John D. Gonglewski

Atmospheric structure function measurements with a Shack–Hartmann wave-front sensor

The phase structure function has been used as a convenient way to characterize aberrations introduced on optical propagation by the atmosphere. It forms the theoretical basis for the calculation of such things as the long- and short-exposure atmospheric transfer function. The structure function is difficult to measure directly and is usually assumed to follow Kolmogorov statistics. We present here a technique for direct measurement of the structure function through the use of a Shack-Hartmann wave-front sensor. Experiments confirm that the atmosphere behaves according to Kolmogorov theory most of the time. However, some instances of non-Kolmogorov behavior have been noted.

First astronomical application of postdetection turbulence compensation: images of α Aurigae, ν Ursae Majoris, and α Geminorum using self-referenced speckle holography

We describe a postdetection turbulence compensation technique for obtaining high resolution imagery through the atmosphere. We present preliminary results fromfield experiments.

Image synthesis from nonimaged laser-speckle patterns: experimental verification.

We have performed laboratory experiments that demonstrate that unspeckled images of coherently illuminated, diffuse objects can be formed from measurements of backscattered laser-speckle intensity. We show that images revealing gross target features can be successfully recovered with relatively few snapshots of the observed laser-speckle pattern.

Image synthesis from nonimaged laser-speckle patterns: comparison of theory, computer simulation, and laboratory results.

The performance of an imaging technique relying on the spatial correlation of laser-speckle intensity measurements is evaluated on the basis of theoretical analysis, computer simulation, and laboratory results. A theoretical expression for the signal-to-noise ratio of the recovered imaging targets power spectrum is used to estimate the imaging performance expected in the computer simulation and laboratory experiment. Power-spectrum estimates for an imaging target, obtained both in the laboratory and through simulation, are compared with the theoretical results and with the true spectrum of the target. Images recovered from the simulation data and the laboratory data are also compared. Our results suggest that the signal-to-noise ratio expression provides an accurate means for estimating the recoverable frequency content of a simple target.

Image synthesis from wave-front sensor measurements of a coherent diffraction field

We demonstrate that images of laser-illuminated objects can be formed from measurements of the wave-front slope (gradient) associated with the backscattered, coherent laser-speckle field. A digital wave-front recovery and image synthesis procedure is described, and the results of computer-simulation experiments are presented in which coherent images are reconstructed from digitally simulated Fourier-plane laser-speckle measurements. Images are recovered from noisy wave-front-difference data to illustrate the effect that measurement noise has on recovered image quality.

Design of a wide field of view phased array telescope

The design of the Multipurpose Multiple Telescope Testbed (MMTT) is described. The MMTT, whose construction is now nearly complete, is believed to be the first wide field of view phased array imaging telescope. The design philosophy and alignment requirements are discussed and the telescope is described. The array consists of four 20 cm telescopes and operates with visible light. The coherently combined image has a 30 arcmin field of view. A key design goal is the maintenance of lateral pupil geometry to within 1µm of the ideal. Two mirrors behind each afocal telescope combine the four beams into one and are actuated to control both tilt and piston. Together, the two beam combiner mirrors control lateral pupil geometry. A control system senses the lateral pupil geometry error by continuously measuring piston phase changes over the telescope field of view. A discrete time varying Kalman filter then processes these measurements to estimate lateral pupil geometry errors.

SCIP computer simulation and laboratory verification

Sheared Coherent Interferometric Photography (SCIP) is an active imaging technique that allows near-diffraction limited imaging of objects through turbulent media. This paper presents computer simulation and laboratory results that illustrate the viability of the technique.

System Design Of A Wavefront Sensing Package For A Wide Field Of View Optical Phased Array

This paper discusses the system design of a wavefront sampling system for a four-telescope optical phased array. This sampling system provides the input to an electromechanical feedback control system that dynamically phases the telescope array. We begin by presenting the basics of the optical phasing problem for a telescope array and then discuss design solutions that provide the necessary wavefront sensing for a feedback control system. The overall system design concentrates on simplicity despite the number of channels, while the optical design embodies a philosophy of invariance to field of view, the use of the first-order optical properties of the entrance/exit pupil relationship, and the exploitation of beam polarization for power conservation and electro-optical chopping.

Passive SWIR airglow illuminated imaging compared with NIR-visible for low-light nighttime observations

It is well known that luminance from photo-chemical reactions of hydroxyl ions in the upper atmosphere (~85 km altitude) produces a significant amount of night time radiation in the short wave infra-red (SWIR) band with wavelength between 0.9 and 1.7 μm. By examining images in an urban and a rural setting, we investigate the correlation between the appearances of passive dark of night images in the SWIR with NIR- visible. The experimental setup consists of two sensors, a NIR-visible CCD and an InGaAs array sensitive in the SWIR, both colocated on an AZ-EL mount, and both co-boresighted so that different viewing angles of the sky and terrestrial scenes are possible. By making corrections for focal length and pixel size, the visible and SWIR data can be compared. After taking several nights of data in the urban environment of Albuquerque, NM, the entire system was then re-located to a rural location on the island of Kauai in a rural setting with very low ambient light. It is shown that under most conditions the SWIR sensor produces significantly better imagery using the airglow illumination source.

Coherent image synthesis from wave-front sensor measurements of a nonimaged laser speckle field: a laboratory demonstrations

We report what are to our knowledge the first coherent images recovered in the laboratory from measurements made with a Shack-Hartmann wave-front sensor of the phase and amplitude of a laser speckle wave front. We discuss the design of our wave-front sensor, which can obtain the phase and amplitude of an optical field with a single intensity measurement, and we point out a particular type of phase jump that cannot be detected by the Shack-Hartmann sensor. We also discuss implementations of this technique that may permit near-diffraction-limited imaging through turbulent media.