James R. Rotge

Optical evaluation of membrane mirrors with curvature

Thin membranes with curvature are investigated as mirror substrates for use in large optical telescopes. These films are mounted on an optically flat circular ring and stretched over a smaller optically flat circular ring where pressure or vacuum is applied to create the doubly curved surface as shown in figure 1. The films may vary in thickness from 20 to 200 microns. This particular experiment examines an aluminum coated 125 micron thick homogeneous, planar, isotropic membrane with a clear aperture of 28 centimeters. The nature of a flexible membrane implies that the surface curvature will result in an assorted array of gross surface figure issues associated with deterministic shape limits, probabilistic imperfections, nonlinear constitutive effects, and long-time- dependent effects. This report will focus on the empirical deterministic shape limits of a doubly curved membrane. Theoretical work on thin films inflated or evacuated into a doubly curved surface has a long history, and remains an active area of research. A number of articles [1,2,3,4,7] include summaries of this history, and offer insight on the deterministic membrane shapes.

Progress toward large-aperture membrane mirrors

The Air Force Research Laboratory (AFRL) is exploring the feasibility of large-aperture, deployable, space-based membrane telescopes operating in the visible and/or near- infrared spectral regions. One of the near-term goals of this work is to develop an understanding of available and achievable membrane materials, specifically concentrating on practical techniques to form large aperture membranes with the necessary surface quality and economy. When this research began a little more than three years ago, the conceptual design was based upon a totally inflatable structure. An inflatable structure has been used for space solar power collection and radio frequency antennas. This totally inflatable lenticular design is simple and relatively easy to demonstrate, but maintaining inflation during an extended lifetime in near-earth orbit may not be feasible. Recently, a new concept for a membrane telescope has emerged which does not depend on sustained inflation during operation. Thin membranes on the order of 10 to 100 micrometer thick will be packaged and deployed, maintaining their surface figure by means other than inflation. Given the fact that the sub- wavelength level surface tolerances required of imaging telescopes will probably not be practical with a membrane- based telescope, such systems will probably rely on real-time holography or some other wavefront correction or compensation technique. We will discuss the primary experimental work ongoing in the AFRL Membrane Mirror Laboratory, and in doing so, some of the issues relevant to demonstrating a practical, large-aperture membrane mirror system.

Minimum strain requirements for optical membranes

Thin membrane inherently require a certain minimum amount of strain to adequately perform as optical elements. This minimum strain can be established by simultaneously considering the effects of strain on the reflective surface, film thickness variations, and the corrective range of the adaptive optics (AO) scheme. To show how strain and the optimal optical surface are related, 75 and 125-micron thick polyimide films were examined under various strain conditions. Thickness variations were also mapped and correlated. The limits of the AO correction scheme set the films surface topography requirement. Our results will help to partially define an optical quality membrane, which is an important initial step toward the manufacturing of such a film.

Polymer material and casting process development for reduced manufacturing cost of spaceborne optics

There is a significant amount of research devoted to developing materials and processes for spaceborne mirrors. Carbon fiber mirrors and advanced ceramic mirrors such as SiC are being developed. These materials provide excellent stiffness to weight ratios and thermal stability. The principal problem with using these lightweight materials for mirrors is the difficulty of polishing the surface to achieve the required optical quality finish. Carbon fiber mirrors also suffer from fiber print through and ceramic mirrors are difficult and costly to polish due to the material hardness and porosity. SRS has been developing processes for depositing a very thin, optical-quality membrane layer of space-qualified polymer onto the surface of a mirror still in a rough-polished state to eliminate the need for expensive and time consuming final surface finishing of lightweight mirrors. By flow casting a polymer onto the surface, remaining peaks and valleys are filled in resulting in an extremely smooth surface. Initial research has shown that the membrane mirror surface can have a significantly better surface finish than the casting substrate, thus eliminating the need for costly final polishing.

Concave membrane mirrors from aspheric to near-parabolic

The surface of an initially planar membrane, which is subsequently subjected to a pressure difference, can be manipulated into a variety of shapes. This report discusses two methods by which optically desirable deterministic shapes might be achieved. The first involves pre-straining of the membrane, a technique which has already been demonstrated to reduce the spherical aberration in such a mirror. However, near-parabolic shapes at low f-numbers appear not to be achievable with this method, i.e., using pressure differences and pre-strain alone. The second technique is a somewhat novel one involving the use of a plunger to translate the central region of the membrane along the optical axis. Preliminary results suggest that attainment of a near-parabolic shape over a substantial area of the membrane may indeed be possible with this method. The experiments described here use an aluminum coated 125 micron thick polyimide membrane with a clear aperture of 11 inches.

Large optically flat membrane mirrors

The feasibility of forming very thin (approximately 100 um), flexible membranes into low-cost, low-mass, large diameter optical elements is being explored. While spherical or parabolic shapes are the ultimate goal for imaging and other light-gathering applications, there are potential applications for large, planar surfaces. Also, knowledge gained while working with planar membranes is being applied to concave structures. Recent efforts have concentrated on measuring and understanding the behavior of currently available materials. This paper discusses experimental results, and describes measurement techniques and membrane materials used. Highlighted are our most recent results on a 11-inch diameter membrane mirror which we measured to be flat to approximately 0.1 um rms.

Large lightweight optical quality windows and filters

The Air Force Research Laboratory, Directed Energy Directorate, together with SRS Technologies Inc., Huntsville, AL, and Surface Optics Corporation, San Diego, CA, have developed meter-class optical quality membranes with dielectric coatings and custom spectral filtering. The windows range in thickness from 5 to 20 µm and can operate in the visible and the near-infrared. To date the largest membrane manufactured is slightly less than one meter in diameter and its optical thickness variation is on the order of 35 nanometers rms. Surface roughness, optical density, and other optical data will be presented. The intent of this article is to expose this technology to optical designers with the expectation that significant design opportunities for observatories, telescopes, and experiments will result.

Deployable near-net-shaped membrane optics

The Air Force Research Laboratory is developing a large space-based optical membrane telescope. When this research began a little more than three years ago, the conceptual design was based upon a totally inflatable structure. An inflatable structure has been used for space solar power collection and radio-frequency antennas. To place the development of the membrane optical telescope in perspective, a short history of past inflatables will be presented. The totally inflatable lenticular design used in a variety of space-based applications in radio and radar antennae, solar power for propulsion applications and solar shields is of particular interest. Recently, a new version of a membrane telescope has emerged. Thin membranes on the order of 10 to 100 micrometers thick will be packaged and deployed without using inflation to maintain the surface figure. The move away from a pure inflatable is driven by several factors, including wavelength-level tolerances required of optical telescopes, even when real-time holography is invoked as the adaptive optics correction technique. Issues that led us to de-emphasize an inflatable, lenticular design and concentrate on a near-net shape film using stress coatings and dual boundary edge control are discussed.

Membrane telescopes: useful in ground-based astronomy?

There is a considerable and growing interest in the use of thin membrane mirrors for optical and infrared systems in space. The possibility of very large, monolithic light-collecting apertures with extremely low areal densities is strong motivation for developing the various technologies that might allow the realization of such systems. During the course of working toward space deployable, large mirror systems, we are also aware of possible ground-based applications of membrane optical elements. This paper briefly discusses some of our recent work and progress with membrane mirror elements, and possible future directions.

Net-shape polymer mirrors

The Directed Energy Directorate is developing a large space-based optical membrane telescope. The goal is to develop technologies that will enable 20-meter, or greater, diameter telescopes, with areal densities of less than 1 kilogram per square meter. The challenges include the development of a new material process that dramatically improves the optical quality of available films, choosing a process that is conceivably scalable to these larger diameters, and designing new structural concepts to meet surface accuracy requirements and areal density restrictions. A significant part of the realization of these goals relies on the development of a stress-coated net-shape film. A stress-coated net-shape film is a bilaminate system comprised of a pre-shaped polymer substrate coated with a compressive dielectric coating. This article is restricted to a discussion of surface data information on a 40-centimeter diameter, 10 tm thick, uncoated net-shape film. Passively forming these films to a near final shape (i.e. net-shape) will reduce the force, power, and range burden of the actuation system required to acquire and maintain the optical figure. Additionally, passively maintaining the form of these film structures will reduce the stiffness requirements of the supporting structure. The union of the polymer substrate and dielectric coating is still under development and will be reported on at a later date.