Christopher C. Wilcox

Active zoom imaging for operationally responsive space

Deployment costs of large aperture systems in space or near-space are directly related to the weight of the system. In order to minimize the weight of conventional primary mirrors and simultaneously achieve an agile system that is capable of a wider field-of-view (FOV) and true optical zoom without macroscopic moving parts, we are proposing a revolutionary alternative to conventional zoom systems where moving lenses/mirrors and gimbals are replaced with lightweight carbon fiber reinforced polymer (CFRP) variable radius-of-curvature mirrors (VRMs) and MEMS deformable mirrors (DMs). CFRP and MEMS DMs can provide a variable effective focal length, generating the flexibility in system magnification that is normally accomplished with mechanical motion. By adjusting the actuation of the CFRP VRM and MEMS DM in concert, the focal lengths of these adjustable elements, and thus the magnification of the whole system, can be changed without macroscopic moving parts on a millisecond time scale. In addition, adding optical tilt and higher order aberration correction will allow us to image off-axis, providing additional flexibility. Sandia National Laboratories, the Naval Research Laboratory, Narrascape, Inc., and Composite Mirror Applications, Inc. are at the forefront of active optics research, leading the development of active systems for foveated imaging, active optical zoom, phase diversity, and actively enhanced multi-spectral imaging. Integrating active elements into an imaging system can simultaneously reduce the size and weight of the system, while increasing capability and flexibility. In this paper, we present recent progress in developing active optical (aka nonmechanical) zoom and MEMS based foveated imaging for active imaging with a focus on the operationally responsive space application.

Characterization of the lightweight telescope developed for the NPOI

A 0.4 meter lightweight telescope has been developed as a prototype for a future 1.4 meter telescope to be implemented at the Naval Prototype Optical Interferometer (NPOI). Using carbon fiber construction for all components, including optics, an order of magnitude reduction in weight is easily obtainable, with the estimated weight of the 1.4 meter telescope being less than 300 pounds. However, lightweight composite materials traditionally offer certain drawbacks, such as different material behavior and vibration characteristics from conventional materials and difficulty in obtaining optical surface quality. This paper describes the characterization of the mechanical properties of the advanced materials used in the construction of these telescopes and includes measurements of the optical figure obtained with carbon fiber construction.

Optical testbed for comparative analysis of wavefront sensors

An optical testbed has been developed for the comparative analysis of wavefront sensors based on a modified Mach Zender interferometer design. This system provides simultaneous measurements of the wavefront sensors on the same camera by using a common aberrator. The initial application for this testbed was to evaluate a Shack-Hartmann and Phase Diversity wavefront sensors referenced to a Mach-Zender interferometer. In the current configuration of the testbed, aberrations are controlled using a liquid crystal spatial light modulator, and corrected using a deformable mirror. This testbed has the added benefit of being able to train the deformable mirror against the spatial light modulator and evaluate its ability to compensate the spatial light modulator. In the paper we present results from the wavefront sensors in the optical testbed.

A lightweight adaptive telescope

Adaptive optics systems are commonly added onto conventional astronomical telescopes to improve the wavefront quality in the presence of atmospheric turbulence. Recent successes in the development of carbon fiber reinforced polymer telescopes have significantly reduced the weight of meter class telescopes making them portable, however, most adaptive optics systems continue to be constructed on large optical benches. The Navy Prototype Optical Interferometer is developing several 1.4 m portable telescope with internal wavefront correction. As part of this upgrade, a prototype 0.4 m aperture telescope has been constructed and a light weight, compact adaptive optics system is being developed. We present in this paper the design of an adaptive optics system for the lightweight telescope. The key to this system is the incorporation of a compact wavefront correction device and a novel collimation optic within the base of the telescope.

Low altitude horizontal scintillation measurements of atmospheric turbulence over the sea: Experimental results

We present the current status and developments of a horizontal beam path laser propagation experiment over the sea performed off the coast of Puerto Rico. Atmospheric turbulence effects have been measured by a Shack-Hartmann wavefront sensor with a Dalsa CCD camera and by a scintillometer from Optical Scientific, Inc* (OSI). We present preliminary scintillation measurements for an approximate period of two days from the two optical systems during the month of July 2005, also suggestions for improvement in the software, data acquisition protocol and hardware are presented.

Adaptive optics using MEMS and liquid crystal devices

In the past two decades, the use of adaptive optics has been validated in many different observatories around the world. However, the availability of new technologies like liquid crystal modulators (LCM) or micro-electro-mechanical-systems (MEMS) deformable mirrors (DM) are providing a revolution in the field. These devices are lower in cost and complexity and are opening the door to applications of adaptive optics that are beyond the astronomical use. In this paper we will present a review of our experience with both MEMS and LCM. Both theoretical and experimental results will be presented.

Ultra-light weight telescope coupled with portable AO system for laser communications applications

In this paper we present some preliminary results of an ultra-light weight telescope manufactured entirely with Carbon Fiber Reinforced Polymer (CFRP), including the optics, coupled with a light weight Adaptive Optics (AO) system. This research has many scopes, ranging from long baseline interferometry to laser communications. In this paper we will examine some of the mechanical properties of the telescope and describe the testing that the system is undergoing.

Mounting a deformable mirror onto a controllable tip/tilt platform

In most adaptive optics systems, there are two elements that control wavefront correction. These are a tip/tilt platform and a deformable mirror. The tip/tilt platform can correct the lower order aberrations like piston, tip and tilt. The deformable mirror can correct the higher order aberrations like defocus, coma, spherical, etc. Currently in this method, two conjugate planes must be created by the two elements. It is also a necessity that these two conjugate planes be identical. This requires more optics and a more complicated alignment process. In this project a deformable mirror is mounted onto a tip/tilt platform resulting in the two correction elements having the same conjugate plane, automatically. This is made possible by the use of a lightweight deformable mirror, as traditional deformable mirrors tend to be quite large and bulky. Results of this experimental project will be presented.

Performance of a flexible optical aberration generator

The Naval Research Laboratory has developed a method for simulating atmospheric turbulence and a testbed that simulates its aberrations using a liquid crystal (LC) spatial light modulator (SLM). This testbed allows the simulation of so-called atmospheric seeing conditions ranging from very poor to very good and different algorithms may be easily employed on the device for comparison. Many models for simulating turbulence often neglect temporal transitions along with different seeing conditions. Using the statistically independent set of Karhunen-Loeve polynomials in conjunction with Kolmogorov statistics in this approach provides a spatial and temporal model for simulating turbulence. An added benefit to using an LC SLM is its low cost; and multiple devices can be used to simulate multiple layers of turbulence in a laboratory environment. Current testing using multiple LC SLMs is under investigation at the Naval Research Laboratory.

Performance of a MEMS reflective wavefront sensor

An all reflective Shack Hartmann style wavefront sensor has been developed using a Sandia National Laboratory segmented Micro-Electro-Mechanical (MEM) deformable mirror. This wavefront sensor is presently being explored for use with adaptive optics systems at the Naval Prototype Optical Interferometer and other experimental adaptive systems within the Naval Research Laboratory. The 61 MEM mirror segments are constructed in a hexagonal array and each segment can be constructed with either flat or optically powered surfaces. The later allows each mirror segment to bring its subaperture of light to a focus on an imaging array, creating an array of spots similar to a Shack Hartmann. Each mirror segment has tip, tilt and piston functionality to control the position of the focused spot such that measurement of the applied voltage can be used to drive a deformable mirror. As the system is reflective and each segment is controllable, this wavefront sensor avoids the light loss associated with refractive optics and has larger dynamic range than traditional Shack Hartmann wavefront sensors. This wavefront sensor can detect large magnitude aberrations up to and beyond where the focused spots overlap, due to the ability to dither each focused spot. Previous publications reported on this novel new technique and the electrical specifications, while this paper reports on experiments and analysis of the open-loop performance, including repeatability and linearity measurements. The suitability of using the MEM deformable mirror as a high dynamic range reflective wavefront sensor will be discussed and compared to current wavefront sensors and future work will be discussed.