Edward Louis Cuellar

Experimental study of spatial structure of turbulence at Maui Space Surveillance Site (MSSS)

We investigated the spatial structure of atmospheric turbulence at Maui Space Surveillance Site (MSSS) using a 3.6 m telescope and a spatial filtering receiver. This receiver simultaneously records four star images on one camera frame. The star images are formed through pupil masks representing aperture diameters of 0.1 m, 0.5m, 1.5 m, and 3.6 m. We determined the camera orientation for each data set by moving the telescope at a given angle in azimuth and elevation. We calculated the horizontal and vertical components of the image centroid and evaluated the statistics of the horizontal and vertical wavefront tilt as a function of the aperture diameter and seeing conditions. We found several evidences of anisotropy of turbulence at MSSS. On four nights we observed that the variance of on-axis horizontal tilt exceeded the variance of the vertical tilt by a factor of 1.3-3.3. We believe that this is due to anisotropy of large-scale turbulence, where the horizontal scale of the turbulent inhomogeneities exceeds their vertical scale. The estimates of the horizontal and vertical turbulence outer scale confirmed this conclusion. In addition, in several data sets the horizontal image spot diameter of the long-exposure star image exceeded the vertical image spot diameter. We also found that large apertures are more likely to have higher anisotropy coefficient values than small apertures. This is because the contribution of small-scale isotropic turbulence to the image centroid reduces with increasing telescope diameter. In the case of isotropic turbulence, the power spectral densities (PSDs) of wavefront tilt are consistent with theoretical models. The telescope vibration modes were observed at 20 Hz. In the case of anisotropic turbulence, the PSDs of the horizontal tilt component have lower slope in the high frequency range, and difference between PSDs for large and small apertures is reduced. The anisotropy of turbulence and atmospheric tilt may affect the design and performance analysis of both active and passive optical systems.

Recent results for visible Rayleigh guide-star atmospheric experiments

We report on the operation and performance of a complete integrated 1 m adaptive optics systems for compensation of atmospheric distortion of optical wavefronts. Both visible artificial laser guide stars (doubled Nd:YAG laser with wavelength of 0.532 micrometers ) and natural stars can be used as sources for reference wavefronts. A polarization shearing interferometer which uses a narrow optical bandwidth and has 500 subapertures is employed to sense wavefront distortion. These measurements are used to compute a conjugate wavefront to the distorted input light. The computed conjugate is then imprinted on a deformable mirror which consists of 500 individual square mirror segments. The effectiveness of the compensation is determined from a measured PSF of the system. Both indoor benchtop and atmospheric experiments are under way to test the performance of the integrated system. The results of these tests so far are very promising, yielding short-exposure images at 0.532 microns which contain discernible energy at the diffraction limit of 0.1 arcsec.

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.

Real-time wavefront reconstruction for a 512 subaperture adaptive optical system

This paper describes a system capable of real-time wavefront reconstruction for a 512 subaperture shearing interferometer. The system was designed to interface with a 1536 channel (512 segment) deformable mirror for atmospheric compensation using an artificial beacon. The phase gradients were measured using a shearing interferometer operating at two distinct shear lengths with quantum limited performance at 200 photons per subaperture. A 128 node parallel processor performed a sparse matrix multiply to reconstruct the phasefront in real time. The matrix truncation technique used allowed 90% of the elements to be removed with only minor penalty in wavefront accuracy.

High-bandwidth long-stroke segmented mirror for atmospheric compensation

Segmented Adaptive Optic Mirrors have been developed, fabricated, and demonstrated in real time atmospheric compensation systems. Until recently, most Segmented Adaptive Optic Mirrors have been designed for single wavelength applications and have not required more than 1.5 (mu) of surface motion since absolute phasing of the surface is not required for very narrow bandwidth compensation. Requirements for astronomical and imaging systems have required the design and fabrication of long stroke (6 - 10 (mu) ) segmented mirrors capable of absolute phasing of the segments, optical response from 0.4 to 3.5 (mu) and bandwidths above 2.5 KHz.

Adaptive optics system for laser power beam forming

We report on the design and operation of an integrated 1 meter adaptive optics system for compensation of a visible wavelength laser for satellite beamforming applications. A visible artificial laser guide star (frequency-doubled Nd:YAG laser with wavelength of 0.532 micrometers ) is used as the source for the reference wavefront. A shearing interferometer which uses a narrow optical bandwidth and has 500 subapertures is employed to sense wavefront distortion. These measurements are used to compute a conjugate wavefront to the distorted input light. The computed conjugate is then imprinted on a deformable mirror which consists of 500 square mirror segments. The deformable mirror is integrated with a 1 m Cassegrain telescope. The tracking system is designed to track and illuminate low Earth orbit (LEO) satellites. Computer control of both the adaptive optics and tracking systems are done via two terminals, and the entire adaptive optics/tracking system can be run by only two operators. We have used this system for both compensated imaging and compensated illumination applications. In this paper, we will present an overview of the system architecture and discuss computer control of the adaptive optics and tracking systems.

Point-ahead-compensated illumination demonstration using BMDO/IST adaptive optics system

Using a large adaptive optical system designed for the correction of a visible wavelength laser, we report the results for the first known compensated laser illumination demonstration of an uncooperative low Earth orbit (LEO) satellite using an active point ahead mirror. The results of these tests using a low-power laser beacon at D/ro equals 25 show improvement of over a factor of 5 in peak energy at the diffraction limited resolution of 0.1 arc sec for stellar images. The amount of light reflected from a large, diffuse LEO satellite was improved by a factor of approximately 2 using the adaptive optical system.

Point-ahead-compensated illumination tests using the 500-channel Innovative Science and Technology Experimental Facility adaptive optics system

In a program with the Office of Naval Research and the Innovative Science and Technology directorate of BMDO, we have designed, built and tested a large adaptive optical system for the correction of a visible wavelength laser beam. A visible artificial laser guide star (frequency-doubled Nd:YAG laser with wavelength of 0.532 micrometers ) is used as the source for the reference wavefront. A shearing interferometer which uses a narrow optical bandwidth and has 500 subapertures is employed to sense wavefront distortion. The sensor is used to control a 500-segment deformable mirror which is integrated with a 1 m telescope. In this paper, we will present results for the first known compensated laser illumination tests of an uncooperative low Earth orbit (LEO) satellite using an active point ahead mirror. The results of these tests using a first generation low-power laser beacon at D/ro equals 25 show improvement of over a factor of 5 in peak energy at the diffraction limited resolution of 0.1 arc sec for stellar images. The amount of light reflected from a large, diffuse LEO satellite was improved by a factor of approximately 2 using the adaptive optical system. In the Spring of 1994, a custom, high powered illuminator laser will be implemented to facilitate novel active tracking and imaging applications.

Performance tests of a 1500 degree-of-freedom adaptive optics system for atmospheric compensation

Results from a benchtop experiment to demonstrate phase compensation using a 512 segment, 1500 degree-of-freedom adaptive optic system are presented. Atmospheric phase distortion is simulated by a static Kolmogorov spectrum aberration plate with r0 equal to the subaperture size. The phase gradients are measured using a Poisson-limited, self-referenced shearing interferometer which operated at two distinct shear lengths. A parallel processor is then employed utilizing a sparse matrix multiply to reconstruct the phase front in realtime. The performance of the compensation was determined by measuring the normalized half lambda/D intensity ratio in the Fourier transform plane. Corrections to a Strehl ratio of 0.55 were performed, consistent with the measured sensitivity of the system.