“Va-et-Vient” (“Back-and-Forth”) CCD Spectroscopy: A New Way to Increase the Limiting Magnitude of Very Large Telescopes

Abstract

Over the last decade, the quantity of scientific results brought by the observations of very faint objects has been quite spectacular. In particular, they concern the photometry of faint galaxies up to B = 27 (Tyson 1988) or K = 22 (Cowie et al. 1994). The consequences of these observations are the detection of a large population of faint galaxies more numerous than any prediction given by standard galaxy evolution and probably a new vision of the distant universe. For faint object spectroscopy, the most recent surveys of field galaxies reach a magnitude range of 23–24 (Colless et al. 1990, 1993, Lilly and Cowie 1993, Tresse et al. 1993) with a reasonable S/N ratio that allows a redshift measurement from absorption-line identification. But in this magnitude range, the sky background flux is dominant with respect to the source, being at least 10 times brighter. It is well known that in that case, with a low readout noise detector and a “perfect” spectrograph, the S/N ratio scales as: