James M. Spinhirne

Starfire Optical Range 3.5-m telescope adaptive optical system

A 941 channel, 1500 Hertz frame rate adaptive optical (AO) system has been installed and tested in the coude path of the 3.5m telescope at the USAF Research Laboratory Starfire Optical Range. This paper describes the design and measured performance of the principal components comprising this system and present sample results from the first closed-loop test of the system on stars and an artificial source simulator.

Progress toward a 50-watt facility-class sodium guide star pump laser

We report on the development of a 50-W, continuous-wave, sodium wavelength guidestar excitation source for installation on the azimuth gimbal structure of the 3.5-m telescope at the Starfire Optical Range. The laser is an all solid-state design employing two diode-pumped Nd:YAG sources operating at 1064 and 1319 nm that are combined to generate 589-nm radiation using a lithium triborate non-linear crystal. Key features of the system include single-frequency, injection-locked high-power oscillators, a doubly resonant sum frequency generator cavity, a short-term 10 kHz wide 589 nm spectrum, excellent beam quality and power stability, and turn-key operation using computer control and diagnostics. The laser beam is projected from the side of the 3.5-m telescope. A novel elevation beam dither approach is employed to determine range to the centroid of the guidestar formed in the column of mesospheric sodium and maintain focus of the wave front sensor.

Performance measurements of generation III wavefront sensors at the Starfire Optical Range

The adaptive optics system for the 1.5-m telescope at the Starfire Optical Range, Kirtland AFB, New Mexico has recently been upgraded. Two of the key components in the new system are improved Generation III Shack-Hartmann Wavefront Sensors (WFSs) built by Adaptive Optics Associates (AOA). The performance of the new WFSs has been measured. Measurements indicate a factor of two improvement in noise performance and less inter- subaperture pixel crosstalk resulting in improved closed loop stability. System design and performance measurements are presented.

Sky tests of a laser-pumped sodium guidestar with and without beam compensation

Three sets of sky tests have been conducted at the Starfire Optical Range with a continuous-wave, single-frequency, 20-W laser in preparation for a 50-W facility-class laser. Brightness measurements were made of the sodium guidestar produced with and without adaptive optics (AO) correction to the outgoing laser beam when it was either linearly or circularly polarized. Correcting for the transmission of our V filter at the sodium wavelength, a circularly polarized laser beam of 12 W out the telescope produced a guidestar of V=7.1 (1015 ph/s/cm2 at the top of the telescope). In general, a circularly polarized beam produces a guidestar between 75 and 100% brighter than a linearly polarized beam, indicating a significant degree of optical pumping of the sodium D2-line magnetic sublevels. However, guidestars produced with beams launched with tip-tilt correction only were 11% brighter than with beams launched with full AO correction. From deconvolved images of the guidestar taken with the 3.5-m telescope, the smallest spot, produced from a beam with 8.5 W of power out the telescope, circular polarization, and launched closed loop, had a Gaussian FWHM of 0.85 arcsec, or 38 cm at an altitude of 92 km. This corresponds to a peak Gaussian intensity of 3.8 mW/cm2.

Adaptive optics using the 3.5-m Starfire Optical Range telescope

The 3.5 meter telescope at the U. S. Air Force’s Starfire Optical Range in Albuquerque, New Mexico, is being equipped with an adaptive optics system for compensation of atmospheric phase disturbances. An optical relay using toroidal mirrors interfaces the telescope to the steering mirror(s), the 941 actuator deformable mirror, and the optical sensors. The methods developed for optimizing this toroidal mirror design are presented. The all reflective design accommodates a wide spectral band and provides near diffraction limited performance over the full one milliradian field of view and for object distances from 12.5 kilometers to infinity. A spectrally flexible and optically efficient design for interfacing the wavefront sensor and science camera(s) to the optical train is used. Alignment techniques have been developed for accurate alignment of the toroidal mirrors and for registration of the deformable mirror and wavefront sensor of the adaptive optics system.

First observations with the Starfire Optical Range 3.5-meter telescope

This paper summarizes the design and initial operation of the Starfire Optical Range 3.5-meter telescope. This facility is the centerpiece of the U.S. Air Forces strategic optical research program for high resolution imaging and laser beam propagation. Areas of research include high resolution imaging of low earth orbit satellites, laser power beaming to satellites, and deep space laser communications. The telescope and mount form the worlds largest optical telescope capable of tracking low earth orbit satellites. A major emphasis in the research programs at the SOR is the development of adaptive optics, especially laser beacon adaptive optics, for large aperture telescopes.

Active tracking with moderate power lasers

Progress on active tracking at the Starfire Optical Range (SOR) has been significant in the years 2003-2004. We have obtained laser returns from a number of retro-reflector and also unaugmented satellite objects, and compared the signal returns to theories presented in previous SPIE papers (ref. 1-3). These results have concentrated on very low-power, sinusoidally-modulated lasers and a large-aperture, phase-sensitive detection receiver to discriminate the return signal from background and noise. This year, we have installed and used a much higher average power, high-repetition-rate pulsed laser in order to increase the signal-to-noise ratio. Results from these laser engagements will be presented along with simulation and theoretical comparisons. Techniques for diagnosing the laser uplink and the receiver systems will be discussed.

Laser beaming demonstrations to high-orbit satellites

A team of Phillips Laboratory, COMSAT Laboratories, and Sandia National Laboratories plans to demonstrate state-of-the-art laser-beaming demonstrations to high-orbit satellites. The demonstrations will utilize the 1.5-m diameter telescope with adaptive optics at the AFPL Starfire Optical Range (SOR) and a ruby laser provided by the Air Force and Sandia (1 - 50 kW and 6 ms at 694.3 nm). The first targets will be corner-cube retro-reflectors left on the moon by the Apollo 11, 14, and 15 landings. We attempt to use adaptive optics for atmospheric compensation to demonstrate accurate and reliable beam projection with a series of shots over a span of time and shot angle. We utilize the return signal from the retro- reflectors to help determine the beam diameter on the moon and the variations in pointing accuracy caused by atmospheric tilt. This is especially challenging because the retro-reflectors need to be in the lunar shadow to allow detection over background light. If the results from this experiment are encouraging, we will at a later date direct the beam at a COMSAT satellite in geosynchronous orbit as it goes into the shadow of the earth. We utilize an onboard monitor to measure the current generated in the solar panels on the satellite while the beam is present. A threshold irradiance of about 4 W/m2 on orbit is needed for this demonstration.

Current laser guide-star adaptive optics systems and concepts for the future

Laser guide star (LGS) adaptive optics systems can dramatically improve the resolution of ground-based astronomical telescopes but introduce a variety of novel optical engineering requirements. We describe how these requirements have been addressed in the Starfire Optical Range (SOR) Gen II adaptive optics system and review sample experimental results illustrating the degree of atmospheric turbulence compensation achieved with natural and laser guide stars. Although adequate for a 1.5 m aperture diameter telescope, the level of compensation possible with a single low altitude LGS is limited by anisoplanatism and will not be adequate for larger aperture telescopes operating at visible wavelengths. We evaluate these limitations numerically for a sample problem involving a four meter aperture diameter telescope and estimate the performance improvements possible through the use of mesospheric sodium guide stars, multiple guide stars, and multiple deformable mirrors. Implementing these advanced concepts introduces new challenges in the areas of laser guide star generation, wavefront sensing, and beam train optical design.

Laser beaming demonstrations at the Starfire Optical Range

The ability to acquire, track, and accurately direct a laser beam to a satellite is crucial for power-beaming and laser-communications. To assess the state of the art in this area, a team consisting of Air Force Phillips Laboratory, Sandia National Laboratories, and COMSAT Corporation personnel performed some laser beaming demonstrations to various satellites. A ruby laser and a frequency-doubled YAG laser were used with the Phillips Lab Starfire Optical Range (SOR) beam director for this activity. The ruby laser projected 20 J in 6 ms out the telescope with a beam divergence that increased from 1.4 to 4 times the diffraction limit during that time. The doubled YAG projected 0.09 J in 10 ns at 20 Hz. The SOR team demonstrated the ability to move rapidly to a satellite, center it in the telescope, then lock onto it with the tracker, and establish illumination. Several low-earth-orbit satellites with corner- cube retro-reflectors were illuminated at ranges from 1000 to 6000 km with a beam divergence estimated to be about 20 (mu) radians. The return signal from the ruby laser was collected in a 15-cm telescope, detected by a photomultiplier tube, and recorded at 400 kHz. Rapid variations in intensity (as short as 15 microsecond(s) ) were noted, which may be due to speckles caused by phase interference from light reflected from different retro-reflectors on the satellite. The return light from the YAG was collected by a 35-cm telescope and detected by an intensified CCD camera. The satellite brightened by about a factor of 30 in the sunlight when the laser was turned on, and dimmed back to normal when the 50-(mu) radian point- ahead was turned off. The satellite was illuminated at 1 Hz as it entered the earths shadow and followed for about 10 seconds in the shadow. In another demonstration, four neighboring GEO satellites were located and centered in succession with a 3.5-m telescope at a rate of about 16 seconds per satellite.