A FIBER-FED ECHELLE SPECTROGRAPH FOR THE HALE 5-M TELESCOPE

We describe a new fiber-fed echelle spectrograph which is now in operation on the Hale 5-m telescope at Palomar Observatory. The instrument is optimized for high spectral stability, necessary for asteroseismology measurements of small periodic variations in stellar radial velocities. It features a 7-element all-spherical 610-mm focal length f/3.0 lens system and prism cross disperser in a compact double-pass quasi-Littrow configuration. Light enters the system at prime focus, and is channeled to the spectrograph by a fiber-optic cable. The instrument rests on a fixed-orientation optical bench inside the telescope's East Arm. It can be operated in a low-resolution mode, with resolution R = 20,000 and overall efficiency (including atmospheric seeing) e approximate to 5 percent at 550 nm, or a high-resolution mode, with R=40,000 and e approximate to 1.5 percent at 550 nm; both modes have R(pixel)=100,000. With a large-format CCD detector (2048X2048 with 27 mu m pixels), the entire visible spectrum from 400-1000 nm can be recorded in two exposures, with no gaps. We have also incorporated a novel imaging system to produce a circularly symmetric guide image of the input fiber tip. Using a fiber-optic double scrambler, the instrument produces radial-velocity measurements which are stable at the similar to ms(-1) level over short (less than or similar to 30 min) time periods.