Echidna - a multi-fiber positioner for the Subaru prime focus

The aim for a spectrograph feed from the Subaru prime focus is to have 400 fibers. Since the area of the field is only approx. 1/10 that of the 400 fiber two degree field (2dF) system on the Anglo-Australian Telescope, placement of magnetic buttons by a robot, as done for 2dF, was not considered applicable. Instead, a concept has been developed in which each fiber is held on a spine which can be tilted to position its tip anywhere within a circle. With targets randomly scattered over the field and the radial range for each spine equal to the spine pitch (7 mm to give approx. 400 fibers in the field), the success rate in reaching targets is acceptably high. At the f/2 focus, a spine tilt of 1/20 radian is just acceptable and requires the spines be 140 mm long. Two basic mechanisms for tilting and holding such a spine have been investigated experimentally. The first uses three commercial miniature linear actuators set parallel and linked to the base of the spine through simple flexures. A prototype has been built and demonstrated to perform satisfactorily. Another approach is to mount the spine on a ball joint and drive it directly in tip and tilt using a bending piezo impact drive. A prototype of this form has been built; initial tests are promising.