Optical design of new generation compact, high resolution and high Doppler precision optical spectrograph

Abstract

We report optical design of new generation compact, high resolution, high throughput and high Doppler precision optical spectrograph. This spectrograph uses cross-dispersed echelle design with white pupils and also takes advantage of a fiber image slicer to slice one 2 arcsec telescope input fiber image (80 micron at f/4 at the KPNO 2.1 meter telescope) into four 1 arcsec images (40 micron). The small sliced images coupled with slow optics play a key role in achieving high spectral resolution within very compact instrument design to substantially reduce construction cost while increasing the instrument stability for high Doppler precision over a long time. This optical spectrograph is called EXtremely high Precision ExtrasolaR planet Tracker III (EXPERT-III). The coupling of the fiber sliced images with an R4 echelle with a 98mm diameter pupil produces R=110,000 in the entire optical wavelength region. It also uses a two-prism cross-disperser to produce nearly homogeneous spectral order coverage while taking advantage of the anamorphic magnification of the prisms to allow large wavelength coverage (380nm-900nm) in a single exposure with a 4kx4k CCD detector. This very high resolution mode is designed to reach extremely high Doppler precision for radial velocity measurements of bright solar type stars. The spectrograph is also directly coupled with an 80 micron telescope fiber-fed image to obtain high throughput with R=60,000 for stellar spectroscopy. Details about the optical design and performance are reported.