Mirrors and their supporting structures, the mirror systems, come in many shapes and sizes. Each system must operate in an environment that imposes unique constraints when combined with the operating system dynamics and other requirements. These constraints make the choice of materials for mirror systems a not so simple task. Many materials are available for use in mirror systems. An overview of the available choices, with emphasis on system compatibility and dimensional stability, is presented. Factors influencing the choice include optical and mechanical requirements, the environment, available fabrication methods and economics. Most available materials are discussed with emphasis on aluminum, beryllium, silicon carbide and silicon carbide/aluminum composites. Properties comparisons are made and fabrication methods discussed. Finally, a methodology for materials selection is presented.

The potential for using silicon carbide as an optical substrate has been recognized for a number of years. The primary characteristics silicon carbide offers relative to other more traditional materials include high stiffness, high toughness, low toxicity, low thermal distortion and potential cost and schedule advantages. These attractive properties become accentuated as the size of the mirror blank increases, especially when considering space borne applications. In this paper, we report on the continuing development of silicon carbide for use in large lightweight mirror applications. In particular we describe the design, fabrication and testing of a 0.8 multiplied by 1.1 meter open backed, 'egg-crate' reaction bonded silicon carbide mirror blank. Several process demonstration (sub-scale blank fabrication repeatability and optical finishing) and material evaluation (coefficient of thermal expansion, modulus of rupture, and chemistry) tasks were performed to initiate a database for future optical designs and evaluation. Finally, the results of cryogenic testing of the blank are presented. All of the silicon carbide blank fabrication work in support of this program was performed by Carborundum Specialty Products. The results indicate that reaction bonded silicon carbide is an excellent material for large lightweight mirror substrates.

A new grade of silicon carbide has been developed with properties that make it very attractive for a variety of applications in precision optical structures. Its microstructural homogeneity makes it capable of accepting an optical finish with subnanometer surface roughness. Its strength and fracture toughness, on a bulk scale, exceed all previous silicon carbide materials. This hot-pressed silicon carbide can be produced in single blocks up to 50 cm square and up to 20 cm thick. Two bonding techniques have been developed for fusing large segments of hot pressed silicon carbide together into a large monolith for constructing large optical structures without using a metallic braze. Bonding structure and bonding strength are discussed.

The lightweight SiC mirrors that are fabricated by the CVD process at Morton Advanced Materials contain graphite core enclosed in the SiC backstructure. A finite element analysis of a lightweight SiC model mirror was performed to assess the effect of the graphite core on the thermal stability of the SiC mirror in the temperature range of 77 - 1623 K. The results indicate that for both no-slip (SiC deposit adhered to graphite core) and slip cases, the maximum stresses in the SiC mirror structure are significantly less than the flexural strength of SiC. Further, the maximum stress in graphite core is close to the tensile strength of graphite indicating that graphite core will probably fracture. Finally, the SiC faceplate figure distortion due to the presence of the graphite core is quite small, on the order of a few tens of nanometers.

During the past several years there has been significant progress in the technology validation, fabrication, and high-power testing of uncooled optics for high-energy chemical lasers. When compared to the past practice of using cooled, molybdenum components, uncooled optics offer the potential for lower component weights, significantly reduced fabrication cost and schedule, and improved performance due to the elimination of the coolant flow-induced jitter. An initial effort, performed under the Uncooled Optics Scaleability Demonstration program, focused on the development and production of large diameter (17-inch) turning flats. In January 1992, the Ballistic Missile Defense Organization (BMDO) initiated Special Study 007, Resonator Optics Materials Assessment (ROMA), to extend this technology to high-power chemical laser resonator optics. The objective of this study was to demonstrate the critical techniques and processes required to fabricate resonator optics for the Star LITE Space-Based Laser (SBL) Advanced Technology Demonstrator and, ultimately, to an operational space-based laser weapon system. This paper provides a background of the technology evolution, an overview of the substrate material selection process, and the development of processes used to fabricate large-diameter uncooled optics. It also provides the framework for four additional papers presented in this session on uncooled optics technology development.

Large aperture single crystal silicon turning flat mirrors provide a new method of high energy laser beam sampling for optical performance diagnostic measurements. This new sampling technique has been successfully implemented on the Alpha Laser Optimization program where transmitted beams through two, uncooled silicon mirrors have been used to measure total energy, instantaneous power, spectral content, and beam jitter. The accuracy of these measurements has been confirmed by independent measurements of these same parameters, and the results fall within expected error bars.

Recent advancements in single crystal silicon material science and fabrication capabilities and very low absorption (VLA) multi-layer dielectric coating technology have led to the development of uncooled, large aperture, high power mirrors for high energy laser (HEL) systems. Based on this success, a segmented single-crystal silicon substrate concept has been selected as the baseline fabrication approach for uncooled 1.2 meter diameter resonator annular optics for the Alpha space based high energy laser. The objective of this Resonator Optics Materials Assessment (ROMA) task was to demonstrate all of the key fabrication processes required to fabricate the full sized annular optics for the Alpha space based high energy laser. This paper documents the fabrication of a half-scale annular optic prototype (AOP) of the Alpha laser rear cone.

Basic design guidelines for beryllium precision structures are discussed and a thorough presentation is given on the design evolution of the RGA integrating structure for ESA's XMM Mission. Beryllium producibility issues, manufacturing flowcharts and flight heritage data is reviewed.

This paper presents an interesting view of beryllium and aluminum-beryllium optics from Russia. It covers the basic design philosophy of why selecting beryllium and compared Russian beryllium to other optical materials. Fabrication processing and equipment are discussed and presented using photographs. Detailed property data for Russian materials is presented in both pure and alloy forms. Unique casting capabilities combining beryllium with aluminum for large optical structures is disclosed along with photographs of actual hardware produced.

The use of continuous fiber reinforced plastic, CFRP, composite materials is introduced here as a viable material for optical telescopes. The thermal characteristics of CFRPs make them attractive as dimensionally stable materials for all-composite telescope structures and mirrors. Composite mirrors have only recently shown promise as replacements for heavier and more fragile glass mirrors. The areal density of a CFRP mirror can be as much as 10 times less than that of a glass mirror. Optical test results show CFRP composite mirrors can be fabricated with an average surface roughness of less than 10 angstroms. Concept models of scope and CFRP optics with associated figure and roughness data are presented.

Many satellite-borne measurement applications require lightweight, reflective near diffraction limited telescopes with wide spectral bands (UV/visible to LWIR), for operation in space environments and over wide temperature ranges. Emerging silicon carbide (SiC) technology provides an attractive material for these telescope applications. It offers (1) the lightweight and stiffness of beryllium, (2) the diffraction limited visible optical performance of glass, (3) superior thermal stability to cryogenic temperatures, and (4) the cost and rapid fabrication advantages of aluminum. This paper describes the design, development, and test for a 50 cm dia. aperture on-axis imaging SiC telescope, and a 'goes like' 0.5 multiplied by 0.3 m scan flat for a geo-stationary earth observatory (GEO) mission. Optical, thermal, and structural design and analyses are described for the demonstration hardware. The scan mirror and telescope system have been optically tested over the temperature range of plus or minus 50 degrees Celsius. Temperature gradients have been induced on the scan mirror and telescope simulating non uniform thermal loading from the earth and sun. Test results are presented which show the excellent optical/thermal stability properties of SiC.

Advances in optical coating and materials technology have made possible the development of instruments with substantially improved efficiency in the extreme ultraviolet (EUV). For example, the development of chemical vapor deposited (CVD) SiC mirrors provides an opportunity to extend the range of normal incidence instruments down to 60 nm. CVD-SiC is a highly polishable material yielding low scatter surfaces. High UV reflectivity and desirable mechanical and thermal properties make CVD-SiC an attractive mirror and/or coating material for EUV applications. The EUV performance of SiC mirrors as well as some strengths and problem areas are discussed.

The next generation of space-borne optical sensor will have to meet tight weight limitation, in order to be viable on smaller, less expensive, launch platforms, while supporting a wide range of mission scenarios. Wide spectral coverage, near-diffraction limited visible quality performance, and increased thermal and structural stability are becoming important features for future space-hardware. SiC represents an emerging technology which is gaining wider acceptance as the leading candidate for the next generation of space flight hardware. As a material for all-reflective flight telescopes and optical benches, SiC offers: the lightweight and stiffness characteristics of beryllium; glass-like inherent stability consistent with visible quality performance levels; superior thermal properties down to cryogenic temperatures and in the presence of large thermal gradients; and an existing, commercially based material and processing infrastructure like aluminum. This paper describes an all-SiC off-axis, three mirror anastigmatic telescope system which promises to meet these stressing technical requirements. The system described maintains a 35 cm entrance aperture with a weight of 14 kgs.

This paper describes a telescope with an aperture diameter of 60 cm, for which the mirrors and mirror mounts are being fabricated. The telescope is a three-mirror anastigmat with an offset field and consists of three powered aspheric mirrors made of silicon carbide and two folding flats made of silicon. The mirrors and mirror mounts are being fabricated at the Vavilov State Optical Institute in St. Petersburg, Russia, as part of a collaborative program with the Lockheed Martin Palo Alto Research Laboratories. The designs of the mirrors and mounts, and the results of interferometric tests of the mirrors, are discussed.

This paper presents the results of interferometric tests of two silicon carbide mirrors tested at room temperature and 6 K. The first mirror has a spherical f/1.73 surface, a diameter of 170 mm, and is of solid, plano-concave construction. The other mirror, a plano measuring 308 mm by 210 mm, is of lightweighted, closed-back construction. The mirrors were manufactured by the Vavilov State Optical Institute, St. Petersburg, Russia, and were loaned to Lockheed for these tests. Optical tests on both mirrors were performed using the Lockheed cryogenic optical test facility at liquid helium temperature and a Zygo Mark II interferometer. There was no change in the surface figure of the mirrors, within the test uncertainty of approximately plus or minus 0.02 waves at 0.6328-micrometer wavelength.

Silicon carbide is one of the few materials developed over recent years which exhibits a combination of mechanical and physical properties that equal or exceed those of most all other materials over a thermal range well above room temperature to the cryogenic extremes. As such, it has all of the excellent qualities demanded for high acuity optical systems for use as both an optic which can be made lightweight and a structural support bench or metering material. Of particular interest is the behavior of this material for cryogenic application. Current available data for thermal expansion characteristics is limited to the region between room temperature and 80 degrees Kelvin. The measurement technique and results of a characterization program for accurately quantifying the nominal thermal strain characteristics of reaction bonded optical grade (RBO) silicon carbide down to 4 degrees Kelvin are presented and discussed. Hysteresis data, important for knowledge of predictable structure motions and optical figure after the extremes of thermal excursion cycling, also are discussed. The presentation concludes with discussions of the implications of these properties as they pertain to the design and performance of cryogenic optical systems.

The properties of silicon carbide (low CTE, high modulus, high conductivity, low density) are ideal for mirrors performing at cryogenic temperatures. Test data at cryogenic temperatures indicate high thermal strain homogeneity as well as low hysteresis (critical properties for high quality optical performance). Until recently, the largest SiC mirrors tested at liquid helium temperatures have been only a few centimeters in diameter. Recently a lightweighted (6 kg) 20-inch-diameter SiC mirror manufactured by United Technologies Optical Systems was tested for figure change at 10 K. Hysteresis was quantified upon return to room temperature. The results indicate high thermal strain homogeneity and low hysteresis. These optical results are applied to a parametric model developed from numerous previous cryogenic tests to estimate the thermal strain variability. Quantitative comparisons to other cryogenic materials are made based on reported test data.

A wide range of next generation spaceborne strategic and measurement/science applications require very lightweight, low cost, reflective telescopes with wide wavelength coverage, capable of near diffraction limited image quality over wide temperature ranges to cryogenic operation. A silicon carbide (SiC) telescope is an extremely attractive technology which offers (1) the lightweight and stiffness features of beryllium, (2) the optical performance of glass to visible quality, (3) superior optical/thermal stability to cryogenic temperatures, and (4) the low cost, fast fabrication processes of aluminum. This paper describes an ultralightweight (less than 1.5 kg), 18 cm aperture, 4 mirror off axis re-imaging infrared SiC telescope assembly designed in a 'flight-worthy' configuration. The overall design approach is described including comparison of telescope structural analysis to vibration test data. The telescope optical performance has been measured from 300 degrees Kelvin to less than 100 degrees Kelvin, including repeated thermal cycles; the rms wavefront change over this temperature range at 10.6 micrometer is less than 0.02 waves with no observed hysteresis, which demonstrates SiC technology to satisfy near diffraction limited infrared capability at cryogenic temperatures.

An experimental database of the optical, electrical, physical, and structural properties was generated for as-grown samples from large diameter silicon boules. Fourier transform infrared spectroscopy (FTIR) was utilized to evaluate the oxygen content and infrared absorbance in the HF laser bandwidth (2.6 to 3 micrometer). The bulk absorption coefficient over this bandwidth was quantified by performing laser absorption calorimetry. Electrical properties were obtained by performing Hall measurements at room temperature and 77 K. The carrier concentration and impurity type were determined, and simple photoconduction experiments were performed to ascertain the presence of bound carriers. The thermal conductivity was measured directly utilizing the Fourier technique, and the coefficient of thermal expansion was determined for room temperature to 600 degrees Celsius via dilatometer measurements. Finally, crystallographic studies involving chemical etching were performed to delineate structural defects and determine their densities. Some of these procedures were repeated after the samples had been defect engineered, i.e., given a high temperature heat treatment, followed by a rapid quench. The simple heat treatment was shown to reduce the bulk absorption coefficient of the material by a factor of four over the HF laser bandwidth (2.6 to 3 micrometer). All results from this material assessment are reported.

MicroRaman, Raman, photoluminescence and x-ray diffraction spectra were measured for different SiC splices. It has been shown that Raman and especially MicroRaman scattering gives us a gain over other diagnostic techniques on the way to define polytype structure of the splice.

Advanced optical systems, currently under consideration, propose the use of lightweight, segmented, long radii elements. Reaction-bonded silicon carbide (RB SiC) is quickly becoming a contender in the materials market for these large, lightweight mirrors due, in part, to its relatively low thermal expansion and high stiffness to weight ratio. The RB SiC manufacturing processes can be considered advanced in comparison to the polishing and finishing processes on bare RB SiC, or on clad RB SiC substrates. In an effort to improve the polishing and finishing techniques, Eastman Kodak Company has investigated a polishing process for physical vapor deposited silicon (PVD Si) coated RB SiC optics and has demonstrated this process on a lightweight, concave, PVD Si-coated RB SiC hexagonal optic. This discussion includes the modification and redesign of an existing conventional planetary table, durability test results on the PVD Si/RB SiC coating bond, process approach and results for the hexagonal demonstration piece, and the thermal stability analysis of the hexagonal mirror upon completion of polishing and ion figuring the front surface of the mirror.

Optical fabrication methods for a one meter reaction bonded optical grade silicon carbide (RBO SiC) substrate have been successfully developed and demonstrated by Itek Optical Systems in Lexington, Massachusetts. A 1.125 m multiplied by .825 m RBO SiC panel, the largest SiC ever fabricated, has been ground and polished to a surface figure of lambda/20 rms at lambda equals .6328 nm. The single most critical technology area for RBO SiC optics has been attaining a high quality optical surface finish; this bimodal material requires particular processing techniques due to the hardness differences between the silicon and silicon carbide components. The measured surface finish quality for this one meter mirror is 15.5 angstrom rms, nearly twice as good as the specified 25 angstrom rms. Furthermore, the variation of surface finish over the mirror area is very low, sigma equals 2 angstrom. The mirror represents the state-of-the-art for lightweight, high quality SiC optical mirrors.

There is a need for thin optical coatings that can be produced at low temperatures and have a high reflectance in the extreme ultraviolet (EUV). Currently, the best such material is silicon carbide (SiC) sputtered onto optical surfaces from targets of beta-SiC which were produced by chemical vapor deposition (CVD). The EUV reflectance of these films, however, is not as high as that of polished CVD SiC, and in addition it degrades with time. In this study the nonreversible reflectance degradation is quantified, and efforts to ameliorate it via ion-assisted deposition (IAD) and other techniques are detailed.

A 50 cm diameter, beryllium mirror was fabricated and cryogenically tested as a joint project between NASA-Ames Research Center and Jet Propulsion Laboratory. The purpose of this project was to determine the cryogenic distortion and hysteresis of a large, state-of-the-art beryllium mirror when cooled to liquid helium temperatures. The mirror blank was HIPed from I-70 special beryllium and machined to a plano-concave sphere with a 200 cm radius of curvature. The blank was annealed, acid etched, and thermally cycled may times during machining, figuring, and polishing to reduce stress. The mirror was tested twice to liquid helium temperature using the Ames Research Center Cryogenic Test Facility. No hysteresis or temporal instability was measured in the two tests. The cryogenic distortion was 0.5 p-v. This distortion is comparable to fused silica and is the lowest for any beryllium mirror tested by this facility.