Multiple object spectroscopy - The Medusa spectrograph

An instrument has been built to obtain simultaneous spectra of many objects in the field of view of the Steward 90 inch (2.29 m) telescope. Short lengths of fused silica fiber 300 microns in diameter are used to bring the light from galaxy images at the Cassegrain focus into a line along the spectrograph slit. From a single exposure of the cluster Abell 1904, which has a redshift of 20,000 km/s, the redshifts of 26 individual galaxies were determined, each with a precision of 100 km/s. The present device, while already giving a sixfold reduction in the mean telescope time per galaxy, has significant light losses because it is not ideally matched to the telescope. An instrument being designed for the prime focus will transmit light from each object as efficiently as a conventional spectrograph.