Optical Physics Company (OPC) has developed under SBIR Phase II funding an advanced laser pointing sensor system (LPSS) for future integration into the NASA Integrated Radio and Optical Communications (iROC) program. This would enable deep space pointing of high rate communications terminals to earth terminals via beaconless lasercom. The heart of the LPSS is OPC’s patented interferometric star tracker which can provide better than 300 nrad pointing accuracy. This paper will report on the LPSS technology and the upcoming ground field testing to verify the pointing accuracy required for deep space high rate laser communications.
The Air Force is interested in high stiffness, lightweight technologies for beam control systems. The corrected system
wavefront error can be minimized using low figure error/surface finish, low print-through, high-stiffness, Silicon
Lightweight Mirror Systems (SLMSTM) technology with high-reflectivity, very low absorption (VLA) coatings. We
report on the fabrication of an F/1.0 mirror/mount weighing 26 pounds and with a first mode in excess of 1 kHz.
The Terrestrial Planet Finder mission requires extreme dynamic stability at cryogenic temperatures in order to carry out its objectives of searching for and observing extraterrestrial planets. As a result, the ability to meet its ambitious science goals will be significantly enhanced by increasing its vibrational damping at cryogenic temperatures. Given the low inherent structural damping at cryogenic temperatures, significant reduction in vibration amplitude could be gained with only modest increases in damping on the structure. To examine the use of vibrational damping options to improve the dynamic stability of cryogenic structures, Jet Propulsion Laboratory has conducted a series of experiments to measure the damping levels of various materials at cryogenic temperatures and to search for the materials with higher cryogenic damping. This paper summarizes our experimental observations on the material damping of silicon foam and silicon carbide foam materials at cryogenic temperatures. These foam materials have been independently developed by Schafer Corporation and have properties that enable their applications in space environments with a range of temperature from 25K to 500K. These materials have been used for mirrors, and uses for foam based structures such as optical mounts and benches are currently in development. As observed from the measured damping, these two foam materials have higher damping than aluminum at cryogenic temperatures, and the damping level is relatively insensitive to temperature change from room to cryogenic temperatures. As a result, these materials may be potential candidates to achieve increased levels of cryogenic damping for the Terrestrial Planet Finder mission.
Silicon and silicon carbide foams provide the lightweighting element for Schafer Corporation's silicon and silicon carbide lightweight mirror systems (SLMSTM and SiC-SLMSTM). SLMSTM and SiC-SLMSTM provide the enabling technology for manufacturing lightweight, athermal optical sub-assemblies and instruments. Silicon and silicon carbide foam samples were manufactured and tested under a Schafer-funded Internal Research and Development program in various configurations to obtain mechanical and thermal property data. The results of the mechanical tests that are reported in this paper include Young's modulus, compression strength, tensile strength, Poisson's ratio and vibrational damping. The results of the thermal tests include thermal conductivity and coefficient of thermal expansion.
Schafer has demonstrated two different methods for actively cooling our Silicon Lightweight Mirror System (SLMSTM) technology. Direct internal cooling was accomplished by flowing liquid nitrogen through the continuous open cell core of the SLMSTM mirror. Indirect external cooling was accomplished by flowing liquid nitrogen through a CTE matched Cesic® square-tube manifold that was bonded to the back of the mirror in the center. Testing was done in the small 4-
foot thermal/vacuum chamber located at the NASA/MSFC X-Ray Calibration Facility. Seven thermal diodes were located over the front side of the 5 inch diameter mirror and one was placed on the outlet side of the Cesic® manifold. Results indicate that the mirror reaches steady state at 82K in less than four minutes for both cooling methods. The maximum temperature difference of the eight diodes was less than 200 mK when the mirror was internally cooled and covered with MLI to insulate it from the large 300 K aluminum plate that was used to mount it.
This paper discusses the design, manufacture and cryogenic testing of a Lightweight Athermal SLMS Innovative Telescope (LASIT) under a Schafer funded Internal REsearch and Development program. The 25 cm aperture, 10X magnification LASIT is of Cassegrain design with structural components manufactured from carbon fiber reinforced silicon carbide (Cesic), while the primary mirror uses silicon lightweight mirror system (SLMS) technology. A fourteen pound, dimensionally stable telescope is the result. LASIT cryogenic testing will be performed at NASA/MSFC under Schafer's Space Act Agreement later this year.
SLMS athermal technology has been demonstrated in the small 4-foot helium cryogenic test chamber located at the NASA/MSFC X-Ray Calibration Facility (XRCF). A SLMS Ultraviolet Demonstrator Mirror (UVDM) produced by Schafer under a NASA/MSFC Phase I SBIR was helium cryo tested both free standing and bonded to a Schafer designed prototype carbon fiber reinforced silicon carbide (Cesic) mount. Surface figure data was obtained with a test measurement system that featured an Instantaneous Phase Interferometer (IPI) by ADE Phase Shift. The test measurement system's minimum resolvable differential figure deformation and possible contributions from test chamber ambient to cryo window deformation are under investigation. The free standing results showed differential figure deformation of 10.4 nm rms from 295K to 27K and 3.9 nm rms after one cryo cycle. The surface figure of the UVDM degraded by lambda/70 rms HeNe once it was bonded to the prototype Cesic mount. The change was due to a small astigmatic aberration in the prototype Cesic mount due to lack of finish machining and not the bonding technique. This effect was seen in SLMS optical assembly results, which showed differential figure deformation of 46.5 nm rms from 294K to 27K, 42.9 nm rms from 294K to 77K, 28.0 nm rms from 294K to 193K and 6.2 nm rms after one cryo cycle.
A lightweight athermal optical system has been demonstrated using silicon lightweight mirrors (SLMS) manufactured by Schafer Corporation, mounted in carbon fiber reinforced silicon carbide mounts (C/SiC or Cesic) produced by IABG and ECM. SLMS are quickly and inexpensively super-polishable (figure of 0.021 waves rms @ 633 nm, 4 Å rms surface finish), stiff (first mode greater than 1500 Hz), lightweight (areal density <10 kg/m2), have superior thermal properties at cryogenic temperatures, and do not out-gas. Under a 1998 NASA Phase I SBIR, Schafer and IABG engineered the so-called A-3 formulation of Cesic, which has a near-perfect CTE match with silicon over the temperature range of 8-300 K, making it the ideal material to athermally mount SLMS. This paper presents results of the cryogenic testing of a 6-inch diameter flat SLMS-Cesic mount assembly at NASA GSFC from room temperature to 80K and back again.
Schafer Corporation is manufacturing silicon lightweight mirrors (SLMS) for both cryogenic and high-energy laser applications. SLMS are quickly and inexpensively super- polishable, stiff and lightweight, have superior properties at cryogenic temperatures, and do not out-gas. This paper presents results of the cryogenic testing of a 6-inch diameter flat SLM at NASA GSFC. Testing was done initially with the SLM integrated into a Schafer designed, Industrieanlagen-Betriebsgesellschaft (IABG) produced carbon fiber reinforced silicon carbide (C/SiC) optical mount and then later, with the SLM attached directly to the cryostat cold finger. The advantage in using a C/SiC optical mount is that the so-called A-3 formulation has a near-perfect CTE match with silicon.
For complex shaped, lightweight, high precision opto- mechanical structures that must operate in adverse environments and over wide ranges of temperature, we consider IABG's optical grade silicon carbide composite ceramic (C/SiC) as the material of choice. C/SiC employs conventional NC machining/milling equipment to rapidly fabricate near-net shape parts, providing substantial schedule, cost, and risk savings for high precision components. Unlike powder based SiC ceramics, C/SiC does not experience significant shrinkage during processing, nor does it suffer from incomplete densification. If required, e.g. for large-size components, a fully-monolithic ceramic joining technique can be applied. Generally, the thermal and mechanical properties of C/SiC are tunable in certain ranges by modifying certain process steps. This paper focuses on the thermo-mechanical performance of new, high precision mounts designed by Schafer Corporation and manufactured by IABG. The mounts were manufactured using standard optical grade C/SiC (formulation internally called A-3). The A-3 formulation has a near-perfect CTE match with silicon, making it the ideal material to athermally support Schafer produced Silicon Lightweight Mirrors (SLMs) that will operate in a cryogenic environment. Corresponding thermo- mechanical testing and analysis is presented in this manuscript.
We contend that carbon fiber reinforced silicon carbide material (C/SiC), developed by IABG, represents the state- of-the-art for ultra-lightweight, high precision optomechanical structures that must operate in adverse environments and over wide ranges of temperature. C/SiC employs conventional NC machining/milling equipment to rapidly fabricate near-net shape parts, providing substantial schedule, cost, and risk savings for high precision components. Unlike power based SiC ceramics, C/SiC does not experience significant shrinkage during processing, nor does it suffer from incomplete densification. By modifying certain process steps, the thermal and mechanical properties of C/SiC are tunable in certain ranges. This paper focuses on recent advances in C/SiC technology and application of this technology to high precision, lightweight applications such as meter-class optics and optical mounts. We also introduce a design for new, high precision mounts based upon standard optical grade C/SiC (formulation A-3) and a custom formulation of C/SiC (D-4) which was engineered for Schafer Corporation by IABG. The A- 3 and D-4 formulations have a near-perfect CTE match with silicon, making them the ideal material to athermally support ultra-lightweight silicon optics that will operate in a cryogenic environment.
Silicon offers significant advantages over other optical substrate materials such as beryllium, silicon carbide and glass for both cryogenic and high-energy laser applications. Silicon is quickly and inexpensively super-polishable (surface figure < (lambda) 10 p-v at (lambda) equals 632.8 nm; surface roughness < 5 angstroms rms), has superior thermal properties at cryogenic temperatures, and can be lightweighted. This paper updates our progress towards producing dimensionally stable ultra-lightweight silicon optics for both cryogenic instruments and high-energy infrared laser systems. We review cryogenic figure test results for three-inch diameter coupons, present analysis results for a half-meter diameter silicon foam-core mirror and tell how these results apply to a Silicon Lightweight Demonstration Mirror, and describe optics being designed for an Offner Relay System.
This paper describes the design, fabrication and cryogenic testing of lightweight silicon mirrors. Silicon offers significant advantages over other optical substrate materials. It has superior thermal properties at cryogenic temperatures, is quickly and inexpensively super-polishable, and is comparably lightweight to other substrates such as beryllium, silicon carbide, graphite epoxy and carbon. both bonded single crystal silicon (SCSi) and lightweight composite mirrors have been produced. The bonded mirrors were fabricated from two SCSi half disks joined using a proprietary process to form a slightly oblong three-inch diameter mirror. The nominally three-inch diameter composite mirrors consists of a silicon foam core, with SCSi faceplates bonded to the front and back surfaces. All optics were tested at the JPL's Cryogenic Test Facility. They were mounted in a custom and proprietary OFHC copper test fixture designed to prevent figure loss over the range of test temperature, and provide excellent thermal contact with the cryogenic chamber's cold plate. Results so far indicate that the bonded mirrors maintain their ambient figure at temperatures down to -183 degrees C.
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