Current strategies for detecting and deflecting/destroying small to medium size asteroids that may possibly enter earth’s atmosphere are in their infancy. Projects such as Sentinel for detection and DE-STAR for deflecting of asteroids are gaining momentum while others are in design stages.
Single crystal silicon (SC-Si) with its unique set of mechanical and thermal properties is eminently suitable for deployment in any and all of these applications. But the superior thermal properties are particularly suited for high energy applications. We show that the exceptionally high thermal conductivity and diffusivity combined with relatively low thermal expansion enable SC-Si mirrors to handle the energy, often without active cooling, while maintaining figure. Both cooled and uncooled SC-Si mirrors have been employed successfully in high energy laser applications.
For IR detection of asteroids, SC-Si lenses, prisms and filters are invaluable in the 1-5 μm range along with SC-Si mirrors at any wavelength. This space qualified material has zero defects and is readily available in a range of sizes to 300 mm diameter from several vendors, but while boules to 460 mm have limited availability, they can be obtained. For applications requiring larger sizes, McCarter’s proprietary glass frit bonding is utilized. Components approaching one meter have been produced.
Zero defect single crystal silicon (Single-Crystal Si), with its diamond cubic crystal structure, is completely isotropic in most properties important for advanced aerospace systems. This paper will identify behavior of the three most dominant planes of the Single-Crystal Si cube (110), (100) and (111). For example, thermal and optical properties are completely isotropic for any given plane. The elastic and mechanical properties however are direction dependent. But we show through finite element analysis that in spite of this, near-isotropic behavior can be achieved with component designs that utilize the optimum elastic modulus in directions with the highest loads. Using glass frit bonding to assemble these planes is the only bonding agent that doesn’t degrade the performance of Single-Crystal Si. The most significant anisotropic property of Single-Crystal Si is the Young’s modulus of elasticity. Literature values vary substantially around a value of 145 GPa. The truth is that while the maximum modulus is 185 GPa, the most useful <110< crystallographic direction has a high 169 GPa, still higher than that of many materials such as aluminum and invar. And since Poisson’s ratio in this direction is an extremely low 0.064, distortion in the plane normal to the load is insignificant. While the minimum modulus is 130 GPa, a calculated average value is close to the optimum at approximately 160 GPa. The minimum modulus is therefore almost irrelevant. The (111) plane, referred to as the natural cleave plane survives impact that would overload the (110) and/or (100) plane due to its superior density. While mechanical properties vary from plane to plane each plane is uniform and response is predictable. Understanding the Single-Crystal Si diamond cube provides a design and manufacture path for building lightweight Single-Crystal Si systems with near-isotropic response to loads. It is clear then that near-isotropic elastic behavior is achievable in Single-Crystal Si components and will provide subsecond thermal equilibrium and sub-micron creep.
Mankind loves space and is drawn to explore its vastness. Existing space telescopes routinely
encounter data losses and delayed data collections during the constantly changing temperature and
load disruptions of space missions. The harsh environment of space thermal cycles and spacecraft
motion loads create unwanted activity such as spacecraft slew, acquisition slew, and temperature
induced blur. In order to compensate for the low performance of the materials currently used for
telescope optics, engineers and designers are using costly on-board coolers, mechanical actuators,
and deformed mirrors, for example, with limited success. However, Zero-defect Single Crystal
Silicon (SCSi) can perform in space environments without coolers, actuators, and other such devices
because SCSi is not ductile and is homogeneous and therefore is not subject to creep, and will not
jitter, or blur during operations. To take advantage of the unique advantages of Zero-defect SCSi,
we are developing and fabricating a Cryostable All-Silicon Imaging Telescope (CAIT). In this
paper, we will discuss the basis for selecting SCSi for our space telescope design, the status of the
CAIT design and fabrication progress, and compare SCSi thermal and strength properties with other
typical space optical materials.
Single crystal silicon (SCSi) is light, strong, has excellent thermal properties, is readily available and cost and delivery
are competitive with, and probably better than, either beryllium or silicon carbide. In addition, SCSi's zero-defect crystal
structure enables polishing to near-perfect surfaces.
Recent developments in direct bonding have led to simple methods of attaching SCSi, a brittle material, to enhance its
high compressive strength and avoid tensile/brittle failures. Dynamic testing of a bonded assembly has demonstrated
high resonant frequencies and damping capacity. Other recent test results have shown the excellent temporal and thermal
stability of both monolithic and bonded mirror specimens.
So why not choose silicon?
Single Crystal Silicon (SCSi) is proving to be an excellent material for the fabrication of lightweight optical components
for use in space. As part of the feasibility studies performed prior to space flight applications, it is important to
determine the mechanical properties of complex structures manufactured from individual sections of SCSi. As an
additional integral building block for future multi-component SCSi structures, the behavior of the McCarter Machine
proprietary frit-bonded metal insert technology was examined. Here we report vibration test results, the objective of
which was to measure the structural damping characteristics of a typical silicon structure and verify its structural stability
after exposure to random vibration. The tests were designed to better understand SCSi, not only as a mirror substrate,
but also as a structural material. The success of this test, combined with the already proven McCarter Machine
manufacturing techniques, give us the ability to now manufacture new lightweight and stable opto-mechanical
assemblies entirely out of SCSi. But since requirements for larger and more sophisticated SCSi structures are limited by
the practical size of available boules, the behavior of these frit-bonded SCSi structures needs to be better understood.
This understanding will be obtained from planned testing of larger frit bonded SCSi opto-mechanical structural
components and assemblies.
Silicon components such as mirrors and infrared lenses have been manufactured for many years, primarily from polycrystalline
silicon (poly). There are inherent advantages that Single Crystal Silicon, (SCSi), has over poly, such as strength and dimensional
stability, that make it more suitable for telescopes. However, there are challenges in the design of an all-SCSi telescope. SCSi is
brittle and has low tensile strength compared to its compressive strength. These properties therefore dictate designs that
minimize tensile stresses and eliminate direct mechanical attachments. McCarter has accepted these challenges and has designed
and is fabricating a lightweight telescope that can replace one of beryllium at substantial savings of cost and schedule.
The challenge of direct attachment has been solved with the use of bonded threaded inserts of low expansion metal. Bonding has
been studied extensively as described in a companion paper, but the proprietary frit-bonding technique developed by Frank
Anthony proved to be the most predictable, stable, and reliable. This technique is also used to fabricate complex components
from an assembly of simpler parts.
To minimize tensile stresses, the mechanical design had to be modified from the original without changing the optical
prescription. This has been successfully accomplished through a "design for manufacturing" approach teaming designers, the
stress analyst and manufacturing personnel. This approach has provided a design that is being produced at lower risk, lower cost
and with higher predicted reliability with no loss in performance.
A very low mass cryogenic telescope of moderate size is needed for the Space IR Telescope Facility (SIRTF). We evaluated multiple concepts for the JPL's SIRTF precursor, the 0.85 m IR telescope technology testbed with an emphasis on simultaneously achieving excellent image quality, minimum mass, and design simplicity. We selected an all-beryllium approach over one employing either silicon carbide or fused quartz mirrors. Based upon recent advances in beryllium powder metallurgy, including techniques for the reduction of residual stress, we are demonstrating that the telescope when cooled to 5 kelvins is capable of simultaneously meeting both the 6.5-micrometers diffraction-limited image quality requirement and the 30-kg mass goal. The design employs very few components and uses a single arch mirror to minimize telescope mass and simplify cryogenic mirror design, analysis, and testing. The telescope's inherently stiff metering tower combines the functions of secondary mirror support and stray light baffling. We describe the design and the trades by which we arrived at a final configuration.
Current optical finishing technology limits the choices for synchrotron radiation materials to a relatively few materials: fused silica and ULE<SUP>TM</SUP>, silicon, CVD silicon carbide, and electroless nickel. We review, in a general way, those materials and several others that can be finished to the required figure and finish levels, generally considered to be < 3 microradians rms and < 5 angstroms rms. With the objective of material choices for synchrotron beam line mirrors in mind, we briefly discuss dimensional stability, cooling, bending, polishing, and manufacturing procedures. After discussing specific materials: those previously mentioned and aluminum, Glidcop<SUP>TM</SUP>, invars, and steels, we conclude that metals are best from an engineering and cost standpoint, but ceramics, including silicon are best from a polishing standpoint.
Optical systems come in many shapes and sizes. Each system must perform in an environment that imposes unique constraints when combined with the operating system dynamics and other requirements. These constraints make the choice of materials a not so simple task.<p> </p> An overview of the available choices in advanced materials, with an emphasis on system compatibility and dimensional stability, is presented. Materials covered include: metals, glasses and glass ceramics, composites including metal and polymer matrix materials, and plated nickel and aluminum coatings. Refractive materials have not been included, the emphasis being on mirror systems. Properties comparisons are made and fabrication methods briefly discussed. There is never a material that meets all the requirements for a particular application. This paper, together with the others in this volume, provides guidelines for selecting the most suitable material or combination of materials for almost any optical system.
We consider the materials choices available for making optical substrates for synchrotron radiation beam lines. We find that currently the optical surfaces can only be polished to the required finish in fused silica and other glasses, silicon, CVD silicon carbide, electroless nickel and 17-4 PH stainless steel. Substrates must therefore be made of one of these materials or of a metal that can be coated with electroless nickel. In the context of material choices for mirrors we explore the issues of dimensional stability, polishing, bending, cooling, and manufacturing strategy. We conclude that metals are best from an engineering and cost standpoint while the ceramics are best from a polishing standpoint. We then give discussions of specific materials as follows: silicon carbide, silicon, electroless nickel, Glidcop, aluminum, precipitation-hardening stainless steel, mild steel, invar and superinvar. Finally we summarize conclusions and propose ideas for further research.
The Infrared Technology Testbed Telescope (1T1T) is a demonstration telescope meeting the needs of the SIRTF mission. It is a Ritchey-Cretien form designed for diffraction limited performance at 6.5 pm, at 5.5 K with an 85 cm. clear aperture. The mirror and system focal ratios are f/1.2 and f/12 respectively. This paper describes the design and fabrication of the efficient, ultra-lightweight, all-beryllium telescope. The design incorporates a central metering tower and single arch primary mirror to achieve a total telescope mass of less than 30 kg. Cryogenic testing of the primary mirror demonstrates the stability of the I-70-H (special) Be and the fabrication process. No thermal hysteresis was observed after repeated cycling to 5 K, and cryo-null figuring was utilized to overcome the small thermal instability observed at that temperature.
REOSC has been selected for the design, manufacturing and integration of the four ESO very large telescope (VLT) secondary mirrors. The VLT secondary mirrors are 1.12 m lightweight convex hyperbolic mirrors made of beryllium. Despite the VLT active optics correction capabilities, the use of a metal for the mirror structure implies specific manufacturing processes and associated design rules in order to ensure its dimensional stability during the telescope required life time. This paper describes how the fabrication process of the VLT secondary mirror has been optimized in order to maximize the dimensional stability of its structure. The beryllium properties are analyzed in parallel with the mirror requirements, the choices for the manufacturing, at all levels, are presented. A short work progress is presented, with the achieved mirror properties.
Stability of beryllium mirrors is said to be unpredictable. Three recent mirrors demonstrate excellent stability. JPL produced a plano-concave, 0.5 m solid test mirror that was machined from a HIP'ed billet of special I-70 Be powder, polished bare and tested to 4K. It was thermally stable and had no hysteresis. The JPL ITTT 0.85 m primary mirror used similar material and processes, but with more stress relied treatments. Tests of this bare-polished, very lightweight single arch hyperboloidal mirror to 5K showed similar excellent results. The ESO VLT chopping secondary is a 1.12 m, machined lightweight, nickel plated, convex paraboloid. Similar fabrication processes are being used but with higher strength I-220 Be. In-processes testing indicates a stable mirror. The results show beryllium to be a stable and predictable mirror material.
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.
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.
Dimensional instability exists to some extent in all components no matter what the materials may be. So the question is not "how can we eliminate instability?" but rather, "how can we reduce it to a tolerable level?" The maximum allowable dimensional instability will vary with application and depends on the particular component and its role in the optical instrument. The purpose of this paper is to provide the basis for deciding how much can be tolerated and for making intelligent choices in the selection of materials and processes for components that will achieve stability design goals with which to meet optical instrument performance specifications. This basis is a better understanding of the causes of instability and methods for minimizing instability.<p> </p> After a discussion of tolerable levels of instability, four types of dimensional instability are defined: thermal, temporal, cycling and hysteresis, with examples given for each. The principal causes of these instabilities: external stress, changes in internal stress, microstructural changes and inhomogeneity/anisotropy of properties, are explained in some detail along with a discussion of material types and properties. Most importantly, methods for minimizing the instabilities are shown. This discussion includes specific recommendations for commonly used materials including: processing techniques to minimize instability, specific problems observed in some materials and how to avoid the problems, and some general guidelines regarding the effects of fabrication methods on stability.<p> </p> It is most important to realize that increasingly tighter specifications for optical instruments mean that the optomechanical designer must work concurrently with other engineering disciplines, particularly materials and processes engineers, to insure the desired thermal and temporal stability of the product.
Silicon carbide (SiC) has become a legitimate competitor to beryllium (Be) for large lightweight mirrors due to its high stiffness to weight ratio and low thermal expansion. However, there are many kinds of SiC and some are better suited for mirrors than others. After a comparison of the various SiC types we described one type of reaction bonded SiC that can be near-net-shape fabricated into very lightweight mirrors in sizes ranging from a few centimeters to greater than two meters. Lightweight spherical mirrors of 0.18 and 0.50 m diameter have been fabricated and polished to very low surface roughness. The blank and optical fabrication techniques are described, and characterization data are presented for uncoated polished surfaces. These mirrors have surface roughness of 8 - 15 angstroms rms and (lambda) /10 figure
A program to fabricate a large, optically fast, aspheric lightweight Be mirror was initiated in order to demonstrate state-of-the-art technology. The mirror blank was fabricated as a 1.0 m diameter, f/0.58 ellipse directly from IP-70 grade powder using near-net-shape hot isostatic pressing (HIPing) and a patented tooling approach that produced a closed back, honeycomb- cored mirror weighing less than 18 kg. Details of the mirror design and of the assembly for HIPing are given. The blank was HIPed, leached, and machined to final shape with all design goals met. The as-HIPed blank was within +/- 0.5 mm in all dimensions and the radius of curvature was within 0.2 of target. The mirror was loose-abrasive ground using plunge grinding with a full-size tool, then polished using a full-size flexible pitch lap. In-process metrology utilized a special-purpose swing-arm profilometer with demonstrated accuracy and repeatability of
A program to fabricate a large, optically fast, aspheric lightweight Be mirror has demonstrated state-of-the-art technology. The mirror blank was fabricated as a 1.0-m diameter, f/0.58 ellipse, directly from IP-70 grade powder, using near-net-shape HIPing as well as a patented tooling approach that produced a closed back, honeycomb-cored mirror weighing less than 18 kg. Details of the mirror design and of the assembly for HIPing are given. The blank was HIPed, leached and machined to final shape with all design goals met; as-HIPed, the blank was within +/- 0.5-mm in all dimensions, and the radius of curvature was within 0.2 percent of target. The mirror was loose-abrasive ground using plunge grinding with a full-size tool, then rough polished using a full-size flexible pitch lap. In-process metrology used a special swing-arm profilometer with demonstrated accuracy and repeatability below one micron.
Reaction bonded silicon carbide (RB SiC) is a two phase mirror material which is readily formable to near-net-shape but can be difficult to polish to a high quality optical surface. The usual solution is the addition of a thick layer of silicon (Si) which may be polished to very high optical quality but which may have a thermal distortion problem due to the mismatch in thermal expansion of the two materials. The second solution is the application of a thick layer of chemically vapor deposited (CVD) SiC which can be polished to high quality but not as readily as the Si. The CVD SiC can also have a mismatch since it is deposited at high temperature and is beta SiC compared to the alpha in the substrate. We have chosen to develop a low temperature method for depositing amorphous SiC which should provide both a polishable surface and a better match of properties. To determine the levels of thermally induced distortion in SiC mirrors we have cryogenically tested 6-inch diameter spherical RB SiC mirrors bare and with polished coatings of amorphous SiC Si and CVD SiC. Results from this program are presented which show that all but the CVD SiC coated mirror are thermally stable. 1.
The choice of a substrate material for large mirrors is a complex engineering task that must account for structural and thermal properties as well as mirror blank fabricability polishability and surface scatter. Nuclear hardness is also a consideration in some applications. Cost is almost always a concern. The standard material for mirror substrates has always been glass. Beryllium technology however is well developed and offers distinct advantages over glass in many applications. Reaction-bonded silicon carbide is a relatively new material that has matured to the point where it can now be considered as an alternative to either beryllium or glass in some large optics applications. The availability of these three different substrate materials offers the system designer a great deal of flexibility in optimizing the material for each particular application. In this paper we present a methodology for comparing the structural properties of mirror substrate materials and lightweighting designs. This methodology is used to compare glass beryllium and silicon carbide. 1. LIGHTWEIGHT MIRROR DESIGN PARAMETERS Basic lightweight mirror design parameters are illustrated in Figure 1. Sandwich mirrors comprise two faceplates with a structured core in between. Although the faceplates are shown as having equal thicknesses tf they can in general be unequal. Open-back mirrors have a single faceplate. For both sandwich and open-back mirrors the mirror core is characterized by its height hc the web thickness tw of the core elements
Reaction bonded silicon carbide (RB SiC) can be readily fabricated to near net shape and mirror blanks produced by this method can potentially be less costly than those fabricated by chemical vapor deposition (CVD). However, RB SiC is two phase, SiC and up to 30% silicon (Si), and can not normally be directly polished to low scatter and roughness levels due to the difference in hardness of the two phases. We have investigated the polishability of RB SiC as a function of Si content and microstructure. Our results show that with a favorable microstructure, RB SiC can be polished to less than 10A rms. For reduced roughness and lower scatter surfaces, we have developed low temperature deposition techniques to apply polishable, single phase coatings to the figured two phase surfaces. Scatter and roughness measurements show levels are comparable to CVD SiC.
In order to fabricate dimensionally stable components the causes of dimensional instability must be understood and controlled to the level required by the specifications. This paper reviews four types of dimensional instability and the sources that cause dimensional change including the effects of external and internal stress on both macro- and micro-levels. Stress relief and stress relaxation as well as the use of low expansion materials are discussed. Examples are given of both unstable components and methods employed for fabricating stable components from materials such as aluminum beryllium glass and composites. 1.
4 August 2003 | San Diego, California, United States
Advanced Materials for Optics and Precision Structures
27 July 1997 | San Diego, CA, United States
Optomechanics and Dimensional Stability
25 July 1991 | San Diego, CA, United States
12 July 1990 | San Diego, CA, United States
SC016: Materials for Precision Instruments
This course covers selection and processing of materials for enhanced performance in optical and instrument systems. Emphasis will be on choosing the right material for the application. The importance of various thermal and mechanical properties, and related figures of merit will be presented and they will be compared for various materials. The dependence of these properties on temperature and other variables will be discussed with respect to system design and operation. Issues such as dimensional stability, options for mirrors and structures, and athermalization will be discussed. Fabrication methods will be described for most materials. Materials covered include: glasses and glass-ceramics, silicon carbide, composites, aluminums, invars, and other metals. Specific materials problems of attendees will be discussed as time permits.
SC219: Materials: Properties and Fabrication for Stable Optical Systems
This course describes materials and their properties as used in instrument and optical systems, emphasizing sources and solutions of dimensional instability of components.It covers issues such as, options for mirrors, benches and other structures, UHV &cryo compatibility and athermalization of both refractive and reflective systems. Optical materials such as glasses, aluminum, beryllium silicon and silicon carbide will be reviewed. Structural materials covered include: aluminum alloys, steels &invars, and composites. Fabrication methods will be described for each material with emphasis on fabrication for stability.
SC139: Telescope Systems: Materials Choices for Performance & Stability
This course describes materials for instruments and telescopes, emphasizing dimensional stability of components and covering issues such as cryo compatibility, options for large instrument benches and structures, and athermalization. Materials covered include: aluminum alloys, steels & invars, graphite/polymers and aluminum/beryllium. Refractive and reflective optical materials such as glasses, glass-ceramics, beryllium and silicon carbide will be reviewed. Fabrication methods will be described for each material. Specific materials problems of attendees will be discussed as time permits.
Many applications call for mirrors with metal, ceramic or composite materials for their mirror faceplates and/or structures. This course describes many options available for mirror, benches and structures. Materials properties, availability, and fabrication methods will be described with emphasis on processing for dimensional stability. Materials discussed include: aluminum alloys, beryllium, invars & steels, silicon carbides and metal & polymer matrix composites.