J. Elon Graves

Seeing at Mauna Kea: a joint UH-UN-NOAO-CFHT study

During two short campaigns intensive coordinated measurements have been performed to determine the various contributions to image degradation on Mauna Kea. Some of the results already obtained are presented here.

First light for Hokupa'a 36 on Gemini North

The University of Hawaii adaptive optics program has recently moved its 36 actuators system, named Hokupaa 36, to the Gemini North Telescope. First light for Hokupaa 36 was in time for the dedication of this telescope during June 1999 and most of the images presented were taken with this adaptive optics system. This paper will cover the modifications to the CFHT, Hokupaa 36 system that were necessary to accommodate the larger 8 meter aperture of the Gemini Telescope. Performance at the telescope has now been measured and compares favorably with that predicted.

Gemini near-infrared imager

We discuss the main design features of the Gemini Near-IR Imager (NIRI) and its scientific capabilities. NIRI is designed to fully exploit the excellent image quality and low telescope emissivity expected from the Gemini telescope on Mauna Kea. It offers a range of pixel scales matched to different scientific objectives and has spectroscopic as well as polarimetric capabilities. One of its main design features is the use of a near-IR 2 X 2 Shack-Hartmann wavefront sensor for tip-tilt and focus control.

University of Hawaii adaptive optics system: III. Wavefront curvature sensor

The structure of a wavefront sensor suitable for adaptive optics is described. The sensor scans rapidly between extra focus images with a vibrating membrane mirror acting as a variable curvature active optical element. The membrane mirror and driver are described. The optical system is coupled to an array detector optimized to match the response of a bimorph mirror and directly produces wavefront error signals. Results are presented and compared with other techniques for curvature sensing. Tip-tilt corrections achieved using the wavefront sensor are discussed.

University of Hawaii adaptive optics system: I. General approach

The adaptive optics system being developed to sharpen images produced by telescopes at Mauna Kea is discussed. An approach based on new components developed and optimized for astronomical applications is described. The approach is limited to low-order wavefront compensation and is used for image stabilization. Avalanche photodiodes were used as sensors and reference stars were employed for sensing wavefront errors in a novel sensing technique based on wavefront curvature measurements. The instrument is described and expected performance is discussed.

Design and performance of an 85 actuator curvature system

The UH 36 element curvature AO system, Hokupaa-36, was recently moved to the Gemini 8m telescope, where it was used with great success obtaining images for the telescope dedication. Since the 36 actuator system was optimized for performance on a 4 m (CFHT) telescope it does not provide full near IR wavelength converge on the Gemini 8m telescope. In order to address this issue we are planning to upgrade the system to 85 actuators. Given the slightly better seeing expected at the Gemini telescope, the move to 85 actuators will give Strehl ratios commensurate to those obtained with 36 actuators on the CFHT. The limiting magnitude will scale with the telescope aperture giving considerably better sky coverage than at the CFHT. Curvature AO systems can scale considerably beyond 85 actuators, at this point technology presents the most important limitations to scaling.

Curvature-based laser guide star adaptive optics system for Gemini South

The Gemini Observatory and University of Hawaii are planning to install an 85-element curvature adaptive optics system with a laser guide star system on its Cerro Pachon telescope in 2001. This paper discusses the motivation, issues on implementing a laser guide star with a curvature-based system, the implementation of a laser guide star based on a commercially available 2W ring-dye laser, and the expected performance of the system. Detailed simulations show very promising results for system performance down to natural guide star magnitudes of 19 - 20th magnitude. The performance cross- over point between NGS and LGS is between 13 - 16th magnitude depending on the performance parameter of interest (e.g. Strehl, energy through a slit, etc.).

Seeing monitor based on wavefront curvature sensing

A new wavefront sensing technique called curvature sensing is described. It maps the wavefront total curvature rather than its slope and has been applied to an experimental seeing monitor which detects turbulence induced fast focus fluctuations. Some of the advantages this monitor presents, as compared to DIMMs, are: (1) sensitivity is increased by the use of a circular pupil, (2) the cost is lowered by the use of a photomultiplier, (3) the loss of signal is prevented by the systems fast run, (4) the system runs continuously, and (5) the noise bias is continuously measured and subtracted out.

Curvature-based Adaptive Optics for the NASA-IRTF

The IRTF is a 3.0 meter, f/38, infrared optimized, cassegrain telescope operated under contract from NASA with the primary mission of providing ground-based support for NASAs planetary missions. We are currently in the design and construction phase of a 36 element, curvature-based, natural guide star, adaptive optics facility installation for the IRTF. System architecture will be modeled on the highly successful AO systems developed at the University of Hawaii. The system should achieve an AO efficiency, q >= 0.4. The Strehl ratio is expected to exceed 0.8 in the K band. We estimate a limiting guide star magnitude for full correction of mR equals 14.4.

Adaptive secondary for the 2.1-m telescope at SPM Observatory

Atmospheric turbulence distorts the wavefront of the incoming light from an astronomical object and so limits the ability of a telescope to form a perfect image. The AO systems for astronomy had come the most powerful tool for infrared observation in the near thermal domain. A conventional AO system requires quite a few reflections that are needed to transfer and correct an image. A typical system would have a collimator, deformable mirror and a camera at the bare minimum. For the thermal region the gains are substantial where one can eliminate extra optical surfaces and their associated thermal background, that occurs when you put the deformable mirror at the secondary. We study the possibility of development an adaptive secondary with the techniques of a Current Bimorph mirror with the necessaries number of actuators for control the edge slope. Also we simulate the performance of a 19 channels curvature adaptive optics system in order to demonstrate the gain achievable with an adaptive secondary. The adaptive secondary for the 2.1 m Telescope at SPM Observatory is designed for a f/50 beam, 100 mm in diameter with 19 actuators necessaries to control the edge slope and curvature.