Impact of optical distortions on fiber positioning in the dark energy spectroscopic instrument

ABSTRACT The Dark Energy Spectroscopic Instrument, to be locat ed at the prime focus of the Mayall telescope, includesa wide “eld corrector, a 5000 “ber positioner system, and a “ber view camera. The mapping of the sky to thefocal plane, needed to position the “bers accurately, is described in detail. A major challenge is dealing withthe large amount of distortion introduced by the optics (of order 10% scale change), including time-dependentnon-axisymmetric distortions introduced by the atmospheric dispersion compensator. Solutions are presented tomeasure or mitigate these eects.Keywords: Optical Distortion, Fiber View Camera, Wide Field Corrector, Dark Energy, Focal Plane Mapping,Baryon Acoustic Oscillations 1. INTRODUCTION The Dark Energy Spectroscopic Instrument (DESI 1 ), currently under construction, will measure the expansionhistory of the Universe using the Baryon Acoustic Oscillation technique. The spectra of 40 million galaxiesover 14000 sq deg will be measured during the life of the experiment. A new prime focus corrector for theKPNO Mayall telescope will deliver light to 5000 “ber optic positioners. The “bers in turn feed ten broad-bandspectrographs. A six- element corrector