VA-ET-VIENT SPECTROSCOPY - A NEW MODE FOR FAINT OBJECT CCD SPECTROSCOPY WITH VERY LARGE TELESCOPES

After a review of the sources of noise in faint object low resolution spectroscopy, we discuss the limitations in the SIN ratio due to the f lat-fielding errors and sky line sampling residuals after sky subtract ion. These errors become rapidly dominant with high flux, and limit th e S/N at very low resolution. In order to partly cancel these difficul ties, we propose a new observing mode for faint object spectroscopy, c alled ''Va-et-Vient'' spectroscopy. Thanks to multiple back-and-forth shifts of charges on the CCD, synchronized with the telescope shifts, the object and a close reference sky are integrated in the same physic al conditions on the CCD through the spectrograph. This method enables us to stay in a photon noise regime on long sky background noise limi ted exposures. A laboratory experiment has been developed to test the feasibility of the method and has been fully exploited. The results ar e presented in the paper, as well as some preliminary comments on test s obtained on the ESO NTT to evaluate the merit of this approach for t he next generation of very large telescopes. A gain of one to two magn itudes, compared with limited actual performances, should be achieved with the new generation of very large telescopes.