To ensure that science surveys having very different observational requirements can be observed simultaneously, a few constraints are imposed on the targets and the observing strategy:
The sky is divided into regions that will be observed as much as possible in similar observing conditions (sky brightness, seeing) such that targets in the appropriate magnitude range can be chosen. Bright time will be primarily devoted to the area dominated by stellar targets (i.e. the Galactic plane and bulge, plus the Magellanic Clouds), while dark/grey time is devoted to the extragalactic sky. A large fraction of the sky in the range -70<Dec<+20° will be observed by 4MOST. The 4MOST survey footprint will be visited multiple times, with an expectation that each field will be observed for a minimum of six times 20min exposures (or “tiles”). Some fields with high target densities will receive many more than six tiles. After each 20min tile, any fibre previously placed on a target that has received sufficient exposure time to reach its signal-to-noise requirement will be moved to a new target. Targets that need more exposure time will be assigned a fibre in subsequent tiles until the required time has been reached.
4MOST Facility Simulator
To demonstrate the feasibility of the above operations concept, and to prepare for the actual observations, a detailed simulator has been developed. The 4MOST Facility Simulator (4FS) consists of two main components: the Throughput Simulator and the Operations Simulator.
4FS Throughput Simulator
The Throughput Simulator (TS) is a software tool used to fold spectral templates through the system response of the 4MOST facility taking account of the range of possible atmospheric conditions. The TS derives its description of the 4MOST instrument response using outputs of the full instrument simulator TOAD.
The TS is used to make estimates of the exposure time that will be required (under a range of observing conditions) to obtain spectra of science targets that have a sufficiently high signal-to-noise ratio to satisfy the specific data quality requirements associated with each target. The TS is coded in such a way to allow rapid bulk processing of a large number of spectral templates through a large parameter space of observing conditions.
4FS Operations Simulator
The Operations Simulator (OpSim) is a software tool used to simulate the progress of the 5-year 4MOST survey. The OpSim is used to plan, optimise and verify many aspects of operations planning. The OpSim is also used as a system engineering tool, to examine the impact of instrument design choices on science survey outcomes. The OpSim contains a detailed parameterised model of the 4MOST facility, including representations of the instrument focal plane, the various constraints/limitations, and a statistical model of the operating environment (e.g. the long-term atmospheric/environmental conditions at Paranal). Where possible, the OpSim uses prototype versions of the various algorithms that will be used to operate the real 4MOST survey, e.g fibre→target assignment, survey strategy, survey scheduling and progress balancing/feedback algorithms. The OpSim serves as the primary test-bed for development and optimisation of these algorithms and their optimal control parameters.
The OpSim takes as its main input a set of target catalogues associated with the science surveys, together with a description of how the success of each science survey may be quantified, a so called
“survey figure-of-merit” (FoM). One “run” of the OpSim simulates the night-time tile-to-tile operations of 4MOST over the 5-year survey duration, right down to the decision tree that results in the placement of individual fibres on individual science targets. The OpSim outputs a large quantity of diagnostic information, both high-level and detailed. At one end of the scale, the OpSim reports the survey success FoMs that can be readily compared when for example the parameters describing the survey strategy have been adjusted. At the other extreme the OpSim outputs very detailed information, such as catalogues describing which targets have been observed and for how long, together with detailed diagnostic information about fibre movements and usage.
We show some example figures produced by the OpSim that provide diagnostic information about survey success, fibre usage, predicted survey sky coverage and target completeness.