Free-space laser communication systems are increasingly implemented on state of the art satellites for their high-speed connectivity. This work outlines a demonstration of the Modular, Agile, Scalable Optical Terminal (MAScOT) we have developed to support Low-Earth Orbit (LEO) to deep-space communication links. In LEO, the MAScOT will be implemented on the International Space Station to support the Integrated Laser Communications Relay Demonstration (LCRD) LEO User Modem and Amplifier Terminal (ILLUMA-T) program. ILLUMA-T's overarching objective is to demonstrate high bandwidth data transfer between LEO and a ground station via a geosynchronous (GEO) relay satellite. Outside of LEO, the MAScOT will be implemented on the Artemis-II mission to demonstrate high data rate optical communications to and from the moon as part of the Optical to Orion (O2O) program. Both missions leverage the same modular architecture despite varying structural, thermal, and optical requirements. To achieve sufficient performance, the terminal relies on a nested tracking loop to realize sub-arcsecond pointing across a ±120 ° elevation and ±175° azimuth field of regard.
During qualification thermal testing of the Transiting Exoplanet Survey Satellite (TESS) lens assemblies, an anomalous focus change was observed during thermal-vacuum testing at its cold operating temperature (-75°C to - 85°C). Optical testing of the lens assemblies performed in the thermal-vacuum chamber indicated the magnitude and direction of the focus change, but did not identify the specific changes in the lens elements that were causing the focus shift. Individual lens motions measured using an interferometer indicated that lens vertices were moving relative to one another in a way that was inconsistent with predictions from detailed structural/thermal/optical (STOP) modeling. Further STOP analysis indicated the focus and vertex motion data were consistent with changes in lens curvature, suggesting that radial forces were deforming the lens. Finite element modeling showed that material property changes in the silicone adhesive material (room temperature vulcanizing, RTV 566) used to bond the lenses to the aluminum bezels could produce the necessary forces. The root cause of the focus shift was suspected to be unanticipated crystallization of the RTV 566 which has not previously been documented. Despite its widespread use, very little information has been published about the mechanical properties of RTV 566, and typical thermomechanical testing of its behavior has always utilized temperature sweeps. For this investigation, extensive testing was performed to characterize material property changes of RTV 566 samples under isothermal conditions at low temperatures (-75 to -85°C), for extended periods and at different levels of applied strain. The data presented here provide conclusive evidence that RTV 566 experiences time-dependent changes in mechanical properties that are consistent with crystallization phenomena.
Grazing-incidence optics for X-ray applications require extremely smooth surfaces with precise mirror figures to provide well focused beams and small image spot sizes for astronomical telescopes and laboratory test facilities. The required precision has traditionally been achieved by time-consuming grinding and polishing of thick substrates with frequent pauses for precise metrology to check the mirror figure. More recently, substrates with high quality surface finish and figures have become available at reasonable cost, and techniques have been developed to mechanically adjust the figure of these traditionally polished substrates for ground-based applications. The beam-bending techniques currently in use are mechanically complex, however, with little control over mid-spatial frequency errors. AOA-Xinetics has been developing been developing techniques for shaping grazing incidence optics with surface-normal and surface-parallel electrostrictive Lead magnesium niobate (PMN) actuators bonded to mirror substrates for several years. These actuators are highly reliable; exhibit little to no hysteresis, aging or creep; and can be closely spaced to correct low and mid-spatial frequency errors in a compact package. In this paper we discuss recent development of adaptive x-ray optics at AOA-Xinetics.
Grazing-incidence optics for X-ray applications require extremely smooth surfaces with precise mirror figures to provide well focused beams and small image spot sizes for astronomical telescopes and laboratory test facilities. The required precision has traditionally been achieved by time-consuming grinding and polishing of thick substrates with frequent pauses for precise metrology to check the mirror figure. More recently, substrates with high quality surface finish and figures have become available at reasonable cost, and techniques have been developed to mechanically adjust the figure of these traditionally polished substrates for ground-based applications. The beam-bending techniques currently in use are mechanically complex, however, with little control over mid-spatial frequency errors. AOA-Xinetics has been developing been developing techniques for shaping grazing incidence optics with surface-normal and surface-parallel electrostrictive Lead magnesium niobate (PMN) actuators bonded to mirror substrates for several years. These actuators are highly reliable; exhibit little to no hysteresis, aging or creep; and can be closely spaced to correct low and mid-spatial frequency errors in a compact package. In this paper we discuss recent development of adaptive x-ray optics at AOAXinetics.
The search for extrasolar habitable planets is one of
three major astrophysics priorities identified for the next decade.
These missions demand very high performance visible-wavelength
optical imaging systems. Such high performance
space telescopes are typically extremely expensive and can be
difficult for government agencies to afford in today's economic
climate, and most lower cost systems offer little benefit because
they fall short on at least one of the following three key
performance parameters: imaging wavelength, total system-level
wavefront error and aperture diameter. Northrop
Grumman Xinetics has developed a simple, lightweight, low-cost
telescope design that will address the near-term science
objectives of this astrophysics theme with the required optical
performance, while reducing the telescope cost by an order of
magnitude. Breakthroughs in SiC mirror manufacturing,
integrated wavefront sensing, and high TRL deformable mirror
technology have finally been combined within the same
organization to offer a complete end-to-end telescope system in
the lower end of the Class D cost range. This paper presents
the latest results of real OAP polishing and metrology data, an
optimized optical design, and finite element derived WFE
Large optics for space will undergo a revolutionary change in the early 21st century. Conventional discrete manufacturing technology will be replaced by an integrated materials approach conducted on the mesoscale level. Integrated Zonal Meniscus is a revolutionary design and manufacturing approach for ultra-lightweight active mirrors that departs radically from the prevailing composite mirror designs and optical processing methods. The integration of actuators, sensors and electronics directly into a silicon carbide thin meniscus mirror substrate has decided advantages over conventional passive and active mirror configurations. The mirror is given the ability to compensate for optical wavefront errors in a configuration that reduces the overall areal density below 10 kg/m2. Our design and manufacturing approach are scaleable to 8-meter class mirrors and beyond.
First and foremost, an optical material must be able to be cost effectively made to the desired size and shape and then be polished to optical tolerances. Over the past 15 years, reaction bonded silicon carbide has been scaled from 2 inches to 1.2 meters, improved in finish from 100 Å to 10 Å and been reduced in areal density from 40 kg/m2 to 10 kg/m2. Its low thermal distortion, high stiffness, high optical quality, and its dimensional stability make CERAFORM SiC ideal for applications such as high energy laser mirrors, space-borne cryogenic mirrors, fast response scan mirrors, and high heat flux applications. In addition CERAFORM SiC products are fabricated using a cost-effective, net shape, fugitive core casting process which can be used to make complex, open or closed back, lightweight substrates. Silicon carbide is unique in that it is competitive with beryllium as a structural material, glass as an optical material, and Invar or graphite-epoxy as a metering material. This paper details the progress that has been made towards scaling facilities and optics to 2-meters and 2 Kg/m2 areal density.
Over the past decade, the principle focus in the lead magnesium niobate (PMN) family of relaxor ferroelectric ceramics has been on the electrostrictive formulation due mainly to its application in deformable optics. Electrostrictive response of the PMN material is very low in hysteresis, contains near-zero creep, and features subnanometer precision. Temperature dependence is its major drawback. Highly desirable is a material which has the broad temperature response of piezoelectric lead-zirconate- titanate with the stable, accurate response of electrostrictive PMN. In this paper we report the development of novel piezoelectric and composite formulations of PMN which provide temperature stabilized response with moderate hysteresis and enhanced structural response with high authority.
Recent advances in polishing the bare CERAFORM SiC surface to finishes as smooth as 1OA rms has enhanced the viability of SiC as a mirror material those applications requiring thermal stability over a broad temperature range. In addition PVD silicon claddings have been developed to provide a low cost polishing option for more environments which are less severe. With the ability to make complex shapes in sizes up to 1 meter, CERAFORM SiC provides a cost-effective alternative to beryllium and glass.
Silicon carbide mirrors have been made ultra lightweight with an areal density below 10 kg/mm2 and have been made in sizes as large as 1.2 meters in diameter. The CERAFORM SiC process provides a cost-effective means to make lightweight substrates in either the open back or closed back form. Optical finishes below 10 angstrom rms have been achieved on both the chemical vapor deposited beta phase and the silicon infiltrated alpha phase. COmplex structures with triangular, square, or hexagonal core geometry has been produced with web thicknesses as thin as 0.015 inches and depth to diameter aspect ratios as large as 50:1. By designing to specific sectional stiffness, SiC offers performance which exceeds that of beryllium and glass, especially in extreme thermal environments. By polishing bare CERAFORM SiC to better than 10 angstrom rms, the single greatest impediment to SiC being used as an optical material was resolved.
The cost of good, reliable deformable mirrors has made them inaccessible to the astronomical community and other would-be commercial users. Historically, the deformable mirror has cost nearly $DOL2500. per channel and comprised about 25% of the cost of an adaptive optics system. Xinetics has created a deformable mirror technology which costs less than $DOL1000. per channel including the electronic driver. In addition, the mirrors feature novel actuator cartridges which contain a simple thermal mechanism to allow simple replacement of damaged actuators. The current mirrors use piezoelectric actuators for broad temperature response and operate at 100 volts. They feature 7-mm interactuator spacing, 4- micrometers displacement, and interactuator coupling tailorable between 5 to 15%. Complete NASTRAN and Weibull analyses has been conducted to insure sufficient safety margin. The results of the 37, 97, and 349 channel mirror manufacturing efforts are reported in this paper.
KEYWORDS: Mirrors, Actuators, Glasses, Optical alignment, Hubble Space Telescope, Optical benches, Packaging, Space telescopes, Space mirrors, Atomic force microscopy
The correctability of the primary mirror spherical error in the Wide Field/Planetary Camera (WF/PC) is sensitive to the precise alignment of the incoming aberrated beam onto the corrective elements. Articulating fold mirrors that provide +/- 1 milliradian of tilt in 2 axes are required to allow for alignment corrections in orbit as part of the fix for the Hubble space telescope. An engineering study was made by Itek Optical Systems and the Jet Propulsion Laboratory (JPL) to investigate replacement of fixed fold mirrors within the existing WF/PC optical bench with articulating mirrors. The study contract developed the base line requirements, established the suitability of lead magnesium niobate (PMN) actuators and evaluated several tilt mechanism concepts. Two engineering model articulating mirrors were produced to demonstrate the function of the tilt mechanism to provide +/- 1 milliradian of tilt, packaging within the space constraints and manufacturing techniques including the machining of the invar tilt mechanism and lightweight glass mirrors. The success of the engineering models led to the follow on design and fabrication of 3 flight mirrors that have been incorporated into the WF/PC to be placed into the Hubble Space Telescope as part of the servicing mission scheduled for late 1993.
The Articulating Fold Mirror (AFM) for the Wide Field/Planetary Camera-II (WF/PC-II) instrument is a very compact, complicated, highly precise mechanism. The AFM's basic function is to provide tip and tilt correction in the optical paths of the WF/PC-II instrument. Its necessity is brought about indirectly by the spherical aberration of the primary mirror in the Hubble Space Telescope (HST). Many challenges are created by the necessity of the new mechanism in the optical design. (1) The new mechanism must exhibit high precision in the placement of the mirror surface in two rotations (tip and tilt). (2) The available packaging volume for the AFM is very shallow and requires an innovative approach to achieve the necessary performance requirements. (3) The schedule for delivery of the flight certified AFM's is extremely tight, and as such does not allow for any failures during the qualification phase of the AFM project. Structural design and analysis plays a major role in meeting the stringent performance requirements within the schedule and fiscal constraints. The final result is a qualified mechanism which meets or surpasses all of its requirements.
The ISOFLOW cooled mirror technology was developed at Itek with the goal of producing a high performance heat exchanger with emphasis on coolant efficiency and low jitter. Design and analysis capabilities, as well as manufacturing processes were developed and demonstrated in 8.0 inch diameter mirrors made from single crystal silicon, silicon carbide, and Ultra Low Expansion glass. The ISOFLOW mirror design has been tested and has demonstrated excellent thermal distortion for very low coolant flowrates and pressure drops. The ISOFLOW mirrors have been polished to better than 3 angstroms rms. The mirrors are bonded with a composite lead borosilicate glass frit blended to match the heat exchanger materials. The heat exchanger was designed to operate over a wide flow range with water or ammonia as the coolant. The high performance turbulent flow regime requires 20 gpm with a 40 psi pressure drop through the mirror. The mirror can also provide low flow low jitter operation with 6.6 gpm and 4.5 psi pressure drop. The demonstrated thermal performance obtained with low flowrates and pressure drops is what makes the ISOFLOW design unique.
A wide variety of deformable mirror structures have been studied for wavefront correction since the advent of adaptive optics nearly two decades ago. These structures generally fall into two categories: (1) segmented facesheet and (2) continuous facesheet. The segmented mirror technology features independently activated mirror elements controlled in the piston, tip, and tilt modes. The continuous facesheet designs use discrete electroceramic or electrostatic displacement actuators arranged in either an axial or bimorph position to bend the continuous facesheet. In addition there are two methods of correction: (1) zonal control and (2) modal control. The basic mirror types are discussed and analyzed in terms of wavefront correction capabilities. Curve fitting characteristics are explained in terms of the optical influence function and mirror meshing functions. The continuous facesheet deformable mirror is used as a model to develop basic design equations which are used for parametric trades.
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