Design and performance of an R-θ fiber positioner for the BigBOSS instrument

The BigBOSS instrument is a proposed multi-object spectrograph for the Mayall 4m telescope at Kitt Peak, which will measure the redshift of 20 million galaxies and map the expansion history of the universe over the past 8 billion years, surveying 10-20 times the volume of existing studies. For each 20 minute observation, 5000 optical fibers are individually positioned by a close-packed array of 5000 robotic positioner mechanisms. Key mechanical constraints on the positioners are: empty set12mm hardware envelope, 014mm overlapping patrol zones, open-loop targeting accuracy <= 40 mu m, and step resolution <= 5 mu m, among other requirements on envelope, power, stability, and speed. This paper describes the design and performance of a newly-developed fiber positioner with R-theta polar kinematics, in which a flexure-based linear R-axis is stacked on a rotational theta-axis. Benefits over the usual eccentric parallel axis theta-phi kinematic approach include faster repositioning, simplified anti-collision schemes, and inherent anti-backlash preload. Performance results are given for complete positioner assemblies as well as sub-component hardware characterization.