Revisiting the Impact of Atmospheric Refraction on VIMOS-MOS Observations : Beyond the Two-hour Angle Rule

Multi-object spectroscopic (MOS) observations with VIMOS have traditionally been limited to a narrow two-hour range from the meridian to minimise slit losses caused by atmospheric dispersion and differential refraction. We revisit the impact of these effects on the quality of VIMOSMOS spectra through extensive simulations of slit losses. We show that MOS observations can be effectively extended to plus/minus three hours from the meridian for fields with zenith angles smaller than 20 degrees at culmination — provided a nonstandard rotator offset angle of 0 degrees is used. The increase in target observability will enhance the efficiency of operations, and hasten the completion of programmes — a particularly relevant aspect for the forthcoming spectroscopic public surveys with VIMOS.