The Keck Cosmic Web Imager Integral Field Spectrograph

TL;DR

The Keck Cosmic Web Imager (KCWI) is a versatile optical integral field spectrograph designed for faint, diffuse cosmic objects, offering high efficiency, configurable spectral resolution, and optimized performance for observations from 350 to 560 nm.

Contribution

This paper presents the detailed design, performance metrics, and commissioning results of the KCWI, a novel instrument optimized for observing the cosmic web and intergalactic medium.

Findings

Peak efficiency exceeds 45% across the wavelength range.

Achieved spectral resolution up to 20000.

Successful on-sky commissioning with detailed performance validation.

Abstract

We report on the design and performance of the Keck Cosmic Web Imager (KCWI), a general purpose optical integral field spectrograph that has been installed at the Nasmyth port of the 10 m Keck II telescope on Mauna Kea, HI. The novel design provides blue-optimized seeing-limited imaging from 350-560 nm with configurable spectral resolution from 1000 - 20000 in a field of view up to 20"x33". Selectable volume phase holographic (VPH) gratings and high performance dielectric, multilayer silver and enhanced aluminum coatings provide end-to-end peak efficiency in excess of 45% while accommodating the future addition of a red channel that will extend wavelength coverage to 1 micron. KCWI takes full advantage of the excellent seeing and dark sky above Mauna Kea with an available nod-and-shuffle observing mode. The instrument is optimized for observations of faint, diffuse objects such as the intergalactic medium or cosmic web. In this paper, a detailed description of the instrument design is provided with measured performance results from the laboratory test program and ten nights of on-sky commissioning during the spring of 2017. The KCWI team is lead by Caltech and JPL (project management, design and implementation) in partnership with the University of California at Santa Cruz (camera optical and mechanical design) and the W. M. Keck Observatory (observatory interfaces).