The instrumental profile of the 4MOST facility

The 4-metre Multi-Object Spectroscopic Telescope (4MOST) is a new high-multiplex, wide-field spectroscopic survey facility under development for the Visible and Infrared Survey Telescope for Astronomy (VISTA) at Paranal. Its key specifications are: a large field of view (FoV) of 4.4 square degrees and a high multiplex capability, with 1624 fibres feeding two low-resolution spectrographs (R =6500), and 812 fibres transferring light to the high-resolution spectrograph (R ≈ 20000). For the end-to-end characterization of the 4MOST facility, we analyze the impact of the atmosphere at Paranal, VISTA telescope, wide field corrector, atmospheric dispersion compensator, tilting spine positioner, fibre system, spectrographs and detector systems. We present an exhaustive analysis of the most influential characteristics on the transmission efficiency for a 4MOST observation. Many environmental, telescope, and instrumental effects can be characterized in isolation, such as glass transmission. But there are also many effects that are caused by a combination of multiple components. For example, the residual atmospheric dispersion in combination with fibre positioning errors; or the fibre field position in combination with fibre tilt angle as well as the fibre focus position. To capture this complexity, we present a coherent quantitative assessment of each significant individual effect, as well as a relevant selection of effect combinations. To quantify the impact on the survey nature of the 4MOST instrument, we also introduce parts of the optical performance simulator TOAD, which was used to compute the impact each effect.