The MOMFOS fibre positioner

The Anglo-Australian Observatory has undertaken a design study for a ≈1000 fibre positioner for the prime focus of the Giant Segmented Mirror Telescope (GSMT) as part of the MOMFOS (Multi-Object Multi-Fibre Optical Spectrograph) instrument. To our knowledge this is the first design study funded for a prime focus instrument for an ELT. It offers a particularly elegant and efficient solution for wide-field multiobject spectroscopy on extremely large telescopes (>30m) for the acquisition of large scale high-redshift surveys (1<z<10) with sufficient area (25 deg²), density coverage (≈2.5 million galaxies) and limiting magnitude (R≤26.5). Closely based on the innovative Echidna positioner under construction for Subaru's FMOS system, the MOMFOS positioner uses piezoelectric microrobotic actuators able to position accurately all 1000 fibres simultaneously. The FMOS-Echidna design is extended to include a novel position feedback system offering radical benefits in cost and speed. We present positioning results for a design capable of a higher packing density than that developed for FMOSEchidna, providing ≈1000 fibres across the 175 mm diameter field of view (20 arcmin).