Dan K. Marker

Surface precision of optical membranes with curvature

Space-based inflatable technology is of current interest to NASA and DOD, and in particular to the Air Force and Phillips Laboratory. Potentially large gains in lowering launch costs, through reductions in structure mass and volume, are driving this activity. Diverse groups are researching and developing this technology for radio and radar antennae, optical telescopes, and solar power and propulsion applications. Regardless of the use, one common requirement for successful application is the accuracy of the inflated surface shape. The work reported here concerns the shape control of an inflated thin circular disk through use of a nonlinear finite element analysis. First, a review of the important associated Hencky problem is given. Then we discuss a shape modification, achieved through enforced boundary displacements, which resulted in moving the inflated shape towards a desired parabolic profile. Minimization of the figure error is discussed and conclusions are drawn.

Final Testing and Evaluation of a Meter-Class Actively Controlled Membrane Mirror

Abstract : Testing has been completed of a O.7O meter diameter mirror using thin-film polymer membranes. Advances in polymer film science have resulted in polymer membranes less than 24 microns in thickness with excellent surface roughness and sub wavelength thickness variation. The cause of such high quality material production, this has allowed the concept of a lenticular mirror design to be reconsidered. This involves the use of a clear canopy integrated with a reflectively coated membrane and pressurization is used to establish a desired focal length Boundary errors as well as significant spherical aberration are typical aberrations associated with such a mirror system. The membrane mirror described here accounts for these errors by utilizing an active boundary control system to help alleviate any errors near the boundary due to possible uneven stresses and any mounting errors. A varied stress coating is also deposited onto the reflective-polymer membrane to alter the mechanical properties of the film, that when pressurized it pushes more towards a parabola instead of a severely aberrated aspheric mirror. The final test data obtained on this system is presented in this paper.

Meter-Class Membrane Mirror with Active Boundary Control

Work has been ongoing in the design, fabrication, and testing of a 0.75 meter diameter mirror using thin-film polymer membranes. Advances in polymer film production have resulted in membrane less than 24 microns in thickness with excellent surface roughness and sub wavelength thickness variation. This has allowed the possibility of using a lenticular system consisting of a clear polymer canopy with a reflective polymer mirror integrated and pressurized to a desired focal length. Typical aberrations for such a system consist of spherical aberration as well as those associated with the boundary. The membrane mirror currently being developed accounts for these errors by utilizing a technique to reduce the spherical aberration present in the membrane by coating it with a varied thickness stress coating. This alters the mechanical properties of the membrane so that when pressurized it is shaped in a way matching the design prescription. An active boundary control system is also utilized to help alleviate any errors near the boundary due to any uneven stresses and any mounting errors. The progress to date on this system is presented in this paper.

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.

Parametric assessment of material properties, boundary conditions and environmental effects on the performance of membrane optical systems

Previous research has demonstrated the feasibility of manufacturing polymer membranes with surfaces suitable for use as optical elements on scales up to 1.5 meters. These membranes have optical surface finishes characterized by a roughness of 1.2 nanometers (rms) and mid spatial frequency figure errors (caused by thickness variations) of approximately 350 nanometers-adequate for many optical applications. With optical quality membranes fabrication demonstrated, the next technical challenges that must be met before large-aperture, ultra-light membrane mirrors can be practically achieved are to develop (1) light-weight deployable support structures, (2) the ability to control the global figure of large optical quality membranes, and (3) an improved understanding of the effects of membrane material properties (e.g., material in-homogeneities, coatings, and boundary conditions) on global figure. The work reported herein further characterizes several key system properties and their effects on optical aberrations. This analysis helps establish technical requirements for membrane optical systems and provides additional insight required to optimize deployable support structures capable of providing passive figure control for membrane optical elements. The results are also used to investigate the need for an electrostatic control system that can actively control the figure of a large membrane mirror.

Design, fabrication, and validation of an ultra-lightweight membrane mirror

Large aperture optical quality primary mirrors have been developed which are extremely lightweight (areal densities less than 1kg/m2) made from stretched reflective polymer membranes. However, aberrations induced by boundary support errors and pressurization of a flat membrane do not produce a perfect parabolic shape. Modeling studies have shown that active boundary control can be very effective in correcting certain types of figure errors typically seen in membrane mirrors. This paper validates these design studies by applying boundary control on a 0.25-meter pressure augmented membrane mirror (PAMM). The 0.25 meter PAMM was fabricated as a pathfinder for a larger prototype. A combination of displacement actuators and electrostatic force actuators were used to control the shape of the mirror. A varied thickness stress coating prescription was developed by a SRS/AFRL team using nonlinear membrane theory. Based on modeled data, the stress coating should force the membrane into a parabolic shape when pressurized, as opposed to a spherically aberrated shape characteristic of a pressurized flat membrane. Test data from the 0.25-meter PAMM proved that the varied thickness stress coating allows for a better shape than the uniform coating.

Development of a one-meter membrane mirror with active boundary control

Materials and processes have been developed for production of polymer membranes with optical quality surface characteristics. These materials have been successfully used to manufacture large, high quality, ultra lightweight, optical flats for beam splitters, lens covers and other applications. These materials can potentially be used to develop large aperture primary mirrors with areal densities less than 1kg/m2. However, for curved mirrors it is more difficult to establish and maintain desired optical figure from the initial packaged configuration. This paper describes design analysis being performed to support fabrication of a membrane mirror test article. Modeling was performed to evaluate the effectiveness of several different boundary control concepts for correcting different types of figure aberrations. Analyses of different combinations of boundary displacement actuators, electrostatic force actuators, and pressure are presented.

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.