A MULTIOBJECT FIBER SPECTROGRAPH FOR THE HALE-TELESCOPE

A new faint-object spectrograph has been designed around the capabilit ies of fiber optics. This instrument, the Norris Spectrograph, is for exclusive use at the Cassegrain focus (f/16) of The Hale Telescope and is optimized for faint galaxy spectroscopy. There are 176 independent ly positionable fibers that are serially manipulated by a single robot ic system. Each of these fibers sees 1.6 arcsec on the sky and the tot al positionable area is in excess of 300 arcmin2. Unlike most multiobj ect spectrographs which utilize fibers that are several tens of meters long, the philosophy of the design of the Norris was quite the antith esis, i.e., to minimize the fiber lengths; hence, it is an entirely se lf-contained telescope-mounted instrument for the Cassegrain focus. Th e instrument consists of an integrated xy stage, for the fiber positio ning, and an attached optical spectrograph. The design of the spectrog raph is basically classical: spherical collimator mirror, standard ref lection grating, and a newly designed all-transmissive-optics camera l ens. The detector currently used is a thinned, AR-coated 2048 x 2048 T ektronix CCD. Fibers are arranged in two linear opposing banks that ca n access the 20 arcmin diameter field-of-view (FOV) of the instrument. The accuracy of fiber placement (assuming errorless coordinates) is l ess than 0.1 arcsec over the entire FOV. Fibers may be placed as close as 16 arcsec. This permits close pairings of fibers for very faint-ob ject spectroscopy. Beam switching between paired fibers, as was done w ith two-channel spectrographs of yesteryear, will help average out tem poral and spatial variations of the light of the night sky. Actual obs ervations performed in this mode of operation indicate that the qualit y of the sky subtraction improves, as would be expected. The density o f paired fibers within the Norris FOV matches the approximate density of faint field galaxies expected to a blue magnitude of 21. Software e xists to take object lists (alpha,delta) and convert them to rectiline ar (x,y) values (mm) on the xy stage by gnomonic projection and to ass ign fibers. This software also corrects for precession of the equinoxe s, proper motion if epoch differences exist, and corrects for differen tial atmospheric refraction. To place a single fiber takes approximate ly 5 s on the average. A lower limit to the efficiency of the spectrog raph plus telescope has been estimated to be 6.8% at 5500 angstrom. In order to derive the throughput of the instrument, the efficiency of t he telescope, estimated to be approximately 56%, must be divided out. This value is consistent with the expectation that the reduction in ef ficiency from that of a standard CCD spectrograph such as The Hale Tel escope's Double Spectrograph will be about a factor of 2. This results from the 60%-70% transmittance of the fibers and other losses. The sp ectra produced are linear with little distortion. With 10 angstrom spe ctral resolution, fitting residuals on the order of 100 km s-1 are eas ily obtainable by modeling the dispersion by a third-order polynomial. The resolutions currently available range from 1 to about 20 angstrom . The spectra have a FWHM in the direction perpendicular to that of th e dispersion of about 90 mum, or equivalently about three 27 mum pixel s found in the older Tektronix 2048 CCDs. The interorder spacing of 25 0 mum is large enough to permit clean spectrum extractions. The instru ment has been in use for several years. The scientific programs vary f rom high resolution (1 angstrom resolution) spectroscopy of stars in n earby globular clusters to a low spectral resolution (10 angstrom) sur vey of faint field galaxies. In this latter survey, with typical 2-hr exposures, absorption-line redshifts as high as z is similar to 0.5 ha ve been routinely measured. Several heretofore unknown quasars with re dshifts around three have also been discovered serendipitously.