Quasi-static aberrations induced by laser guide stars in adaptive optics

Laser Guide Star Adaptive Optics (LGS AO) systems use the return from an artificial guide star to measure the wavefront aberrations in the direction of the science object. We observe quasi-static differences between the measured wavefront and the wavefront aberration of the science object. This paper quantifies and explains the source of the difference between the wavefronts measured using an LGS and a natural guide star at the W. M. Keck Observatory, which can be as high as 1000 nm RMS

Keck Interferometer status and plans

Keck Interferometer is a NASA-funded project to combine the two 10 m Keck telescopes for high sensitivity near-infrared fringe visibility measurements, nulling interferometry at 10 μm to measure the quantity of exozodiacal emission around nearby stars, and differential-phase measurements to detect "hot-Jupiters" by their direct emission. It is being developed by the Jet Propulsion Laboratory, the W. M. Keck Observatory, and the Michelson Science Center. Recent activity has included formal visibility mode commissioning, as well as science observations, and we briefly review some of the significant technical aspects and updates to the system. We have also completed laboratory development of the nuller. The nuller uses two modified Mach-Zehnder input nullers, a Michelson cross combiner, and a 10 μm array camera to produce background-limited null measurements. To provide required temporal stability for the nuller, the system incorporates end-to-end laser metrology with phase referencing from two 2.2 μm fringe trackers. The nuller recently completed its pre-ship review and is being installed on the summit. After nuller integration and test, the differential phase mode will be deployed, which will use a 2-5 μm fringe detector in combination with a precision path length modulator and a vacuum delay line for dispersion control

Keck Interferometer: from development phase to facility-class instrument

The Keck Interferometer is entering a regular limited observational phase. A restricted number of observers are expected to use the instrument over the course of the next few years in a shared-risk capacity. To facilitate this, the W. M. Keck Observatory and the Jet Propulsion Laboratory are following a Handover procedure consisting of a number of stages related to the science modes of the instrument as they reach completion. The first of these is the Visibility Science mode that involves only the two Keck telescopes. Other modes to follow are Nulling, Differential Phase, Astrometry, and Imaging. The process includes defining a reasonable level of functionality of each mode, training observatory staff to maintain and schedule tasks related to the upkeep of each mode, and defining and documenting each of the subsystems related to each mode. Here we discuss the outline of the Handover plan and report on its progress to date

Radial thresholding to mitigate Laser-Guide-Star aberrations on Centre-of-Gravity-based Shack-Hartmann wavefront sensors

Sodium Laser Guide Stars (LGSs) are elongated sources due to the thickness and the finite distance of the sodium layer. The fluctuations of the sodium layer altitude and atom density profile induce errors on centroid measurements of elongated spots, and generate spurious optical aberrations in closed--loop adaptive optics (AO) systems. According to an analytical model and experimental results obtained with the University of Victoria LGS bench demonstrator, one of the main origins of these aberrations, referred to as LGS aberrations, is not the Centre-of-Gravity (CoG) algorithm itself, but the thresholding applied on the pixels of the image prior to computing the spot centroids. A new thresholding method, termed ``radial thresholding'', is presented here, cancelling out most of the LGS aberrations without altering the centroid measurement accuracy.Comment: 8 pages, 9 figures, accepted for publication in MNRA

Recent progress at the Keck Interferometer

In this paper we report on progress at the Keck Interferometer since the 2004 SPIE meeting with an emphasis on the operations improvements for visibility science

Design of a precision calibration unit for Keck NIRC2 AO instrument

High-precision astrometry has the potential to address questions in planet formation, black hole science, Galactic structure, and more. However, in order to achieve a precision of sub-milli arcseconds (mas), we need a calibration method better than the current techniques such as on-sky calibration using calibrated stellar or stellar cluster systems, which have a precision of ~1 mas. Precision calibration unit with a regular grid of photo-lithographically manufactured pinholes combined with self-calibration techniques, on the other hand, is a new and innovative way to potentially achieve a precision of sub-mas over the entire field of view. This technique is beneficial to adaptive optic (AO) instruments for future telescopes like the Thirty Meter Telescope (TMT). In this work, we present our design for a new astrometric calibration unit to feed the NIRC2 AO instrument at the W. M. Keck Observatory. It allows calibration over a large field of view of 47" x 47"

Design of a precision calibration unit for Keck NIRC2 AO instrument

High-precision astrometry has the potential to address questions in planet formation, black hole science, Galactic structure, and more. However, in order to achieve a precision of sub-milli arcseconds (mas), we need a calibration method better than the current techniques such as on-sky calibration using calibrated stellar or stellar cluster systems, which have a precision of ~1 mas. Precision calibration unit with a regular grid of photo-lithographically manufactured pinholes combined with self-calibration techniques, on the other hand, is a new and innovative way to potentially achieve a precision of sub-mas over the entire field of view. This technique is beneficial to adaptive optic (AO) instruments for future telescopes like the Thirty Meter Telescope (TMT). In this work, we present our design for a new astrometric calibration unit to feed the NIRC2 AO instrument at the W. M. Keck Observatory. It allows calibration over a large field of view of 47" x 47"

Advances in instrumentation at the W. M. Keck Observatory

In this paper we describe both recently completed instrumentation projects and our current development efforts in the context of the Observatory's science driven strategic plan which seeks to address key questions in observational astronomy for extra-galactic, Galactic, and planetary science with both seeing limited capabilities and high angular resolution adaptive optics capabilities. This paper will review recently completed projects as well as new instruments in development including MOSFIRE, a near IR multi-object spectrograph nearing completion, a new seeing limited integral field spectrograph for the visible wavelength range called the Keck Cosmic Web Imager, and the Keck Next Generation Adaptive Optics facility and its first light science instrument DAVINCI

Keck Interferometer status and plans

Keck Interferometer is a NASA-funded project to combine the two 10 m Keck telescopes for high sensitivity near-infrared fringe visibility measurements, nulling interferometry at 10 μm to measure the quantity of exozodiacal emission around nearby stars, and differential-phase measurements to detect "hot-Jupiters" by their direct emission. It is being developed by the Jet Propulsion Laboratory, the W. M. Keck Observatory, and the Michelson Science Center. Recent activity has included formal visibility mode commissioning, as well as science observations, and we briefly review some of the significant technical aspects and updates to the system. We have also completed laboratory development of the nuller. The nuller uses two modified Mach-Zehnder input nullers, a Michelson cross combiner, and a 10 μm array camera to produce background-limited null measurements. To provide required temporal stability for the nuller, the system incorporates end-to-end laser metrology with phase referencing from two 2.2 μm fringe trackers. The nuller recently completed its pre-ship review and is being installed on the summit. After nuller integration and test, the differential phase mode will be deployed, which will use a 2-5 μm fringe detector in combination with a precision path length modulator and a vacuum delay line for dispersion control