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(Abridged) Atmospheric dispersion and field differential refraction imposesevere constraints on widefield MOS observations. Flux reduction and spectraldistortions must be minimised by a careful planning of the observations --which is especially true for instruments that use slits instead of fibres. Thisis the case of VIMOS at the VLT, where MOS observations have been restricted,since the start of operations, to a narrow two-hour range from the meridian tominimise slit losses. We revisit in detail the impact of atmospheric effects onthe quality of VIMOS-MOS spectra. We model slit losses across the entire VIMOSFOV as a function of target declination. We explore two different slitorientations at the meridian: along the parallactic angle (North-South), andperpendicular to it (East-West). We show that, for fields culminating at zenithdistances larger than 20 deg, slit losses are minimised with slits orientedalong the parallactic angle at the meridian. The two-hour angle rule holds forthese observations using N-S orientations. Conversely, for fields with zenithangles smaller than 20 deg at culmination, losses are minimised with slitsoriented perpendicular to the parallactic angle at the meridian. MOSobservations can be effectively extended to plus/minus three hours from themeridian in these cases. In general, night-long observations of a single fieldwill benefit from using the E-W orientation. All-sky or service modeobservations, however, require a more elaborate planning that depends on thetarget declination, and the hour angle of the observations. We establishgeneral rules for the alignment of slits in MOS observations that will increasetarget observability, enhance the efficiency of operations, and speed up thecompletion of programmes -- a particularly relevant aspect for the forthcomingspectroscopic public surveys with VIMOS.

astronomy and astrophysics

Revisiting the Impact of Atmospheric Dispersion and Differential  Refraction on Widefield Multiobject Spectroscopic Observations. From  VLT/VIMOS to Next Generation Instruments