instrument along with other EXPERT instruments (EXPERT and LiJET) will be used for time sensitive extremely high precision Doppler measurements such as hunting for rocky planets and studying stellar oscillations. Our planned rocky planet survey will primarily target habitable zone (HZ) rocky planets around K0-M4 dwarfs. A combination of the ET network HZ planet survey in the optical wavelengths with the FIRST HZ planet survey around M4-M9 dwarfs in the near IR wavelengths (Ge et al. 2012a) will give us a broad view of HZ planets around low mass stars for the first time. The overall planet sample will substantially increase the power for statistical study of planet occurrence and properties and constraining on planet formation models and physical conditions around low mass (0.2-0.8 M-circle dot) stars. The ET network will also be used for following up planet candidates from the SDSS-III Multi-object APO Radial Velocity Exoplanet Large-area Survey (MARVELS) and space missions (such as Kepler).