The Optical Corrector for the Dark Energy Spectroscopic Instrument

• Published in: The Astronomical journal Vol. 168; no. 2; pp. 95 - 143
• Main Authors: Miller, Timothy N., Doel, Peter, Gutierrez, Gaston, Besuner, Robert, Brooks, David, Gallo, Giuseppe, Heetderks, Henry, Jelinsky, Patrick, Kent, Stephen M., Lampton, Michael, Levi, Michael E., Liang, Ming, Meisner, Aaron, Sholl, Michael J., Silber, Joseph Harry, Sprayberry, David, Aguilar, Jessica Nicole, de la Macorra, Axel, Eisenstein, Daniel, Fanning, Kevin, Font-Ribera, Andreu, Gaztañaga, Enrique, Gontcho A Gontcho, Satya, Honscheid, Klaus, Jimenez, Jorge, Joyce, Dick, Kehoe, Robert, Kisner, Theodore, Kremin, Anthony, Landriau, Martin, Le Guillou, Laurent, Magneville, Christophe, Martini, Paul, Miquel, Ramon, Moustakas, John, Nie, Jundan, Percival, Will, Poppett, Claire, Prada, Francisco, Rossi, Graziano, Schlegel, David, Schubnell, Michael, Seo, Hee-Jong, Sharples, Ray, Tarlé, Gregory, Vargas-Magaña, Mariana, Zhou, Zhimin
• Format: Journal Article
• Language: English
• Published: Madison The American Astronomical Society 01.08.2024; IOP Publishing
• Subjects: Alignment; Assembly; Astronomical instrumentation; Astronomical instruments; ASTRONOMY AND ASTROPHYSICS; Atmospheric correction; Atmospheric diffusion; Atmospheric dispersion; Celestial bodies; Dark adaptation; Dark energy; Diameters; Error correction; Fabrication; Galaxies; Image quality; INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY; Lenses; Measuring instruments; Optical telescopes; Pollution dispersion; Primary mirror; Prime focus; Quasars; Trussed structures; Wide-field telescopes
• ISSN: 0004-6256; 1538-3881
• DOI: 10.3847/1538-3881/ad45fe

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 m Mayall telescope at Kitt Peak National Observatory has been adapted for DESI, including the construction of a 3.°2 diameter prime focus corrector that focuses astronomical light onto a 0.8 m diameter focal surface with excellent image quality over the DESI bandpass of 360–980 nm. The wide-field corrector includes six lenses, as large as 1.1 m in diameter and as heavy as 237 kilograms, including two counterrotating wedged lenses that correct for atmospheric dispersion over zenith angles from 0° to 60°. 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 we list some lessons learned during the multiyear fabrication phase.