AESOP is the fibre positioner unit for the 4MOST instrument for the 4-metre European Southern Observatories (ESO) VISTA telescope in Chile. The 4MOST project, led by the Astrophysical Institute Potsdam (AIP, Germany), involves a number of European partners.

Astralis is responsible for design, assembly, test, delivery, and commissioning of the complete AESOP fibre positioning subsystem.

AESOP deploys 2400 optical fibres to required positions on the curved focal surface of the telescope. Each fibre can be deployed anywhere within a fixed patrol area. Field reconfigurations are achieved in an iterative closed-loop process with positional feedback from a metrology system. Optical fibres are connectorised at their exit from the positioner system, where they feed a fibre bundle terminated in a series of optical spectrographs.

The design for AESOP is an evolution of Astralis’ tilting spine technology, first designed and implemented in the FMOS-Echidna instrument for the Subaru telescope.