The Keck Observatory began science observations with a laser guide star adaptive optics system, the first such system on an 8-10 m class telescope, in late 2004. This new capability greatly extends the scientific potential of the Keck II Telescope, allowing near-diffraction-limited observations in the near-infrared using natural guide stars as faint as 19th magnitude. This paper describes the conceptual approach and technical implementation followed for this system, including lessons learned, and provides an overview of the early science capabilities.

https://www2.keck.hawaii.edu/optics/aodocs/PWetal2006PASP118_297.pdf

The W. M. Keck Observatory Laser Guide Star Adaptive Optics System: Overview

We present results of near infrared imaging of the disk-galaxy-merger NGC 6240 using adaptive optics on the Keck II Telescope and reprocessed archival data from NICMOS on the Hubble Space Telescope. Both the North and South nuclei of NGC 6240 are clearly elongated, with considerable sub-structure within each nucleus. In K' band there are at least two point-sources within the North nucleus; we tentatively identify the south-western point-source within the North nucleus as the position of one of the two AGNs. Within the South nucleus, the northern sub-nucleus is more highly reddened. Based upon the nuclear separation measured at 5 GHz, we suggest that the AGN in the South nucleus is still enshrouded in dust at K' band, and is located slightly to the north of the brightest point in K' band. Within the South nucleus there is strong H2 1-0 S(1) line emission from the northern sub-nucleus, contrary to the conclusions of previous seeing-limited observations. Narrowband H2 emission-line images show that a streamer or ribbon of excited molecular hydrogen connects the North and South nuclei. We suggest that this linear feature corresponds to a bridge of gas connecting the two nuclei, as seen in computer simulations of mergers. Many point-like regions are seen around the two nuclei. These are most prominent at 1.1 microns with NICMOS, and in K'-band with Keck adaptive optics. We suggest that these point-sources represent young star clusters formed in the course of the merger.

/pdf/the-core-of-ngc-6240-from-keck-adaptive-optics-and-hst-q9bpoynmx1.pdf

The Core of NGC 6240 from Keck Adaptive Optics and HST NICMOS Observations

We report on observations obtained with the adaptive optics system at the 10 m Keck II Telescope during engineering validation and early science observing time for the adaptive optics system. We observed Neptune at near-infrared wavelengths. Angular resolution was 005–006, corresponding to a spatial scale of approximately 1000 km at Neptune. We discuss the latitudinal structure of circumferential cloud bands and of compact infrared-bright features seen in the southern hemisphere, as well as their variation with wavelength. We determine the values of I/F (proportional to the ratio of reflected intensity to incident solar flux) in the J and H infrared-wavelength bands, including narrowband filters where there is strong methane absorption. We use the I/F values inside and outside of methane bands to estimate the altitude of clouds responsible for the brightest compact features in the infrared. Our data show that, on two of our four observing dates, the brightest region on Neptune contained highly reflective haze layers located below the tropopause but not deeper than a few bars.

https://iopscience.iop.org/article/10.1086/344943/pdf

Cloud Structures on Neptune Observed with Keck Telescope Adaptive Optics

The wavefront control strategy for the proposed Gemini Planet Imager, an extreme adaptive optics coronagraph for planet detection, is presented. Two key parts of this strategy are experimentally verified in a testbed at the Laboratory for Adaptive Optics, which features a 32 x 32 MEMS device. Detailed analytic models and algorithms for Shack-Hartmann wavefront sensor alignment and calibration are presented. It is demonstrated that with these procedures, the spatially filtered WFS and the Fourier Transform reconstructor can be used to flatten to the MEMS to 1 nm RMS in the controllable band. Performance is further improved using the technique of modifying the reference slopes using a measurement of the static wavefront error in the science leg.

/pdf/wavefront-control-for-the-gemini-planet-imager-3o2j60o204.pdf

Wavefront control for the Gemini Planet Imager

Adaptive optics is concerned with the correction of turbulence induced aberrations and mis-alignments in optical systems. It is an expensive and complicated technology which has been developed for astronomy and defence applications. Recently, it has found a role within industry and medicine. This thesis deals with a simplified mechanism for adaptive feedback based on the analysis of distorted images, and an iterative feedback process. Simplification is achieved at the expense of speed, but there are particular applications where this technique is suitable. The one considered here is the removal of so-called non-common path errors in a traditional adaptive optics system, which can otherwise degrade the system performance.

Image sharpening metrics and search strategies for indirect adaptive optics

The Keck AO system is nearing completion and is nearly ready for system integration in the optics laboratory at Keck Headquarters. because of the ambitious scientific objectives of this project, the AO system is quite complex in design. The optics bench alone has 33 motion control axes, and potentially 6 image planes. Calibration will play a major role in making the AO system function as planned. We have identified approximately 35 separate calibration procedures that will be needed to be performed during the integration phase in the laboratory. Many of these procedures are quite straightforward. Other calibrations are quite involved and are mini 'research projects' in themselves. It is this last category that will be discussed in this paper.

Laboratory calibration of the W.M. Keck Observatory Adaptive Optics Facility

The non-circular shape of the Keck telescopes primary mirror, combined with the fact that the telescope is on an alt/az mount with the adaptive optics system on the left Nasmyth platform, means that as the telescope tracks a star field the hexagonal pupil image rotates on the wavefront sensor camera and on the deformable mirror. This creates a need for a dynamic control strategy. The control matrix, the list of which subapertures are sufficiently illuminated and the list of which actuators are active and which are slaved, all must change on the fly, with the loops closed and without perturbing a science observation in progress. This control strategy must also be robust in the presence of modest pupil translation. These needs are met in the Keck AO system by the Pupil Rotation Compensation (PRC) software tool. The PRC software is responsible for maintaining system parameters that are functions of the pupil position and orientation. This paper will present the details of the PRC software design including algorithms used for selecting active subapertures, dealing with variable subaperture illumination, calculating control matrices, and slaving actuators outside the pupil illuminated area on the deformable mirror.

Compensating for pupil rotation in the W.M. Keck Observatory adaptive optics system

The Multiple Mirror Telescope on Mt. Hopkins will soon be upgraded to a single 6.5 m primary mirror.An adaptive- optical system, featuring a thin-shell, adaptive-secondary mirror with 330 voice-coil actuators is being developed for this new telescope. The thin-shell mirror is supported by a thick, concave aluminum substrate which also holds the actuator control and monitoring electronics and serves as the reference surface. With the actuator electronics dissipating heat into the substrate, the thermal behavior of the aluminum reference plate becomes as important issue. This paper presents results form tow experiments designed to determine the thermal behavior of the reference plate.

Thermal characterization of the support plate for the 6.5 m MMT thin-shell, adaptive-secondary mirror

The wavefront controller for the Keck Observatory AO system consists of two separate real-time control loops: a tip-tilt control loop to remove tilt from the incoming wavefront, and a deformable mirror control loop to remove higher-order aberrations. In this paper, we describe these control loops and analyze their performance using diagnostic data acquired during the integration and testing of the AO system on the telescope. Disturbance rejection curves for the controllers are calculated from the experimental data and compared to theory. The residual wavefront errors due to control loop bandwidth are also calculated from the data, and possible improvements to the controller performance are discussed.

/pdf/initial-performance-of-the-keck-ao-wavefront-controller-10cfip3f7w.pdf

Initial performance of the Keck AO wavefront controller system

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Papers

Laboratory calibration of the W.M. Keck Observatory Adaptive Optics Facility

Compensating for pupil rotation in the W.M. Keck Observatory adaptive optics system

Thermal characterization of the support plate for the 6.5 m MMT thin-shell, adaptive-secondary mirror

Initial performance of the Keck AO wavefront controller system