Fabrication of the DESI Corrector Lenses

TL;DR

This paper details the design, fabrication, and performance evaluation of the complex lenses used in the DESI corrector, a crucial component for the spectroscopic survey of the universe's expansion.

Contribution

It presents the successful manufacturing, polishing, coating, and integration of large, complex lenses for the DESI corrector, including reoptimization based on final measurements.

Findings

Lenses meet the demanding optical specifications.

Successful polishing and coating processes were achieved.

The final corrector performance aligns with initial design estimates.

Abstract

The Dark Energy Spectroscopic Instrument (DESI) is under construction to measure the expansion history of the Universe using the Baryon Acoustic Oscillation technique. The spectra of 35 million galaxies and quasars over 14000 square degrees 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 describe the DESI corrector optics, a series of six fused silica and borosilicate lenses. The lens diameters range from 0.8 to 1.1 meters, and their weights 84 to 237 kg. Most lens surfaces are spherical, and two are challenging 10th-order polynomial aspheres. The lenses have been successfully polished and treated with an antireflection coating at multiple subcontractors, and are now being integrated into the DESI corrector barrel assembly at University College London. We describe the final performance of the lenses in terms of their various parameters, including surface figure, homogeneity, and others, and compare their final performance against the demanding DESI corrector requirements. Also we describe the reoptimization of the lens spacing in their corrector barrel after their final measurements are known. Finally we assess the performance of the corrector as a whole, compared to early budgeted estimates.