An optimized tiling pattern for multiobject spectroscopic surveys: application to the 4MOST survey

Abstract

Large multi-object spectroscopic surveys require automated algorithms to optimise their observing strategy. One of the most ambitious upcoming spectroscopic surveys is the 4MOST survey. The 4MOST survey facility is a fibre-fed spectroscopic instrument on the VISTA telescope with a large enough field of view to survey a large fraction of the southern sky within a few years. Several Galactic and extragalactic surveys will be carried out simultaneously, so the combined target density will strongly vary. In this paper, we describe a new tiling algorithm that can naturally deal with the large target density variations on the sky and which automatically handles the different exposure times of targets. The tiling pattern is modelled as a marked point process, which is characterised by a probability density that integrates the requirements imposed by the 4MOST survey. The optimal tilling pattern with respect to the defined model is estimated by the tiles configuration that maximises the proposed probability density. In order to achieve this maximisation a simulated annealing algorithm is implemented. The algorithm automatically finds an optimal tiling pattern and assigns a tentative sky brightness condition and exposure time for each tile, while minimising the total execution time that is needed to observe the list of targets in the combined input catalogue of all surveys. Hence, the algorithm maximises the long-term observing efficiency and provides an optimal tiling solution for the survey. While designed for the 4MOST survey, the algorithm is flexible and can with simple modifications be applied to any other multi-object spectroscopic survey.

Optimising a magnitude-limited spectroscopic training sample for photometric classification of supernovae

“…Observation of each object in the catalogue was simulated in grey and dark time (we assume none of our targets will be targeted during bright time) and the success rate was averaged over both 15 https://www.lsst.org/scientists/simulations/opsim curve distributions at each magnitude as the current dark/grey/bright cadence is undecided for 4MOST. However, current simulations for 4MOST's tiling pattern favour dark time over grey (Tempel et al 2020a), so their average can be considered as a lower limit to our success rate. The function describing the success rate is shown in Table 2.…”

“…Current 4MOST simulations combine observing fields of the same sky coordinates and instrument position angle into observing blocks (OBs, Tempel et al 2020a). The duration of the OBs are limited by a total exposure time of 1 h. Success is determined by whether a targeted supernova spectrum's necessary exposure time falls below 1 h for our criterion of SNR = 7 per 15 Å.…”

“…TiDES will be 'piggy-backing' on other surveys and will not be driving where to point 4MOST or for how long. We assume 1 h field visits based on Tempel et al (2020a). For TiDES, a field visit exposure time of 1 h in combination with a spectral success criterion (SSC) effectively imposes a magnitude limit to spectroscopically confirmed supernovae.…”

4MOST: the 4-metre multi-object spectroscopic telescope project in the assembly, integration, and test phase

“…This allows prediction of the needed exposure times in different observing conditions for the tens of millions of targets given the spectral success requirements provided by the different Survey teams. The 4FS-Visit Planner optimally tiles the sky with 4MOST field-of-views pointings, assigning each visit a sky condition and a sequence of exposure times based on the local target density and exposure time needs, aiming to maximise target completion and minimising over exposure [15]. A long-term scheduler distributes the visits over the five survey years such that they can always be observed close to the meridian, while a short-term scheduler picks the optimal visit to schedule in the next minutes given the current and predicted observing conditions.…”