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William J. Broad’s April 27, 1999 New York Times headline read “Private Spy in Space To Rival Military’s.” He then goes on to describe the upcoming launch of an imaging satellite built by Lockhead Martin for Space Imaging. This heralds a new era for electro- optical systems in space as the emphasis migrates from governmental to commercial applications. Though a few academic and industrial remote sensing satellites have been launched since the seventies, their numbers and performance has been limited. Several emerging technologies such as efficient laser sources, robust optical networking, affordable adaptive optics and lightweight optical elements are starting to reverse this trend. While passive imaging and remote sensing have been the prime emphasis in the past, the use of lasers for active global wind sensing and optical free space communications has become a hot area. The key attribute that will entice commercial electro-optical satellite systems is to decrease weight to a point that launch costs are affordable without compromising performance. I.e. more bang-for- the-buck. Several groups are pursuing a variety of techniques to reduce the areal density of primary optical elements from the current 135 kg/m2 to at least 15 kg/m2. Some are even trying to reach 1 kg/m2. The following is a description of the activities of four of these groups.
The University of Arizona is pursuing a more “conventional” approach to lightweighting mirrors1. An ultra-thin glass meniscus attached to a supporting carbon composite base structure. The supports are piezoelectric adjusters and are arranged in a way that allows the highly flexible surface to be adjusted to the desired profile. In effect a large deformable mirror. The meniscus is formed by carefully grinding and polishing a plate of glass to a thickness of 2 mm. The goal is to produce a two meter mirror segment that has an areal density of 15 kg/m2, including the meniscus, actuators, support base and control wires. If selected for use in NASA’s Next Generation Space Telescope, several of these segments will be phased and adjusted to create the eight-meter primary mirror. In NASA’s relatively benign mission, from a mechanical disturbance viewpoint, of seeking the origins of the universe, this adjustment may be made over several hours using the primary mirror actuators only. However, the demands of a low earth orbit electro-optical missions may require the use of additional jitter and figure control optics, thus making alternative lightweighting methods attractive.
One such technique is to remove all of the supporting and control structure and use the meniscus mirror surface only. Researchers at JPL are doing just that2. They are producing thin, preformed doubly-curved, 500m shells that are flexible enough to be rolled for launch, yet have sufficient rigidity to recover and maintain their original shape when unfurled. Candidate materials include composites and nanolaminate structures. The intent is to provide NASA with cost effective telescopes for future missions. In one mission, NASA plans to orbit around Mars and obtain high-resolution (centimeter) images of potential landing sites. Under the NASA Origins program, the Terrestrial Planet Finder, a phase array of 8-m telescopes will be launched to detect planets outside of the solar system. This type of mirror approaches an areal density of 1 kg/m2. Even this amazing performance may be improved upon if one is willing to venture into the realm of gossamer optics.
The ultimate in primary weight and thinness are sheets of cellophane like membranes that are either deployed as segments or inflated. Using aluminum coated 125-micron thick polyamide membrane, the Air Force Research Laboratory3 has constructed a 28 cm clear aperture mirror that deviates from a parabolic surface by as little as 5.5 waves, primarily spherical aberration, in the visible when the appropriate radial stress and strain controls are applied at the mirror edge. To date, films with uniform thickness have been used, however, the use of materials with a radially tapered thickness are under consideration that may bring the final surface profile within a wave of the desired surface profile. Another technique is to use a ‘plunger’ to move the central portion of the mirror. Preliminary analysis has shown that the plunger approach can reduce the residual spherical aberration by a factor of eight. Due to the inherent lack of rigidity, this family of optics requires the use of additional jitter and figure control optics to bring the overall optical system to diffraction limited performance. One novel technique is to use electrostatic control of bimorph membranes.
Bimorphs are constructed by bonding and insulating two layers of piezoelectric membrane material oriented so that the polarizations are opposing. When an electric field is applied across the resulting structure the membrane will bend in proportion to electric field strength. To provide shape control, the field strength needs to vary over the entire membrane surface. Applying individual electrode areas can do this. A large mirror will require 1,000s or 10,000s of electrodes, along with the associated control electronics. The University of Kentucky4 has another approach: Apply a single reflective electrode to the mirror surface and ‘paint’ the back with a scanning electron beam. On 10-cm long strips of bimorph membrane samples, tip deflections of up to .75 cm have been demonstrated. Through the use of conductive and/or dielectric layers the rate of charge removal from the membrane can be varied. This allow the mirror surface to ‘remember’ its position while the electron beam is rastering across the entire surface providing localized adjustment.
These approaches to dramatic lightweighting of optical systems all have one similar aspect: the need for real-time adaptive control. The bulkiness and limited performance of current deformable mirror technology opens the way for innovative control systems such as MEMS, real-time holography and nonlinear optical phase conjugation. On the horizon are optical components that are integrated sensors, processors and correctors; i.e. intelligent optical components. The research described above and other associated subjects will be highlighted during the 2000 IEEE/LEOS’s Summer Topical Meeting.
1. Many thanks to Roger Angel for the helpful discussions and to Chris Stetson for providing the artwork. See http://athene.as.arizona.edu:8000/caao/ for more information
2. Many thanks to James B. Breckinridge for providing the information.
3. J.M. Wilkes, C.H. Jenkins, D.K. Marker, R. A. Carrera, D.C. Duneman and J.R. Rotge, “Concave membrane mirrros: from ashperic to near- parabolic,” To be published, Proceedings of the SPIE International Symposium on Optical Sciences, Engineering and Instrumentation, Denver, CO, vol 3760 (July 19-23 1999)
4. J.W. Martin, J.A. Main and G.C. Nelson, “Shape Control of Deployable Membrane Mirrors,” Proceeding of the International Mechanical Engineering Congress and Exposition, Anaheim, CA, Adaptive Structures and Material Systems, AD vol 57/MD vol 83, pp 217-223 (November 15-20 1998).
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