Brian Patrick

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.

Software for integrated optical design analysis

The Integrated Optical Design Analysis (IODA) program is a software tool being developed to support concurrent engineering design of complex optical systems. IODA provides seamless data fusion between thermal, structural, and optical models used to design the system. The software architecture was developed by reviewing current design processes and developing software to automate the existing procedures. IODA significantly reduces the design iteration cycle time and eliminates many potential sources of error.

Large-aperture fast multilevel Fresnel zone lenses in glass and ultrathin polymer films for visible and near-infrared imaging applications.

The ability to fabricate 4-level diffractive structures with 1 µm critical dimensions has been demonstrated for the creation of fast (∼f/3.1 at 633 nm) Fresnel zone lenses (FZLs) with >60% diffraction efficiency into the -1 focusing order and nearly complete suppression of 0 and +1 orders. This is done using tooling capable of producing optics with 800 mm apertures. A 4-level grating fabricated in glass at 300 mm aperture is shown to have <15  nm rms holographic phase error. Glass FZLs have also been used as mandrels for casting zero-thermal-expansion, 20 µm thick polymer films created with the 4-level structure as a route to mass replication of efficient diffractive membranes for ultralight segmented space-based telescope applications.

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.

Design, test, and evaluation of an electrostatically figured membrane mirror

Significant advances have been achieved in manufacturing optical quality membrane materials with surface quality suitable for use as first surface mirrors. These materials have been used to fabricate test articles demonstrating diffraction limited performance in the laboratory environment. These mirrors are supported using heavy rigid fixtures and pressure forces to tension the membrane. A lighter weight system is required to transition the membrane mirror technology to space hardware applications. Using electrostatic forces to tension and figure the membrane is one promising approach to developing a flight weight membrane mirror system. This paper discusses the design and testing of an experimental membrane mirror system that was developed to evaluate the potential areal density, figure accuracy and stability of a lightweight electrostatically figured mirror manufactured from precision cast optical quality membrane material.

Zero CTE polyimides for athermal optical membranes

Polyimides are attractive mirror candidate materials due to their low mass, solar radiation resistance, and cryogenic flexibility. However, polyimides exhibit high coefficients of thermal expansion (CTE) values (40-70 ppm/K), inducing image distortion from CTE mismatch. Additionally, the temperature of large aperture (10 m) membranes is not uniformly controlled in space, further increasing image distortion from anisotropic deformations. The CTE of the MSRS Novastrat polyimide line was adjusted to exhibit CTEs between -16 ppm/K and 53 ppm/K, including 0 ppm/K, 10 ppm/K, 17 ppm/K, and 25 ppm/K corresponding with CTE matches of graphite/epoxy, carbon steel, copper, and aluminum (respectively). The development of these CTE-matched membranes is presented, as well as the effect of the CTE adjustment on the mechanical properties.

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.

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.