Spine development for the Echidna fiber positioner

The Echidna multi-object fiber positioner is part of the Fiber Multi-Object Spectrograph (FMOS) project for the prime focus of the Subaru telescope. Given the physical size of the focal plane and the required number of fibers (400), a positioning system based on the Anglo-Australian Observatory's 2dF instrument, that incorporates the placement of magnetic buttons by a single X/Y/Z robot, was considered impractical. Instead, a solution has been developed in which each fiber is mounted on a tilting spine that allows the fiber to be positioned anywhere in a circle of radius 7 mm. Each of the 400 fibers therefore has a fixed "patrol" area in the field of view, with a significant overlap between neighbouring spines. A description of a single Echidna spine is presented. Each spine is driven by a quadrant tube piezoelectric actuator (QTP) that, by a ratcheting mechanism, is able to position the fiber to within 10 μm of any coordinate in the corresponding patrol area. Results of positioning tests for eight of the twenty prototype spines reveal better than specification performance, as well as a durability far in excess of the specified lifetime of the instrument.