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 Maunakea, Hawaii. The novel design provides blue-optimized seeing-limited imaging from 350–560 nm with configurable spectral resolution from 1000–20,000 in a field of view up to 20″ × 33″. 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 Maunakea 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 10 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).

Significance: Drug development for Alzheimer’s disease (AD) has been largely unsuccessful to date. Although numerous agents are reportedly effective in vitro, only an inadequate number of them have been tested in vivo, partially because of the lack of reliable and cost-efficient imaging methods to monitor their in vivo therapeutic effectiveness. Several amyloid beta (Aβ)-specific PET tracers have been used for clinical studies. However, their application for monitoring drug treatment in small animals is limited. Near-infrared fluorescence (NIRF) molecular imaging is a cheap, easy-to-use, and widely available technology for small animal studies. In this report we demonstrate, for the first time to our knowledge, that NIRF imaging can be used to monitor the loading changes of Aβs in an AD mouse model.