Design and performance of a F/#-conversion microlens for Prime Focus Spectrograph at Subaru Telescope

TL;DR

This paper presents the design, optimization, and manufacturing of a microlens that converts the fast F/2.2 beam at Subaru Telescope's prime focus to F/2.8, improving spectrograph performance.

Contribution

It introduces a novel concave-plano negative microlens design optimized for maximum ray confinement and details its manufacturing process with broadband anti-reflective coating.

Findings

Optimized lens design achieves efficient F/# conversion.

Manufactured microlenses with broadband AR coating.

Enhanced fiber-fed spectrograph performance.

Abstract

The PFS is a multi-object spectrograph fed by 2394 fibers at the prime focus of Subaru telescope. Since the F/# at the prime focus is too fast for the spectrograph, we designed a small concave-plano negative lens to be attached to the tip of each fiber that converts the telescope beam (F/2.2) to F/2.8. We optimized the lens to maximize the number of rays that can be confined inside F/2.8 while maintaining a 1.28 magnification. The microlenses are manufactured by glass molding, and an ultra-broadband AR coating (<1.5% for lambda=0.38-1.26 um) will be applied to the front surface.