Liger for Next Generation Keck AO: Filter Wheel and Pupil Design

TL;DR

Liger is a versatile next-generation near-infrared imager and spectrograph for Keck Observatory, featuring upgradeable filter wheels and pupil masks to optimize adaptive optics observations over a broad wavelength range.

Contribution

This paper presents the innovative design of filter wheels and pupil masks for Liger, enabling future upgrades and optimized performance in adaptive optics imaging and spectroscopy.

Findings

Design allows for future filter upgrades.

Pupil masks can be added or changed easily.

Fixed masks offer near-ideal performance with simpler mechanics.

Abstract

Liger is a next-generation near-infrared imager and integral field spectrograph (IFS) for the W.M. Keck Observatory designed to take advantage of the Keck All-Sky Precision Adaptive Optics (KAPA) upgrade. Liger will operate at spectral resolving powers between R$\sim$4,000 - 10,000 over a wavelength range of 0.8-2.4$\mu$m. Liger takes advantage of a sequential imager and spectrograph design that allows for simultaneous observations between the two channels using the same filter wheel and cold pupil stop. We present the design for the filter wheels and pupil mask and their location and tolerances in the optical design. The filter mechanism is a multi-wheel design drawing from the heritage of the current Keck/OSIRIS imager single wheel design. The Liger multi-wheel configuration is designed to allow future upgrades to the number and range of filters throughout the life of the instrument. The pupil mechanism is designed to be similarly upgradeable with the option to add multiple pupil mask options. A smaller wheel mechanism allows the user to select the desired pupil mask with open slots being designed in for future upgrade capabilities. An ideal pupil would match the shape of the image formed of the primary and would track its rotation. For different pupil shapes without tracking we model the additional exposure time needed to achieve the same signal to noise of an ideal pupil and determine that a set of fixed masks of different shapes provides a mechanically simpler system with little compromise in performance.