Positioning Errors and Efficiency in Fiber Spectrographs

The use of wide-field multi-object fiber-input spectrographs for largeredshift surveys introduces the possibility of variations in the observedsignal-to-noise ratio across the survey area due to errors in positioning thefibers with respect to the target image positions, leading toposition-dependent errors in the survey catalog. This paper brings together acomprehensive description of the sources of fiber-to-image position errors indifferent instrument designs, and quantifies their effects on the efficiencywith which signal is recorded. For point sources, a function relating afractional efficiency and an equivalent aperture correction to thefiber-to-image position error, the fiber diameter and the image size is plottedfor typical values of fiber and image sizes found in current instruments. Thetools required by observers to maximize the efficiency of fiber-spectrographicsurveys are discussed.Comment: 23 pages, Latex, including two EPS figures using psfig. PASP accepted (2002 August issue). Corrected the X-axis labels on both figures which were a factor of 2 too larg