Physics – Optics
Scientific paper
Nov 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993pasp..105.1308h&link_type=abstract
Astronomical Society of the Pacific, Publications (ISSN 0004-6280), vol. 105, no. 693, p. 1308-1321
Physics
Optics
12
Astronomical Spectroscopy, Cassegrain Optics, Design Analysis, Faint Objects, Optical Fibers, Spectrographs, Charge Coupled Devices, Collimators, Gratings (Spectra), Lenses, Mirrors, Optical Paths
Scientific paper
A new faint-object spectrograph has been designed around the capabilities 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 independently positionable fibers that are serially manipulated by a single robotic system. The instrument consists of an integrated xy stage, for the fiber positioning, and an attached optical spectrograph. The design of the spectrograph is basically classical: spherical collimator mirror, standard reflection grating, and a newly designed all-transmissive-optics camera lens. The detector currently used is a thinned, AR-coated 2048 X 2048 Tektronix CCD. Fibers are arranged in two linear opposing banks that can access the 20 arcmin diameter field-of-view (FOV) of the instrument. The accuracy of fiber placement is less 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-object spectroscopy. Beam switching between paired fibers will help average out temporal and spatial variations of the light of the night sky. Actual observations performed in this mode of operation indicate that the quality of the sky subtraction improves. The density of paired fibers within the Norris FOV matches the approximate density of faint field galaxies expected to a blue magnitude of 21. Software exists to take object lists and convert them to rectilinear values on the xy stage by gnomonic projection and to assign fibers. This software also corrects for precession of the equinoxes, proper motion if epoch differences exist, and corrects for differential atmospheric refraction. To place a single fiber takes approximately 5 s on the average. A lower limit to the efficiency of the spectrograph plus telescope has been estimated to be 6.8% at 5500 A. In order to derive the throughput of the instrument, the efficiency of the telescope, estimated to be approximately 56%, must be divided out. This value is consistent with the expectation that the reduction in efficiency from that of a standard CCD spectrograph such as The Hale Telescope's Double Spectrograph will be about a factor of 2.
Blakee L.
Carr Michael A.
Cohen Julien
Cromer John
Emery E.
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