Back and forth spectroscopy: Optimization of an optical nod-and-shuffle technique to reach fainter objects and increase the multiplex gain on multiobject spectrographs

Based on several on-sky tests using the Multi-Object Spectrograph at the Canada-France-Hawaii Telescope,we present refinements of the faint objects optical spectroscopy technique we proposed in 1994 (initially named "Va-et-Vient" = "Back-and-Forth"), a nod-and-shuffie technique that preserves a sky dominated photon noise regime. We validated schemes that allow 100% of the total telescope time used to integrate the light from the scientific objects. We also investigated how to optimize the configuration of the charge shifting in order to use minimal space on the detector. For typical distant galaxy fields (profile dominated by seeing), we demonstrated an increase by a factor of approximately 2 of the multiplex gain and even higher for more compact sources. The technique proves perfectly apt at eliminating all the systematic errors that may cause the saturation-of the signal-to-noise ratio: slit defects, CCD fringing, and flat-field residuals. This technique is also the most efficient and unique approach to use curved slits.