Samuel C. Rogers

Adaptive optics using a liquid crystal phase modulator in conjunction with a Shack-Hartmann wave-front sensor and zonal control algorithm

Multi-segment liquid crystal spatial light modulators have received much attention recently for use as high-precision wavefront control devices for use in astronomical and non-astronomical applications. They act much like piston only segmented deformable mirrors. In this paper we investigate the use of these devices in conjunction with a Shack-Hartmann wave-front sensor. Previous investigators have considered Zernike modal control algorithms. In this paper we consider a zonal algorithm in order to take advantage of high speed matrix multiply hardware which we have in hand.

ADONIS: daylight imaging through atmospheric turbulence

The AMOS daylight optical near-infrared imaging system, acronym ADONIS, is a sensor system designed for collecting satellite images under daylight conditions and employing speckle post-processing for enhancement of the resulting images. This paper presents our solution (the ADONIS system) to the daylight observation problem by first establishing the issues related to radiometry, daylight detection, and incoherent speckle imaging. System design resolution optimization results are presented. ADONIS imaging results and conclusions based on these results also are presented.

Experimental measurements of estimator bias and the signal-to-noise ratio for deconvolution from wave-front sensing.

Deconvolution from wave-front sensing (DWFS) has been proposed as a method for achieving high-resolution images of astronomical objects from ground-based telescopes. The technique consists of the simultaneous measurement of a short-exposure focal-plane speckled image, as well as the wave front, by use of a Shack-Hartmann sensor placed at the pupil plane. In early studies it was suspected that some problems would occur in poor seeing conditions; however, it was usually assumed that the technique would work well as long as the wave-front sensor subaperture spacing was less than r(0) (L/r(0) < 1). Atmosphere-induced phase errors in the pupil of a telescope imaging system produce both phase errors and magnitude errors in the effective short-exposure optical transfer function (OTF) of the system. Recently it has been shown that the commonly used estimator for this technique produces biased estimates of the magnitude errors. The significance of this bias problem is that one cannot properly estimate or correct for the frame-to-frame fluctuations in the magnitude of the OTF but can do so only for fluctuations in the phase. An auxiliary estimate must also be used to correct for the mean value of the magnitude error. The inability to compensate for the magnitude fluctuations results in a signal-to-noise ratio (SNR) that is less favorable for the technique than was previously thought. In some situations simpler techniques, such as the Knox-Thompson and bispectrum methods, which require only speckle gram data from the focal plane of the imaging system, can produce better results. We present experimental measurements based on observations of bright stars and the Jovian moon Ganymede that confirm previous theoretical predictions.

Hybrid blind deconvolution for high-resolution satellite imaging

The utility of a blind deconvolution algorithm used in conjunction with the Knox-Thompson algorithm is demonstrated with day time observations of the MIR space station.

Atmospheric effects on long-range laser-enhanced imaging from a simulated airborne platform

We describe here a preliminary set of experiments to demonstrate the feasibility of laser illuminated imagery for remote surveillance from an airborne platform. Such an imaging sensor enhances the presently available sensor suite in that it provides high resolution day and night capability.

Atmospheric turbulence monitoring in conjunction with imager-designator operation

The performance of operational military E-O systems including imaging FLIRs, target designators, and laser rangefinders (LRF) is limited by atmospheric refractive- index turbulence. In locations subject to intense daytime heating and significant nighttime cooling, typically an arid desert-like environment, the diurnal change in Cn2 can range over three to four orders of magnitude or larger in some cases. Elevation of the path above the desert floor even at one end can significantly reduce the performance- degrading effects of atmospheric turbulence on FLIRs, designators, and LRFs. In case where operation of these systems at longer wavelengths is possible, performance limitations can, to some extent, be mitigated. This paper discusses the use of multi-wavelength scintillation measurements as a diagnostic, and LRFs. In cases where operation of these systems at longer wavelengths is possible, performance limitations can, to some extent, be mitigated. This paper discusses the use of multi-wavelength scintillation measurements as a diagnostic to infer a path- integrated value for Cn2 which can be related to the performance of various E-O systems. An experimental design utilizing IR wavelengths and several slant-paths ranging in length from 2.8 km to 10 km and elevated approximately 730 m above a desert floor is discussed. The multi-wavelength scintillometer design used is based on the 11.15 micrometers scintillometer described in a paper previously presented at an earlier conference.

System electronic design for a telescope-mounted deformable mirror system

Logicon RDA and Applied Technology Associates have supported PL/LIMI on various imaging experiments during the past years. We are currently supporting an adaptive optics experiment using a 349 actuator deformable mirror. We discuss the system requirements, the design schedule, the final hardware configuration, simulated and actual system performance and planned enhancements. Keywords: Deformable Mirror, Actuator, Electronic system

Electronic control system exploiting both the dual-frequency effect and the transient nematic effect for a fast 127-element nematic liquid crystal spatial light modulator

Nematic liquid crystals have been sued as phase retarders in adaptive optics system for more than 10 years. Liquid crystal spatial light modulators (SLMs) can be very useful both as controlled disturbances for characterizing adaptive optics systems, and as wavefront correctors. Of the available options, SLMs have the advantages of low cost, reliability, compactness, low power, ease of controllability, optical transmissivity, and simple aperture scalability. Moreover, these devices are very non-linear and normally have quite different response times in the increasing versus the decreasing phase direction. The current limitation for using these devices in atmospheric adaptive systems is their relatively slow response times, not only for small phase steps in the excitation direction, but for all steps in the relaxation direction. The response time in both directions can be significantly improved by exploiting two important properties of certain nematic liquid crystals: the Dual-Frequency Effect and the Transient Nematic Effect. This paper presents the details of custom- built electronic hardware that takes advantage of these two effects to implement (lambda) /10 phase accuracy with millisecond settling time for both increasing and decreasing phase shifts.

Adaptive optics using a liquid crystal phase modulator in conjunction with a Shack-Hartmann wavefront sensor and zonal control algorithm

Multi-segment liquid crystal spatial light modulators have received much attention recently for use as high-precision wavefront control devices for use in astronomical and non- astronomical applications. They act much like piston only segmented deformable mirrors. In this paper we investigate the use of these devices in conjunction with a Shack-Hartmann wave-front sensor. Previous investigators have considered Zernike modal control algorithms. In this paper we consider a zonal algorithm in order to take advantage of high speed matrix multiply hardware which we have in hand.

Low-order adaptive optics experiment on 3.5-m telescope at the Starfire Optics Range

It has been demonstrated both with computer simulation and field experiments that low order adaptive optics can improve the signal to noise ration in subsequent computer post- processing algorithms, especially when viewing an extended object. In this presentation we show open and closed loop images of a meteorological satellite taken with a low order adaptive optics instrument package mounted on the side of the Air Forces 3.5 meter telescope. The images are subsequently post-processed using speckle algorithms. It is shown that the boost in post-processing signal to noise ratio, provided by the low order adaptive optics, closely corresponds to computer simulation results.