Brett deBlonk

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.

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.

Design and testing of a one-meter membrane mirror with active boundary control

The use of thin-film membranes is of considerable interest for lightweight mirror applications. The low areal density makes them ideal for large aperture imaging applications. One type of setup looked into in the past has been the lenticular design, which consists of a clear canopy attached to a reflective film that uses positive pressure to set the curvature of the mirror. One drawback to this concept has been the fact that too much error was introduced during the pass through the canopy due to material inhomogeneities and poor optical properties. This is no longer an issue thanks to developments over the past several years in the field of optical-quality polymer development. Thin-films (< 24 microns) can now be routinely made with surface roughness, thickness variation, and very good transmission properties well within specification for many visible and IR applications. The next step in this developmental process has been maintaining a prescribed figure in the mirror. This paper summarizes the current efforts in fabricating and testing a 1-meter class lenticular membrane mirror system utilizing active boundary control and stress-coating applications to form a usable aperture for visible imaging applications.

Deployment Repeatability Testing of Composite Tape Springs for Space Optics Applications

Abstract : Tape springs are of interest to the space structures community because of their high packaging ratios, ability to self-deploy, and high stiffness-to-mass ratios. The current drive to lightweight telescopes has focused mostly on decreasing the mass of the mirrors; yet decreasing the mass of the support structure may also generate significant mass savings. Here the use of carbon-fiber-composite tape springs is examined as a potential support structure of a secondary mirror in a Cassegrain-type telescope configuration. For the tape springs to be useful in this capacity, they must exhibit deployment precision to levels consistent with optical control systems. Deployment repeatability of such structures is investigated through simplified sensing configurations that include a linear structural element and a tripod comprised of carbon-fiber-composite tape springs supporting a simulated secondary mirror. Single tape springs showed deployment repeatability on the order of 100 microns, while the tripod configuration showed deployment repeatability on the order of 50 microns.

Converted silicon carbide technology developments for optics

Silicon carbide structures fabricated by converting near-net-shape graphite preforms via Chemical Vapor Conversion (CVC) phase reaction have long provided improved performance components for electronics processing. In recent years, this same technology has been applied to the fabrication of simple and lightweighted mirrors and is moving into optical bench applications. To support the expanded applications, Poco has further evaluated the material properties of SUPERSiC® silicon carbide, developed technologies to mount silicon carbide mirrors on benches of similar and dissimilar materials, and fabricated complex monolithic geometries using in situ conversion bonding of mating graphite components. Overviews of each of these areas will be presented.

Characterizing the nonlinear dynamic behavior of membrane optics

The use of true-membrane reflectors holds the promise of increasing the size of space-based apertures by decreasing payload mass and reducing launch volumes, but figure acquisition and maintenance of the thin, deployed structure present significant control challenges. The ability to manage both the static and dynamic aberrations defines the utility of these compliant mirrors for resolving quality images. The scope of the current study consists of characterizing the non-linear dynamic behavior of membrane reflectors to visible-optics quality under realistic support and loading scenarios. The basis for quality in the finite element model (FEM) deformed shape predictions is established both by comparing FEM and analytical solutions for linear static problems and by studying the convergence of eigen solutions. Most of the results are shown, too, to be within a previously determined range of optically-accurate solutions. The topographical difference between linear and non-linear dynamic solutions is characterized and correlated to support and loading regimes for eventual inclusion in closed-loop-control schemes. The objective of this paper is thus to study the non-linear characteristics of the dynamic behavior of membrane optics as the basis for future work in system identification and figure control.