Positioning errors and efficiency in fiber spectrographs

The use of wide-field multi-object fiber-input spectrographs for large redshift surveys introduces the possibility of variations in the observed signal-to-noise ratio across the survey area due to errors in positioning the fibers with respect to the target image positions, leading to position-dependent errors in the survey catalog. This paper brings together a comprehensive description of the sources of fiber-to-image position errors in different instrument designs, and quantifies their effects on the efficiency with which signal is recorded. For point sources, a function relating a fractional efficiency and an equivalent aperture correction to the fiber-to-image position error, the fiber diameter and the image size is plotted for typical values of fiber and image sizes found in current instruments. The tools required by observers to maximize the efficiency of fiber-spectrographic surveys are discussed. Subject headings: instrumentation: spectrographs — techniques: spectroscopic — surveys — astrometry