The Optical Design of a Double Pass Cross-Dispersed Echelle Spectrograph for Extremely High Precision ExtraSolar Planet Trackers

摘要

We describe the optical design and performance of a cross-dispersed echelle spectrograph designed to deliver high precision radial velocities. The spectrograph design enables two working modes, a Radial Velocity Mode (RVM) and a Direct Echelle Mode (DEM). The spectra resolving power of the RVM is R=18000 over 390nm-690nm when used with 1 arcsec slit, and delivering a R=27000 over 390nm-1000nm while using 0.6 arcsec slit for DEM. The focal ratio of this spectrograph is f/4 and the collimated beam diameter is 85mm. An R2 Echelle with 87 I/mm groove density and a 63 degree normal blaze angle will be used as the main disperse grating. A 45 degree PBM2Y prism operated in a double pass serves as a cross-disperser to separate the dispersion orders. Two objects spectra will be recorded on the top and bottom half of the one 4k by 4k CCD (15-micron pixel size) respectively in RVM, while one object spectra will be recorded on the same entire CCD. The total throughput of this spectrograph, in which consists of all spherical surface lenses is around 60%.