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Proceedings Paper

Broadband anti-reflection coating for the meter class Dark Energy Spectroscopic Instrument lenses
Author(s): Charles Kennemore III; Debi Archer; Russ Barbaria; Joe Bertolina; Rick Dix; Katy Dodson; Sharon Doi-Okray; Aaron Gallon; Bill Sweet; Timothy N. Miller; Robert W. Besuner; Michael E. Levi; Michael Lampton; Pat Jelinsky; Jerry Edelstein; David J. Schlegel; Jeremy McCauley; Henry Heetderks; Michael J. Sholl; Peter Doel; David Brooks; Stephen Kent
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Paper Abstract

The Dark Energy Spectroscopic Instrument (DESI), currently under construction, will be used to measure the expansion history of the Universe using the Baryon Acoustic Oscillation technique. The spectra of 35 million galaxies and quasars over 14000 sq deg will be measured during the life of the experiment. A new prime focus corrector for the KPNO Mayall telescope will deliver light to 5000 fiber optic positioners. The fibers, in turn, feed ten broad-band spectrographs. We will describe the broadband AR coating (360 nm to 980nm) that was applied to the lenses of the camera system for DESI using ion assisted deposition techniques in a 3 m coating chamber. The camera has 6 lenses ranging in diameter from 0.8 m to 1.14 m, weighing from 84 kg to 237 kg and made from fused silica or BK7. The size and shape of the surfaces provided challenges in design, uniformity control, handling, tooling and process control. Single surface average transmission and minimum transmission met requirements. The varied optical surfaces and angle of incidence considerations meant the uniformity of the coating was of prime concern. The surface radius of curvature (ROC) for the 12 surfaces ranged from nearly flat to a ROC of 611 mm and a sag of 140 mm. One lens surface has an angle of incidence variation from normal incidence to 40°. Creating a design with a larger than required bandwidth to compensate for the non-uniformity and angle variation created the ability to reduce the required coating uniformity across the lens and a single design to be used for all common substrate surfaces. While a perfectly uniform coating is often the goal it is usually not practicable or cost effective for highly curved surfaces. The coating chamber geometry allowed multiple radial positions of the deposition sources as well as substrate height variability. Using these two variables we were able to avoid using any masking to achieve the uniformity required to meet radial and angle performance goals. Very broadband AR coatings usually have several very thin and optically important layers. The DESI coating design has layers approaching 3 nm in thickness. Having sensitive thin layers in the design meant controlling layer thickness and azimuthal variation were critical to manufacturing repeatability. Through use of strategically placed quartz crystal monitors combined with stable deposition plumes, the manufacturing variability was reduced to acceptable levels. Low deposition rates and higher rotation rates also provided some stability to azimuthal variation.

Paper Details

Date Published: 10 July 2018
PDF: 16 pages
Proc. SPIE 10706, Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation III, 107061S (10 July 2018); doi: 10.1117/12.2313006
Show Author Affiliations
Charles Kennemore III, Viavi Solutions Inc. (United States)
Debi Archer, Viavi Solutions Inc. (United States)
Russ Barbaria, Viavi Solutions Inc. (United States)
Joe Bertolina, Viavi Solutions Inc. (United States)
Rick Dix, Viavi Solutions Inc. (United States)
Katy Dodson, Viavi Solutions Inc. (United States)
Sharon Doi-Okray, Viavi Solutions Inc. (United States)
Aaron Gallon, Viavi Solutions Inc. (United States)
Bill Sweet, Viavi Solutions Inc. (United States)
Timothy N. Miller, Lawrence Berkeley National Lab. (United States)
Robert W. Besuner, Lawrence Berkeley National Lab. (United States)
Michael E. Levi, Lawrence Berkeley National Lab. (United States)
Michael Lampton, Lawrence Berkeley National Lab. (United States)
Pat Jelinsky, Lawrence Berkeley National Lab. (United States)
Jerry Edelstein, Lawrence Berkeley National Lab. (United States)
David J. Schlegel, Lawrence Berkeley National Lab. (United States)
Jeremy McCauley, Lawrence Berkeley National Lab. (United States)
Henry Heetderks, Lawrence Berkeley National Lab. (United States)
Michael J. Sholl, Lawrence Berkeley National Lab. (United States)
SpaceX Inc. (United States)
Peter Doel, Univ. College London (United Kingdom)
David Brooks, Univ. College London (United Kingdom)
Stephen Kent, Fermi National Accelerator Lab. (United States)

Published in SPIE Proceedings Vol. 10706:
Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation III
Ramón Navarro; Roland Geyl, Editor(s)

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