Robert W. Presta

Experimental comparison of a Shack–Hartmann sensor and a phase-shifting interferometer for large-optics metrology applications

We performed a direct side-by-side comparison of a Shack-Hartmann wave-front sensor and a phase-shifting interferometer for the purpose of characterizing large optics. An expansion telescope of our own design allowed us to measure the surface figure of a 400-mm-square mirror with both instruments simultaneously. The Shack-Hartmann sensor produced data that closely matched the interferometer data over spatial scales appropriate for the lenslet spacing, and much of the <20-nm rms systematic difference between the two measurements was due to diffraction artifacts that were present in the interferometer data but not in the Shack-Hartmann sensor data. The results suggest that Shack-Hartmann sensors could replace phase-shifting interferometers for many applications, with particular advantages for large-optic metrology.

Sodium-layer laser-guide-star experimental results

We describe a series of experiments to characterize the sodium-layer guide star that was formed with the high-power laser developed for the Lawrence Livermore National Laboratory Atomic Vapor Laser Isotope Separation program. An emission spot size of 3.0 m was measured, with an implied laser irradiance spot diameter of 2.0 m. The rms spot motion at the higher laser powers, with active beam-pointing control, was less than 0.5 arcsec and had little effect on the observed spot size under these conditions. We measured the resonant backscatter from the sodium layer as a function of laser power to obtain a saturation curve. With a transmitted power of 1100 W and an atmospheric transmission of 0.6, the irradiance from the guide star at the ground was 10 (photons/cm2)/ms, corresponding to a visual magnitude of 5.1. The implications for the performance of wave-front sensors with a laser guide star of this magnitude and resulting closed-loop adaptive-optics performance are discussed.

Design, layout, and early results of a feasibility experiment for sodium-layer laser-guide-star adaptive optics

We describe the design and the early results of a feasibility experiment for sodium-layer laser-guide-star adaptive optics. Copper-vapor-laser-pumped dye lasers from Lawrence Livermore National Laboratory’s Atomic Vapor Laser Isotope Separation program are used to create the guide star. The laser beam is projected upward from a beam director that is located ~5 m from a 0.5-m telescope and forms an irradiance spot ~2 m in diameter at the atmospheric-sodium layer (at an altitude of 95 km). The laser guide star is approximately fifth magnitude and is visible to the naked eye at the top of the Rayleigh-scattered laser beam. To date, we have made photometric measurements and open-loop wave-front-sensor measurements of the laser guide star. We give an overview of the experiment’s design and the laser systems, describe the experimental setup, show preliminary photometric and open-loop wave-front-sensor data on the guide star, and present predictions of closed-loop adaptive-optics performance based on these experimental data. The long-term goal of this effort is to develop laser guide stars and adaptive optics for use with large astronomical telescopes.

Group-delay diagnostic for measuring vapor column density

We describe a technique for determining Nfl by measuring the group-velocity delay of a probe laser beam propagating through a vapor. This diagnostic has wide dynamic range, is simple to implement, and can be used as a high-bandwidth vapor rate monitor. In addition, it can be used to measure column density, Nl, number density, N, oscillator strengths, f, or absorption cross sections, collisional line broadening, and vapor group-velocity delay.

Design and performance of a laser guide star system for the Keck II telescope

A laser system to generate sodium-layer guide stars has been designed, built and delivered to the Keck Observatory in Hawaii. The system uses frequency doubled YAG lasers to pump liquid dye lasers and produces 20 W of average power. The design and performance result of this laser system are presented.

Real-time wavefront correction system using a zonal deformable mirror and a Hartmann sensor

We have developed an adaptive optics system that corrects up to five waves of 2nd-order and 3M-order aberrations in a high-power laser beam to less than 1/10th wave RMS. The wavefront sensor is a Hartmann sensor with discrete lenses and position-sensitive photodiodes; the deformable mirror uses piezoelectric actuators with feedback from strain gauges bonded to the stacks. The controller hardware uses a VMIE bus. The system removes thermally induced aberrations generated in the master-oscillator-power-amplifier chains of a dye laser, as well as aberrations generated in beam combiners and vacuum isolation windows for average output powers exceeding 1 kW. The system bandwidth is 1 Hz, but higher bandwidths are easily attainable.

Adaptive optics at Lick Observatory: System architecture and operations

We will describe an adaptive optics system developed for the 1 meter Nickel and 3 meter Shane telescopes at Lick Observatory. Observing wavelengths will be in the visible for the 1 meter telescope and in the near IR on the 3 meter. The adaptive optics system design is based on a 69 actuator continuous surface deformable mirror and a Hartmann wavefront sensor equipped with an intensified CCD framing camera. The system has been tested at the Cassegrain focus of the 1 meter telescope where the subaperture size is 12.5 cm. The wavefront control calculations are performed on a four processor single board computer controlled by a Unix-based system. We will describe the optical system and give details of the wavefront control system design. We will present predictions of the system performance and initial test results.

Wavefront correction system based on an equilateral triangular arrangement of actuators

We are developing an adaptive optics system that is based on an array of actuators arranged with subapertures that are equilateral triangles. The wavefront sensor is a video Hartmann sensor that also uses an equilateral array of lenslets. The controller hardware uses a VME bus. The design minimizes the generation of reflected wavefronts higher than first order across each lenslet for large excursions of actuators from positions where the mirror is flat and, thus maximizes the precision of the slopes measured by the Hartmann sensor. The design is also immune to the waffle mode that is present in the reconstructors of adaptive optics systems where actuators are arranged in a square array.

Adaptive optics package designed for astronomical use with a laser guide star tuned to an absorption line of atomic sodium

We present the design and implementation of a very compact adaptive optics system that senses the return light from a sodium guide-star and controls a deformable mirror and a pointing mirror to compensate atmospheric perturbations in the wavefront. The deformable mirror has 19 electrostrictive actuators and triangular subapertures. The wavefront sensor is a Hartmann sensor with lenslets on triangular centers. The high-bandwidth steering mirror assembly incorporates an analog controller that samples the tilt with an avalanche photodiode quad cell. An f/25 imaging leg focuses the light into a science camera that can either obtain long-exposure images or speckle data. In laboratory tests overall Strehl ratios were improved by a factor of 3 when a mylar sheet was used as an aberrator. The crossover frequency at unity gain is 30 Hz.