Robert Q. Fugate

Experimental study of the effect of non-Kolmogorov stratospheric turbulence on star image motion

The effect of non-Kolmogorov stratospheric turbulence on star image motion is for the first time experimentally investigated with a ground-based telescope. A new approach permitting isolation of star image motion induced solely by atmospheric turbulence is employed. In this technique Polaris image wander is recorded with the telescope bolted in place to minimize uncontrolled telescope motion. High resolution temporal and spatial statistics of wave-front tilt are obtained. The dependencies of tilt variance, tilt power spectra, and tilt temporal correlation on telescope diameter are investigated for five apertures in the range 0.1-1.5 m. The experimental data show the dependence of tilt variance on telescope diameter does not follow the predictions of the Kolmogorov and von Karman models. The graph of the measured dependence has a “knee” which can be explained only by assuming a non-Kolmogorov stratospheric turbulence effect. The difference between tilt components in different axes indicates anisotropy in stratospheric turbulent inhomogeneities. The slopes of the measured tilt power spectra, approximately -1 in the low frequency range and -8/3 in the high frequency range, do not agree with theoretical predictions. The measured tilt temporal correlation scale is in the range 0.1-1.0 s, and the behavior of the correlation coefficients indicates the effect of large scale inhomogeneities not predicted by the conventional model. Uncontrolled telescope motion is manifested as a “bump” in the tilt power spectra in the range 70-90 Hz, but this makes an insignificant contribution to Polaris jitter variance.

Low-power FLC-based retromodulator communications system

On September 15, 1996, researchers from Utah State University/Space Dynamics Lab in conjunction with Phillips Lab/Starfire Optical Range and Kjome Research successfully flew and tested a retromodulator laser communication package on a high altitude balloon. This paper addresses the layout and hardware used for the communication link, as well as presenting some preliminary data collected during the 6 hour flight of the balloon. The package was a proof of concept demonstration system for a low-power laser communications systems for small, low Earth orbiting satellites. The ferroelectric liquid crystal based retromodulator design of Utah State provided test patterns for modulation rates up to 20 kilo bits per second. Data was successfully downlinked using a 1200 bps RS232 format and a simplistic receiver. The Starfire Optical Range 1.5-meter telescope located on Kirtland AFB, tracked the balloon, which reached a float altitude of 31 km and collected the modulated light reflected from the payload.

Observations of faint objects with laser beacon adaptive optics

This paper reports progress in the use of laser beacons for compensating the images of faint objects using adaptive optics. The system describe is located at the USAF Phillips Laboratorys Starfire Optical Range near Albuquerque, New Mexico on a 1.5 m telescope. The adaptive optics system uses a 241 actuator continuous facesheet deformable mirror. A copper vapor laser beacon focused at 10 km range is used to sense higher-order distortions. A natural guide star is used for sensing full aperture tilt. The limiting factor for long exposure performance of the system is tracking--the correction of full aperture tilt. The main goal of the work reported here was to determine the performance of the tracking system under conditions of weak signal but good higher order compensation (similar to conditions encountered in laser beacon adaptive optics). Single axis rms track errors of approximately 20 milliarcsec were achieved for track signal levels equivalent to stars of 13th magnitude. However, we cannot track stars fainter than 9 - 10th magnitude because the laser induces very low level, long lived phosphorescence in the optical train. This appears to be a fundamental limitation for our system as it is presently configured.

Laser beacon adaptive optics for power beaming applications

This paper discusses the laser beam control system requirements for power beaming applications. Power beaming applications include electric and thermal engine propulsion for orbit transfer, station changing, and recharging batteries. Beam control includes satellite acquisition, high accuracy tracking, higher order atmospheric compensation using adaptive optics, and precision point-ahead. Beam control may also include local laser beam clean-up with a low order adaptive optics system. This paper also presents results of tracking and higher-order correction experiments on astronomical objects. The results were obtained with a laser beacon adaptive optics system at Phillips Laboratorys Starfire Optical Range near Albuquerque, NM. At a wavelength of 0.85 micrometers , we have achieved Strehl ratios of approximately 0.50 using laser beacons and approximately 0.65 using natural stars for exposures longer than one minute on objects of approximately 8th magnitude. The resulting point spread function has a full width half maximum (FWHM) of 0.13 arcsec.

Measurements of tilt angular anisoplanatism

A new approach for the experimental study of tilt angular anisoplanatism is developed. This method uses measurements of the random motion of a moon edge image to assess wavefront tilt. This technique provides a wide, continuous range of angular separations which are not available in observations of binary stars. It is determined that the brightness of the moon is sufficient to make observations with a high resolution imaging system. Statistical properties of the tilt angular correlation and tilt averaging function are experimentally investigated. It is shown that tilt angular correlation scale increases from 40 arcsec to 118 arcsec by increasing telescope diameter, indicating that the concept of isoplanatic angle is not applicable to tilt-related phenomena.

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.

Recovery of atmospheric phase distortion from stellar images using an artificial neural network

We report recent experimental verification of an new method to determine atmospheric phase directly from focused images of starlight. An artificial neural network is used to infer the phase from two images of a star, one at the exact focus and another intentionally out of focus. We applied the network to images of Vega obtained on the 1.5 m telescope at Starfire Optical Range (SOR), Kirtland Air Force Base, Albuquerque, New Mexico. Neural network predictions agree well with phase reconstructions using a conventional Hartmann wavefront sensor. The network approach offers a simple, inexpensive way to implement adaptive optics on astronomical telescopes in the near term.

Summary of optical turbulence measurements taken at the Starfire Optical Range

Since 1987, we have measured the transverse coherence length (Frieds ro parameter) and the isoplanatic angle, (theta) o, using optical instruments at the Starfire Optical Range (SOR). Through the end of December 1994, we have accumulated 160, 653 ro measurements and 185,488 (theta) o measurements. The transverse coherence length, ro, was determined by measuring the short exposure modulation transfer function of the atmosphere using a 35.5 cm Celestron transfer observing bright stars. The isoplanatic angle was obtained using a stellar scintillation technique on bright stars with a 10 cm Meade telescope. Prior to 1992, a 20 cm Celestron telescope with an apodized aperture was used. Means and frequency distributions of ro and (theta) o have been determined for the period 1987 through December 1994, for each individual year and for each month using measurement from all years. In addition, for the period November 1993 to December 1994, we used Albuquerque National Weather Service rawinsonde wind data and a model of the Cn2 profile to estimate the Greenwood frequency fG. These estimates generally compare well with measured Greenwood frequency data obtained from the high speed wavefront sensor in the 1.5 m adaptive optics system. The summaries presented are a first attempt to characterize the optical turbulence at the SOR and may be used to plan experiments during months of statistically low atmospheric turbulence and Greenwood frequency.

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.

High-bandwidth interferometer for real-time measurement of deformable mirrors

We report on the design and fabrication of a high bandwidth interferometer suitable for real- time measurement of the figure of a deformable mirror. The design allows for measurement of mirror figure in terms of optical path differences (OPD) between the surface of the mirror and a static reference wavefront. Measurements are made on a 31 by 31 square grid. This instrument is relevant for atmospheric adaptive optics systems because it provides a method for accurately monitoring the figure of a deformable mirror during real-time compensation of atmospheric turbulence. Measured values of OPD on the mirror surface are output in digital form at approximately 10 kHz and can be used as a feedback signal in a digital control-loop for driving the deformable mirror. The system uses a common 4-bucket or 4-measurement interferometric algorithm to compute OPD. The maximum measurable OPD is +/- 7.5 waves. Tests of the completed interferometer indicate that it can routinely measure the dynamic changes in figure of an optical mirror. Preliminary tests indicate that the measurements are accurate to approximately (lambda) /25.