Iwona A. Palusinski

Sag and phase descriptions for null corrector certifiers

Null correctors are important in the fabrication of aspheric optics. Although null correctors simplify testing of aspheric surfaces, they can increase the risk of fabricating aspheric surfaces. Undetected errors in the null corrector will result in an aspheric surface that does not meet design specifications. To prevent such problems, a null corrector is certified prior to using it to test an aspheric surface during fabrication. Certification verifies that the null corrector has been produced properly. Current methods of certification that include computer-generated holograms and diamond-turned mirrors require a mathematical description of the certifier. We provide sag and phase formulas for describing such certifiers. The formulas can be used to define certifiers for a broad range of aspheric surfaces. For each aspheric test surface, the specific certifier parameters are found through optimization using default merit functions. Via two different aspheric surfaces, we demonstrate the usefulness and simplicity of our truncated-series sag and phase formulas to define certifiers.

Developing SiC for optical system applications

Future large-aperture optical space systems will need to use lightweight materials that meet stringent requirements, and that reduce program and launch costs. Lightweight optical systems produced quickly and cost-effectively, and the resultant lighter payloads, can reduce these costs. Mirrors for future systems have areal density goals of less than 5 kg/m2 and will need to use new materials1. A promising one is silicon carbide (SiC) because of its physical and mechanical properties. These enable the production of low areal density, high quality mirrors, as well as lightweight athermal telescope structures. Athermal structures are desirable because they simplify designs and reduce tolerance requirements to maintain performance during on-orbit temperature changes. The use of SiC to make mirrors and structures is in the developmental stage and has limited space heritage. To ensure the use of this material in space applications, qualification and system performance in the space environment must be addressed. This paper provides an overview of SiC, along with recommendations to further the development of SiC into a mature technology that can be successfully integrated into future large-aperture optical space programs.

Influence of low-earth orbit exposure on the mechanical properties of silicon carbide

The influence of the Low-Earth orbit (LEO) environment on the mechanical strength of silicon carbide (SiC) was evaluated on two flight experiments as part of the Materials on the International Space Station Experiment (MISSE). SiC samples for modulus of rupture (MOR) and equibiaxial flexural strength (EFS) testing were flown on the Optical and Reflector Materials experiments (ORMatE) as part of MISSE-6 (launched on STS-123, March 2008; returned on STS-128, September 2009) and MISSE-7 (launched on STS-129, November 2009; returned on STS- 134, June 2011). Two different SiC vendors provided material for each flight experiment. The goal of the experiments was to measure mechanical properties of the flight samples and compare them to an equal number of similar samples in control and traveler sample sets. Complete characterization of the strength of brittle materials typically requires many more test specimens than could be reasonably accommodated on the ORMatE sample tray and statistical models based on few samples include large uncertainties. Understanding the results of the mechanical tests of MISSE samples required comparison to results from a statistically valid number of samples. Prior testing by The Aerospace Corporation of material supplied by the same four vendors was used to evaluate the MISSE results, including flight and control samples. The results showed that exposure to LEO over the durations covered by MISSE 6 and 7 (approximately 18 and 20 months, respectively) did not alter the mechanical strength of the silicon carbide for any of the vendors’ materials.

NDE methods for determining the materials properties of silicon carbide plates

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 Youngs Modulus and calculate the thickness profile and Poissons Ratio of the plates. The Youngs 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 Poissons 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.

Material testing of silicon carbide mirrors

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.

Optical Reflector Materials Experiment‐I (ORMatE‐I) and ORMatE‐II on Board MISSE

The first Optical Reflector Materials Experiment (ORMatE‐I) is on‐board MISSE‐6. The follow‐on experiment, ORMatE‐II, is part of MISSE‐7. Both these projects are a collaborative effort among The Aerospace Corporation, the US Naval Research Laboratory (NRL), and the Air Force Research Laboratory Materials Directorate (AFRL/ML). ORMatE‐I is a study of optically reflective materials focused on SiC for use as a lightweight mirror substrate. Several types of SiC material grown by different methods and vendors are included as well as diverse coating materials and deposition techniques. Advanced glass substrate technologies, like ULE and corrugated borosilicate, are also on‐board. Additional SiC and composite materials will be evaluated on ORMatE‐II along with silver mirrors deposited by various means. A description of both experiment suites and a summary of the pre‐flight optical characterization will be presented.

Development of a systematic approach to space qualification of silicon carbide for mirror applications

Over the last few years significant progress has been made in the development of silicon carbide (SiC) for mirror applications. These improvements include lightweighting techniques, higher production yields, and larger diameter apertures. It is now necessary to evaluate and address the systems engineering challenges facing this material to ensure space qualification and integration into future space applications. This paper highlights systems engineering challenges, suggests areas of future development, and proposes a systematic path forward that will outline necessary steps to space qualify this new material.