SDSS-V focal plane robot positioning metrology

Abstract

The Sloan Digital Sky Survey V (SDSS-V) is an all-sky spectroscopic survey of ≥ 6 million objects, designed to decode the history of the Milky Way, reveal the inner workings of stars, investigate the origin of solar systems, and track the growth of supermassive black holes across the Universe.1 The robotic Focal Plane System (FPS)2 will carry 500 robots each with three fibers for science and metrology. The science fibers feed the BOSS3 and APOGEE4 spectrographs, while the metrology fibers are back illuminated to aid in robot positioning. Blind initial x/y positional precision of the robots is expected to be better than 50µm. The robots must position the fibers to better than 5µm in order to meet the science requirements. The FPS fiber viewing camera (FVC) consists of optomechanical components that look back through the telescope optics at light from back-lit fiducial and metrology fibers to measure the positions of the robots in the telescope focal plane. The FVC takes an image of the robots in the telescope focal plane, measures their positions to an accuracy of better than 3µm, and then feeds back error commands to the robot control system to meet the 5µm positional requirement. This paper details the optomechanical design, and initial results of an engineering run on the du Pont telescope.