Operation of a long fused silica fiber as a link between telescope and spectrograph

Abstract

An experimental fiber link has been set up at the Steward 36-inch (91-cm) telescope to bring light from a star image at the prime focus to a spectrograph located in a laboratory below the observing floor. The link is a single fused-silica fiber of 125 microns diameter and 20 m length. An intensified TV guiding system is used to hold star images on the fiber entrance, which in this configuration subtends 5 arcsec on the sky. A comparison is made of spectra of the nucleus of NGC 4151 obtained with this system and with the same spectrograph at the Cassegrain focus of the 90-inch (2.3-m) telescope. It is shown that the ultraviolet loss is not severe; with H-alpha (where losses are negligible) used as a reference point, the transmission at the 3450-A forbidden line of Ne V is found to be about 50%. These tests demonstrate the practicality of the FLOAT (Fiber Linked Optical Array Telescope), a method suggested by Angel et al. (1977) for constructing a large-aperture (about 500 sq m) optical telescope that would fill many imaging and nonimaging needs at relatively low cost. The fiber system is found to be practical to build and straightforward to operate. The applicability of a fiber link as a high-efficiency alternative to existing coude optics is discussed.