Development of the FOBOS focal plane positioner

The Fibre-Optic Broadband Spectrograph (FOBOS) is a facility-class multi-object spectrograph currently being designed, and to be deployed to the Keck II telescope. FOBOS is able to simultaneously observe 1729-objects across a 20' field of view, with 30% instrument throughput from 0.31-1.0 mu m and a spectral resolution of R>3500 delivered by three, bench-mounted 4-channel spectrographs. The FOBOS focal plane will be configured using 1729 'Starbug' robots, which are vacuum-adhered piezo actuators that 'walk' across the field plate to position fibres. Using Starbugs to position fibres allows fast configuration and flexibility in payloads, with a mixture of Single Fibre, IFUs, and Imaging Bundles (used for guiding) capable of being rapidly positioned across the field. The FOBOS team have recently passed their conceptual design review. The FOBOS positioner design builds on the experience gained from the TAIPAN instrument, which used 150 Starbugs and demonstrated their viability as a science instrument. In this paper we detail the conceptual design of the FOBOS focal positioner system. This includes details of the Starbug design, optomechanics, and optical designs that allow the focal plane positioner to operate. The FOBOS focal positioner design builds on the experience gained from TAIPAN, a prototype instrument built to demonstrate the Starbugs technology.