To maximize the cost-effectiveness of the Mars Laser Communication Demonstration (MLCD), the project is pursuing the use of ground-based astronomical telescopes as large-aperture optical receiving antennae. To facilitate communication as the spacecraft approaches solar conjunction, a large membrane filter is being considered to reject approximately 95% of the sun’s power, while efficiently admitting light at the 1060 nm signal wavelength. Through the use of this filter and some additional facility modifications, the problems of thermally-induced telescope aberrations and dangerous focusing of solar power can effectively be mitigated. The use of a membrane filter is expected to be cost competitive, introduce less scattered light, and provide more flexibility in placement and operations than alternative approaches. This paper addresses the initial design of the filter and preparation of test samples to evaluate candidate materials.
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.
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.
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.