Lick Observatory Laser Guide Star Adaptive Optics System Commissioning

A sodium-layer laser guide star adaptive optics system has been developed at LLNL for use on the 3-meter Shane telescope at Lick Observatory located on Mount Hamilton, near San Jose, California. The system is based on a 127-actuator continuous-surface deformable mirror, a Hartmann wavefront sensor equipped with a fast-framing low-noise CCD camera, and a pulsed solid-state-pumped dye laser tuned to the atomic sodium resonance line at 589 nm.

 

In September, 1996, this system produced the first significant image improvement using high-order adaptive optics with a sodium-layer laser guide star as a reference.


 

Adaptive Optics System Description

The Lick adaptive optics system is mounted at the f/17 Cassegrain focus of the 3 m telescope. The system can feed both an optical 1024x1024 CCD camera and a near-IR 256x256 NICMOS III camera.

The deformable mirror, built at LLNL, is used to correct wavefront phase continuously over the telescope pupil. This thin mirror has 127 electro-restrictive (PMN) actuators arranged in a triangular pattern, each capable of deforming the front surface by up to +-4 microns. In the current configuration, 61 of the actuators are actively controlled. A separate flat fast-steering mirror, built by Physik Instrumente, is used to correct the overall wavefront tip-tilt.

The wavefront sensor is a Shack-Hartmann design with 37 subapertures in the clear aperture of the telescope. The subapertures have a diameter of 44 cm mapped to the telescope primary mirror, and are arranged in a triangular pattern to match the deformable mirror actuators. The wavefront sensor camera, built by Adaptive Optics Associates, uses a fast-framing low-noise 64x64 CCD, designed and built at Lincoln Laboratory. This CCD camera has a read noise of 7 electrons per pixel at a readout rate of 1200 frames per second. A separate tip-tilt sensor is necessary when the wavefront sensing is performed using the laser guide star. The tip-tilt sensor, designed and built at LLNL, uses four photon-counting avalanche photo-diodes operated as a quad cell.

The wavefront control computer is a 160 Mflop Mercury VME system with 4 i860 processors. The wavefront control system can be operated at a sample rate of up to 500 Hz and provides a control bandwidth (0 dB crossover) of up to 30 Hz including the effects of camera integration and readout time as well as the compute time and the transfer time to the deformable mirror drivers. When the wavefront sensing is performed using the laser guide star, the average tip-tilt from the wavefront sensor is utilized by a separate dedicated real-time digital processor to control a high-bandwidth beam-steering mirror in the laser, thereby stabilizing the laser guide star on the wavefront sensor at a bandwidth equal to that for the high-order wavefront control system. Additionally in this case, the fast-steering mirror in the adaptive optics system is controlled at a bandwidth of up to 120 Hz by a dedicated analog system using the signal from the avalanche photo-diode tip-tilt sensor.

 

Laser Guide Star System Description

The Lick laser guide star system is based on a tunable dye laser pumped by a set of flash-lamp-pumped frequency-doubled solid-state (Nd:YAG) lasers. The pump lasers are located in a room below the telescope dome floor and are fiber-optically coupled to the dye laser mounted on the side of the telescope. The dye laser produces light that is tuned to the atomic sodium resonance (D2) line at 589 nm. The dye laser light is projected into the sky by a refractive launch telescope with a 30 cm primary lens also mounted on the side of the telescope. In the current configuration, ~18 W of average laser power is projected into the sky with a pulse width of ~100 ns and a pulse repetition rate of ~11 kHz.

The apparent size of the guide star in the mesospheric sodium layer has been measured to be as small as 1.8 arc seconds for a 10 second exposure, and typically has a size predominantly determined by seeing on the up- and down-link paths. In both September and November 1996, the return signal from the laser guide star was measured to be comparable to that for a natural star with a magnitude of 7.0 in the V band. This corresponds to a photon flux above the atmosphere of ~1.3 photon per sqaure cm per ms. This return signal is a factor of ~2 higher than that measured at LLNL in 1992, for a comparable laser power level, and a factor of ~4 higher than previous measurements made at Lick. Some change in return flux is expected due to the known seasonal variation in the column density of the mesospheric sodium layer.

 

Laser Guide Star Adaptive Optics Test Results

Images at a wavelength of 2.2 microns from a field star, SAO 56102 (V=9.1), with and without correction by the laser guide star adaptive optics system were recorded on October 1, 1996. In this test both the laser guide star and the field star were on-axis. The light from the field star was attenuated on the wavefront sensor using a narrow-band filter centered at the wavelength of the laser guide star, 589 nm. The field star was also used as the tip-tilt guide star to stabilize the corrected image during the 80 s exposure.

The peak intensity of the corrected image was increased by a factor of 3.3 relative to the uncorrected image, and the image FWHM was decreased by a factor of 2.4, from 0.75 arc seconds to 0.31 arc seconds. Similar image improvement using the laser guide star adaptive optics system was obtained for several different stars on September 30 and October 1, 1996, as well as on November 26 and 27, 1996, under seeing conditions that were worse by a factor of 2-3, including surface winds of 30-40 mph.