Liger is a next-generation near-infrared (0.81 - 2.45 μm) integral field spectrograph (IFS) and imaging camera for the W. M. Keck Observatory (WMKO) adaptive optics (AO) system. The instrument concept originated by coupling designs from WMKO AO instruments and the future Thirty Meter Telescope IRIS spectrograph. Liger is designed for the Keck I telescope to operate behind the upcoming Keck All-sky Precision Adaptive optics (KAPA) system. The imaging camera sequentially feeds an IFS that makes use of slicer assembly for the coarsest sampling (75 mas and 150 mas per spaxel) and lenslet array (14 mas and 31 mas per spaxel) for diffraction-limited sampling. The imaging camera makes use of off-axis parabolas and a Hawaii-2RG detector to achieve a 14 mas/pixel plate scale and 20.5"×20.5" field of view. The IFS offers a range of spectral resolving power of R=4,000 { 10,000 and yields large fields of view for an AO-fed IFS. Liger will enable new science by providing enhanced capabilities, including higher spectral resolving power, access to shorter wavelengths, and larger fields of view than any current or planned ground- or space-based IFS system. Liger will be able to unlock previously inaccessible science across a wide range of the cosmos, such as dark matter substructure, supermassive black holes, the Galactic Center, exoplanet atmospheres, and the time-variable Solar System. We present the overall design of the Liger subsystems and review unique science drivers.
We present evaluations of the Keck Telescope’s adaptive optics (AO) performance on Milky Way Galactic center imaging and spectroscopic observations using three different AO setups: laser guide star with infrared (IR) tip-tilt correction, laser guide star with visible tip-tilt correction, and infrared natural guide star with a pyramid wavefront sensor. Observations of the Galactic Center can utilize a bright IR tip-tilt star (K′ = 7.4 mag) for corrections, which is over 10 arcseconds closer than the optical tip-tilt star. The proximity of this IR star enables the comparison of the aforementioned AO configurations. We present performance metrics such as full-width-at-half-maximum (FWHM), Strehl ratio, and spectral signal to noise ratio and their relations to atmospheric seeing conditions. The IR tip-tilt star decreases the median spatial FWHM by 31% in imaging data and 30% in spectroscopy. Median Strehl for imaging data improves by 24%. Additionally, the IR star removes the seeing dependence from differential tip-tilt error in both imaging and spectroscopic data. This evaluation provides important work for ongoing upgrades to AO systems, such as the Keck All sky Precision Adaptive Optics (KAPA) upgrade on the Keck I Telescope, and the development of new AO systems for extremely large telescopes.
We present the status and plans for the Keck All sky Precision Adaptive optics (KAPA) program. KAPA includes (1) an upgrade to the Keck I laser guide star adaptive optics (AO) facility to improve image quality and sky coverage, (2) the inclusion of AO telemetry-based point spread function estimates with all science exposures, (3) four key science programs, and (4) an educational component focused on broadening the participation of women and underrepresented groups in instrumentation. For this conference we focus on the KAPA upgrades since the 2020 SPIE proceedings1 including implementation of a laser asterism generator, wavefront sensor, real-time controller, asterism and turbulence simulators, the laser tomography system itself along with new operations software and science tools, and modifications to an existing near-infrared tip-tilt sensor to support multiple natural guide star and focus measurements. We will also report on the results of daytime and on-sky calibrations and testing.
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