The Optical Corrector for the Dark Energy Spectroscopic Instrument
Timothy N. Miller, Peter Doel, Gaston Gutierrez, Robert Besuner, David Brooks, Giuseppe Gallo, Henry Heetderks, Patrick Jelinsky, Stephen M. Kent, Michael Lampton, Michael Levi, Ming Liang, Aaron Meisner, Michael J. Sholl, Joseph Harry Silber, David Sprayberry, Jessica Nicole Aguilar, Axel de la Macorra, Daniel Eisenstein, Kevin Fanning, Andreu Font-Ribera, Enrique Gaztanaga, Satya Gontcho A Gontcho, Klaus Honscheid, Jorge Jimenez, Dick Joyce, Robert Kehoe, Theodore Kisner, Anthony Kremin, Martin Landriau, Laurent Le Guillou, Christophe Magneville, Paul Martini, Ramon Miquel, John Moustakas, Jundan Nie, Will Percival, Claire Poppett, Francisco Prada, Graziano Rossi, David Schlegel, Michael Schubnell, Hee-Jong Seo, Ray Sharples, Gregory Tarle, Mariana Vargas-Magana, Zhimin Zhou
TL;DR
The DESI project targets a transformative dark energy survey by measuring tens of millions of spectra using a wide-field prime-focus corrector on the Mayall 4-meter telescope. The paper provides a comprehensive account of the corrector's optical design, fabrication, and the associated mechanical structure, including alignment (hexapod), stray-light mitigation, and integration/testing, culminating in on-sky validation of performance. It emphasizes atmospheric-dispersion compensation, ghost analysis, and manufacturability constraints that shaped the design, along with lessons learned from multi-year fabrication. The work situates the corrector within a broader instrument context, with companion papers detailing the focal plane, fiber system, and spectrograph system.
Abstract
The Dark Energy Spectroscopic Instrument (DESI) is currently measuring the spectra of 40\,million galaxies and quasars, the largest such survey ever made to probe the nature of cosmological dark energy. The 4-meter Mayall telescope at Kitt Peak National Observatory has been adapted for DESI, including the construction of a 3.2-degree diameter prime focus corrector that focuses astronomical light onto a 0.8-meter diameter focal surface with excellent image quality over the DESI bandpass of 360-980nm. The wide-field corrector includes six lenses, as large as 1.1-meters in diameter and as heavy as 237\,kilograms, including two counter-rotating wedged lenses that correct for atmospheric dispersion over Zenith angles from 0 to 60 degrees. The lenses, cells, and barrel assembly all meet precise alignment tolerances on the order of tens of microns. The barrel alignment is maintained throughout a range of observing angles and temperature excursions in the Mayall dome by use of a hexapod, which is itself supported by a new cage, ring, and truss structure. In this paper we describe the design, fabrication, and performance of the new corrector and associated structure, focusing on how they meet DESI requirements. In particular we describe the prescription and specifications of the lenses, design choices and error budgeting of the barrel assembly, stray light mitigations, and integration and test at the Mayall telescope. We conclude with some validation highlights that demonstrate the successful corrector on-sky performance, and list some lessons learned during the multi-year fabrication phase.