Optical systems are designed for a great variety of purposes and are influenced by significantly differing requirements. Due to these differences, material trade studies are part of almost all optical system designs. These trade studies must use objective comparative parameters in order to choose the best optical and structural materials for the optical system. Material figures of merit such as specific stiffness and thermal stability are traditional figures of merit used in materials trade studies. In this paper, we explore additional material figures of merit arising from both technical and programmatic concerns. We show how to use all of these figures of merit simultaneously in a systematic approach to optimum materials selection.

During the past several years, ECM carried out a large trade-off project with the objective of comparing CTEmeasurements of samples of our Cesic® MF ceramic material performed by laboratories in Europe, USA, and Japan. Our focus was on CTE measurements in the cryo-environment down to less than 20 K. From past experience, we realized that we needed to pay particular attention to making the samples sufficiently large to obtain realistic measurement results that relate to actual applications, especially for temperatures below that of LN2. Thus, we made the samples as large as allowed by the size limitations of the test equipment at the different laboratories, namely, from 15 mm to 250 mm in length. The scatter in the test data obtained by the different laboratories was so large that the results were unreliable and not very useful, which we interpreted to be principally due to the large length range of the samples. Based on these results, we selected Mitsubishi Electric Corporation (MELCO), Japan, for follow-up cryo-measurements because they were able to test 250-mm long samples down to < 10 K. More recently MELCO enlarged their cryo-test equipment to sample sizes as large as 500 mm and with the rather high measurement accuracy of 0.1 ppm/K. In this paper we present the CTE measurement data obtained by the different laboratories and describe the new cryo-test facility at MELCO.

This review paper summarizes the extensive investigations that have been performed at SCHOTT to achieve a deeper understanding of the CTE homogeneity of ZERODUR® within single blanks and the casted formats (boules). Especially for the upcoming Extremely Large Telescope (ELT) projects like E-ELT or TMT with at least several hundreds of mirror segments the reproducibility of the mean CTE, CTE homogeneity and axial gradient is very important while keeping the CTE quality assurance process economic at the same time. Statistics of CTE homogeneity measurements on a ZERODUR® boule suitable for an economical production of ELT mirror substrates using the improved dilatometer will be presented. It will be shown, that it is possible to achieve tight CTE specifications by utilisation of processes existing at SCHOTT, while at the same time guaranteeing a long term reproducibility. The CTE measurement is optimized for a temperature interval from 0°C to 50°C. We developed a model to extrapolate the CTE behaviour to specific temperature conditions at the telescope site.

A prototype for a compact and relatively simple system capable to measure the refraction index of glasses, between 0.4 to 1.7 micron, at room and cryogenic temperatures (T=100-300 K), with an absolute precision of few parts of 100'000 has been developed at INAF facilities in Merate. It is based onto the measurement of the deviation angle of monochromatic light passing through a prism sample placed in the cryogenic chamber. The precision of the measurements depends on many factors, in particular the angle measurement and the thermal stability. Different subsystems have been studied to keep all these parameters under accurate control. One of the main issues has been the trade-off between the simplicity of the set-up and the precision of the refractive index measurement.

A brief overview is provided on three common methods of nondestructive evaluation (NDE), Ultrasonic Testing (UT), Radiographic Testing (RT) and Eddy Current (EC). These methods vary in the physics applied to the testing or evaluation process while specific techniques vary in the way each method may be applied. Understanding the physics behind each method and constraints of each technique helps us understand the capabilities and limitations of the inspection. Recognizing the capabilities and limitation of these NDE Techniques helps us properly design the inspection, collect and process data and draw appropriate conclusions.

Two types of SiC plates, differing in their manufacturing processes, were interrogated using a variety of NDE techniques. The task of evaluating the materials properties of these plates was a challenge due to their non-uniform thickness. Ultrasound was used to estimate the Young's Modulus and calculate the thickness profile and Poisson's Ratio of the plates. The Young's Modulus profile plots were consistent with the thickness profile plots, indicating that the technique was highly influenced by the non-uniform thickness of the plates. The Poisson's Ratio is calculated from the longitudinal and shear wave velocities. Because the thickness is cancelled out, the result is dependent only on the time of flight of the two wave modes, which can be measured accurately. X-Ray was used to determine if any density variations were present in the plates. None were detected suggesting that the varying time of flight of the acoustic wave is attributed only to variations in the elastic constants and thickness profiles of the plates. Eddy Current was used to plot the conductivity profile. Surprisingly, the conductivity profile of one type of plates varied over a wide range rarely seen in other materials. The other type revealed a uniform conductivity profile.

Silicon carbide mirror blank materials were characterized using ultrasound phased array technology to determine the feasibility of using this method for rapidly characterizing material homogeneity and locate anomalous flaws. A 10MHz 64 element linear probe was used to measure and image the top and bottom surface reflected signal peak amplitudes. C-scan images located the presence of possible sub-surface heterogeneities as well as surface scratches invisible to the eye. Bottom surface scans located variations in homogeneity within the sample bulk. High frequency ultrasound phased array was found to be well suited for detailed characterization of SiC mirror blanks.

The Aerospace Corporation is developing a space qualification method for silicon carbide optical systems that covers material verification through system development. One of the initial efforts has been to establish testing protocols for material properties. Three different tests have been performed to determine mechanical properties of SiC: modulus of rupture, equibiaxial flexural strength and fracture toughness. Testing materials and methods have been in accordance with the respective ASTM standards. Material from four vendors has been tested to date, as part of the MISSE flight program and other programs. Data analysis has focused on the types of issues that are important when building actual components- statistical modeling of test results, understanding batch-to-batch or other source material variations, and relating mechanical properties to microstructures. Mechanical properties are needed as inputs to design trade studies and development and analysis of proof tests, and to confirm or understand the results of non-destructive evaluations of the source materials. Measuring these properties using standardized tests on a statistically valid number of samples is intended to increase confidence for purchasers of SiC spacecraft components that materials and structures will perform as intended at the highest level of reliability.

An important space application for structural ceramics is the large (~1m diameter) silicon carbide mirrors used in telescopes. However, all ceramics have two drawbacks. First, ceramics are brittle and have a low resistance to catastrophic flaw propagation during service. Second, ceramics have a population of preexisting flaws produced during manufacturing. The most modern and successful theory of fracture control is "defect tolerant design", which recognizes that engineering structures are inherently flawed and which is used to predict the structure's service life. As part of defect tolerant design, the size of the inherent flaws is controlled by combining nondestructive evaluation (which has a threshold for the smallest flaw that can be reliably identified) and proof testing (which provides an independent measurement of the largest preexisting flaw). We are developing a novel proof test that is specialized for a lightweighted ceramic mirror. The most promising loading method is identified and an experimental implementation has been proposed and designed for future development.

This annual review documents our progress towards inexpensive mass production of silicon carbide mirrors and optical structures. Results are provided for a NASA Small Business Technology Transfer (STTR) X-Ray Mirror project. Trex partnered with the University of Alabama-Huntsville Center for Advanced Optics (UAH-CAO) to develop fabrication methods for polished cylindrical and conical chemical vapor composite (CVC^TM) SiC mandrels. These mandrels are envisioned as pre-forms for the replication of fused silica x-ray optics to be eventually used in the International X-Ray Observatory (IXO). CVC SiC^TM offers superior high temperature stability, thermal and mechanical performance and polishability required for this precision replication process. In this program, Trex fabricated prototype mandrels with design diameters of 10.5cm, 20cm and 45cm. UAH-CAO was Trex's university partner in this effort and worked on polishing and metrology of the unusual x-ray mandrel geometries. UAH-CAO successfully developed an innovative interferometric method for measuring the CVC SiC^TM x-ray mandrels based on a precision cylindrical lens system. UAH-CAO also developed finishing and polishing methods for CVC SiC^TM that utilized a Zeeko IRP200 computer controlled polishing tool. The three technologies key technologies demonstrated in this program (near net shape forming of CVC SiC^TM mandrels, the x-ray mandrel metrology and free-form polishing capability on CVC SiC^TM) could enable cost-effective manufacture of the x-ray mandrels required for the International X-Ray Observatory (IXO).

To reduce the finishing costs of silicon carbide mirror substrates, silicon claddings are applied allowing the surfaces to be more easily diamond turned and polished than the bare chemical vapor deposited (CVD) silicon carbide or bimodal reaction bonded SiC (RB-SiC). The benefits of using silicon as the optical face will be reviewed as will the process for applying plasma enhanced chemical vapor (PE-CVD) deposited amorphous silicon cladding on substrates. Using one mirror as an example, the successful finishing results will be shared.

In support of a new airborne telescope system the Multi-Spectral Telescope for a new Reconnaissance Program ECM was contracted by Carl Zeiss Optronics to produce 20 mirrors made of HB-Cesic®, each mirror with a diameter of 315 mm. The contract requirements were that the mirrors remain stable under extreme mechanical and thermal loads and that the production schedule is 12 months. The main challenges of the project were achieving consistent optical performance of the mirrors and meeting the tight schedule of delivering the mirrors, in lots of four, every two months for a total delivery time of twelve months. In this paper we present the lessons learned in producing the HB-Cesic® mirrors, including all the steps up to the final integration of the mirrors into the telescope system and establishing reliable repeatability of the production cycles.

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?

SPIRALE is a French Earth observation demonstration project consisting of two satellites. In support of the project and under contract with Thales Alenia Space, ECM manufactured two fully integrated all-Cesic® telescopes, composed of super-light-weighted complex monolithic structures, including two off-axis aspheric mirrors per telescope with integrated interfaces for mounting. The all-Cesic telescope assembly was tested under shock and vibration loads, and by exposure to realistic in-flight thermal environments. In this paper we describe the space-qualified process of manufacturing such high-precision space telescopes based on our Cesic® technology; the advantages of our Cesic® technology compared to traditional materials, such as metals or glass ceramics; and some of the test results. This project demonstrates that all-Cesic® telescopes have great potential for future space applications, especially under cryogenic conditions, due to their athermal characteristics and the great versatility of the Cesic® manufacturing process.

BAE Systems has developed an all-beryllium-aluminum version of the F-9120; a compact, lightweight, dual-band Electro-Optical/Infrared (EO/IR) long range sensor for high altitude tactical reconnaissance applications. The use of beryllium-aluminum as a common mirror and structure material provides a novel sensor solution that satisfies the military's need to gather high quality, long range, simultaneous, visible and infrared imagery at a lower cost. This paper will discuss the formulation and implementation of BAE Systems innovative material approach as well as its manufacturing and performance advantages.

The Near Infrared Camera (NIRCam) Optical Bench Assembly (OBA) is a I-220H beryllium adhesively-bonded structure designed to operate at 35K. To support design activities, an adhesive testing program was performed, with particular emphasis on adhesive allowables at 35K. The geometries of the samples were designed to emulate the structural features of the OBA. The testing program is described, test data presented, and the results applied to the NIRCam OBA.

The Near Infrared Camera (NIRCam) is the primary imaging instrument on the James Webb Space Telescope. The primary structure for NIRCam is called the Optical Bench Assembly (OBA). The OBA is a bonded Beryllium structure designed to operate at 35K. The structure has recently undergone thermal cycling to 35K followed by structural qualification vibration testing. Analytical predictions were made of the structural performance during vibration. These predictions closely matched the actual performance. This paper summarizes the build and assembly of the OBA, and focuses on the qualification thermal and structural testing of the OBA. The qualification testing is described and pre-test analysis is presented and compared with test results.

The zero-expansion glass ceramic ZERODUR® from SCHOTT is widely used for ground-based astronomical mirrors and in industrial applications. This paper points out that it is also well suited for satellite applications, especially with respect to the space radiation environment. Recent developments show that highly lightweighted components can be manufactured and that such structures are strong enough to survive launch vibrations. A series of thirty reference applications, where ZERODUR® has been or is currently used (including METEOSAT, SPOT, ROSAT, CHANDRA, and HST), demonstrate the high and long lasting performance of ZERODUR® components in orbit. The ongoing successful missions and upcoming new satellites continue to enlarge the space heritage of this unique material.

Recently SCHOTT has shown in a series of investigations the suitability of the zero expansion glass ceramic material ZERODUR® for applications like mirrors and support structures of complicated design used at high mechanical loads. Examples are vibrations during rocket launches, bonded elements to support single mirrors or mirrors of a large array, or controlled deformations for optical image correction, i.e. adaptive mirrors. Additional measurements have been performed on the behavior of ZERODUR® with respect to the etching process, which is capable of increasing strength significantly. It has been determined, which minimum layer thickness has to be removed in order to achieve the strength increase reliably. New data for the strength of the material variant ZERODUR K20® prepared with a diamond grain tool D151 are available and compared with the data of ZERODUR® specimens prepared in the same way. Data for the stress corrosion coefficient n of ZERODUR® for dry and normal humid environment have been measured already in the 1980s. It has been remeasured with the alternative double cleavage drilled compression (DCDC) method.

The pupil imaging lens (PIL) assembly is one component within the NIRCam instrument, which is the primary imaging instrument on the James Webb Space Telescope (JWST). The main purpose of the PIL assembly is to form an image of the eighteen primary mirror segments of the JWST onto the NIRCam focal plane arrays (FPAs). NIRCam is the only instrument on the JWST observatory with wave front sensing (WFS) capability, and will use the PIL in conjunction with the WFS measurements1. The purpose of this paper is to introduce the audience to a monolithic PIL assembly created from five unique lenses mounted to an optical baseplate. The lenses are assembled using a direct bonding method, with the aid of a diluted potassium hydroxide solution. Described in this paper is a simple, yet precise method for aligning and assembling the PIL optics.

A. Verneuil developed flame fusion to grow sapphire and ruby on a commercial scale around 1890. Flame fusion was further perfected by Popov in the Soviet Union in the 1930s and by Linde Air Products Co. in the U.S. during World War II. Union Carbide Corp., the successor to Linde, developed Czochralski crystal growth for sapphire laser materials in the 1960s. Edge-Defined Film-Fed Growth (EFG) was invented by H. Labelle in the 1960s and the Heat Exchanger Method (HEM) was invented by F. Schmid and D. Viechnicki in 1967. Both methods were commercialized in the 1970s. Gradient solidification was invented in Israel in the 1970s by J. Makovsky. The Horizontal Directional Solidification Method (HDSM) was invented by Kh. S. Bagdasorov in the Soviet Union in the 1960s. Kyropoulos growth of sapphire, known as GOI crystal growth in the Soviet Union, was developed by M. Musatov at the State Optical Institute in St. Petersburg in the 1970s. Today, half of the world's sapphire is produced by the GOI method.

Due to their very specific set of material properties, silicon nitride and silicon carbide have gained a lot of interest in the last 20 years. Moreover, many new approaches in technical equipment and processes were enabled with corresponding research and production activities. Also large efforts were made at FCT during the last years, to get able to supply even very large and complex shaped components made of sintered silicon carbide (SSiC) and of gas pressure sintered silicon nitride (GPSN) ceramics. This approach has opened new applications and markets for such ceramic materials. On the other side, designers and engineers are now allowed to think much more complex in designing of ceramic components. In this paper, a new rapid prototyping routine for very complex components as well as the corresponding materials will be presented. Components for optical equipment in innovative avionic and space applications, and more conventional technologies are described. Not only their unique key intrinsic properties, like high Youngs Modulus, very low CTE, very high strength and fracture toughness for a ceramic but also newly developed and adopted shaping, sintering and machining technologies in both green and sintered state have let to highly valued products. This enabled FCT to offer Carl Zeiss Optronics using silicon nitride for a newly designed, very complex housing structure of an avionic pod camera. Due to a very low CTE, high stiffness and less weight, an improved performance was reached. Also Thales Alenia Space is engaged since some years in activities to develop and qualify Silicon nitride ceramics for space projects. Extremely stiff, very lightweight and large truss space structures with a very low CTE, high rigidity and no outgasing for satellites can now be realized. Deep tests sequence has been performed to qualify truss beams and end fittings made in the same material. Also advanced dynamic testing equipment for avionic turbine blades requires new approaches. In cooperation with TIRA a series of shaker heads were developed which can operate at much higher frequencies and so reduce fatigue testing time and costs. Last but not least, highly precise and thin walled disc structures with diameters up to 380 mm are produced for wafer handling and testing equipment.

NASA's upcoming ARES V launch vehicle, with its' immense payload capacities (both volume and mass) has opened the possibilities for a whole new paradigm of space observatories. It becomes practical to consider a monolith mirror of sufficient size to permit significant scientific advantages, both in collection area and smoothness or figure at a reasonable price. The technologies and engineering to manufacture and test 8 meter class monoliths is mature, with nearly a dozen of such mirrors already in operation around the world. This paper will discuss the design requirements to adapt an 8m meniscus mirror into a Space Telescope System, both launch and operational considerations are included. With objects this massive and structurally sensitive, the mirror design must include all stages of the process. Based upon the experiences of the Hubble Space Telescope, testing and verification at both component and integrated system levels are considered vital to mission success. To this end, two different component level test methods for gravity sag (the so call zero- gravity simulation or test mount) are proposed, with one of these methods suitable for the full up system level testing as well.

The fabrication of lightweight mirror assemblages via a replication technique offers great potential for eliminating the high cost and schedule associated with the grinding and polishing steps needed for conventional glass or SiC mirrors. A replication mandrel is polished to an inverse figure shape and to the desired finish quality. It is then, coated with a release layer, the appropriate reflective layer, and followed by a laminate for coefficient of thermal expansion (CTE) tailorability and strength. This optical membrane is adhered to a mirror structural substrate with a low shrinkage, CTE tailored adhesive. Afterwards, the whole assembly is separated from the mandrel. The mandrel is then cleaned and reused for the next replication run. The ultimate goal of replication is to preserve the surface finish and figure of the optical membrane upon its release from the mandrel. Successful replication requires a minimization of the residual stresses within the optical coating stack, the curing stresses from the adhesive and the thermal stress resulting from CTE mismatch between the structural substrate, the adhesive, and the optical membrane. In this paper, the results on replicated trials using both metal/metal and ceramic/ceramic laminates adhered to light weighted structural substrates made from syntactic foams (both inorganic and organic) will be discussed.

With the increasing complexity of optical designs flown on satellites, specialty thin film and multilayer filter coatings are being implemented more often. Unfortunately, very little ionizing radiation testing has actually been performed on such coatings, and certainly not completed to very high doses as might be experienced by an unshielded space optic in orbit for many years. In this paper we present results of gamma irradiation testing on the transmission properties of selected multilayer filters and optical coatings performed in an inert argon environment.

Cerium oxides are of interest in many applications including optical coatings, solid-state fuel cells, and catalyst supports. Due to excellent absorption in ultraviolet (UV), these materials are also widely used as UV blocking layers in medical glassware and aerospace windows. In this paper, we present the fabrication and characterization of cerium oxide thin films and their potential application in UV sensing. Cerium oxides were deposited by reactive oxygen ion beam assisted e-beam evaporation. Comparing with the films obtained without ion assistance, oxygen plasma assisted deposition enhances refractive index from ~ 1.8 to ~ 2.2 and improves electrical resistivity from ~ 10⁶ Ωcm to ~ 10¹⁰ Ωcm. More importantly, the film deposited with ion assistance shows stronger blue to UV absorption observed from the transmittance spectra, which presumably is better for UV sensing. X-ray photoelectron spectroscopy and X-ray diffraction measurements further suggest insights in stoichiometry and crystal structures. A concept-proving photodiode was fabricated by employing a p⁺-Si/CeO₂/n-In₂O₃ heterostructure. The current-voltage characteristics exhibit obvious diode-like behavior with a current rectification ratio of ~ 10⁵. A low leakage current in a range of 10^-10 ~ 10^-8 A was achieved at reverse biases of 0 to 10 V, respectively. The diode demonstrates high photocurrent gain of ~ 100 times and fast photoresponse under a 405 nm UV exposure.

We report on the design, application, and testing of custom protected silver and aluminum coatings for use on the Magdalena Ridge Observatory Interferometers (MROI) unit telescopes. The coatings were designed by Optical Surface Technologies (OST), and tested under normal observational conditions on Magdalena Ridge. Mirror coating samples fabricated by OST were given to MRO, and then placed in an insulated automated enclosure at the observatory site. Within the enclosure, environmental conditions such as temperature and humidity were continuously monitored. The automated enclosure was instructed to open during the night dependent upon weather conditions matching those that would occur under normal operations of the interferometer. This paper tracks the affect of the Magdalena Ridge environment on the performance of the coatings, specifically with regards to reflectivity.

The synchrotron radiation (SR) X-ray absorption fine-structure spectroscopy (XAFS) technology was employed on Si Kedge absorption spectra for bulk 6H-SiC with different doping concentration. Their Fourier transform spectra were analyzed, which have shown a parabolic linear distribution of bond lengths. Through combined Raman and XAFS studies, the coincidental results could be obtained. In the Raman spectroscopy, the LO mode intensity becomes weaker and broader as the doping concentration increases. This indicates that the crystallinity is damaged by the heavy doping concentration. The Raman curves have been fitted by theoretical formulas and the accurate information of the intensity, peak position, and FWHM in each TO and LO modes have been obtained. By the EXAFS and the fitting program, the bond length of Si-C in 6H-SiC decreases as the doping concentration increases. It is believed to be caused by the nitrogen atoms substituting for carbon atoms in the SiC lattice. But further research work is needed to identify this. The little change in Si-Si bond length indicates the influence of doping is still under local structure, near the absorbed atoms.

This paper presents a roll-to-roll method to fabricate microlens arrays on a glass substrate by using a cost-effective PDMS (Polydimethylsiloxane) mold. We fabricated microlens arrays mold, which was made by photoresist(AZ4620), on the silicon substrate by thermal reflow process, and transferred the pattern to PDMS film. Roll-to-roll system is a standard printing process whose roller is made of acrylic cylinder surrounded with the PDMS mold. UV resin was chosen to be the material to make microlens in rolling process with UV light curing. We investigated the quality of microlens arrays by changing the parameters, such as embossing pressure and rolling speed, to ensure good quality of microlens arrays.