Present space-based optical imaging sensors are expensive. Launch costs are dictated by weight and size, and system design must take into account the low fault tolerance of a system that cannot be readily accessed once deployed. We describe the design and first prototype of the space-based infrared imaging interferometer (SIRII) that aims to mitigate several aspects of the cost challenge. SIRII is a six-element Fizeau interferometer intended to operate in the short-wave and midwave IR spectral regions over a 6×6 mrad field of view. The volume is smaller by a factor of three than a filled-aperture telescope with equivalent resolving power. The structure and primary optics are fabricated from light-weight space-qualified carbon fiber reinforced polymer; they are easy to replicate and inexpensive. The design is intended to permit one-time alignment during assembly, with no need for further adjustment once on orbit. A three-element prototype of the SIRII imager has been constructed with a unit telescope primary mirror diameter of 165 mm and edge-to-edge baseline of 540 mm. The optics, structure, and interferometric signal processing principles draw on experience developed in ground-based astronomical applications designed to yield the highest sensitivity and resolution with cost-effective optical solutions. The initial motivation for the development of SIRII was the long-term collection of technical intelligence from geosynchronous orbit, but the scalable nature of the design will likely make it suitable for a range of IR imaging scenarios.
Aperture synthesis imaging techniques using an interferometer provide a means to achieve imagery with spatial resolution equivalent to a conventional filled aperture telescope at a significantly reduced size, weight and cost, an important implication for air- and space-borne persistent observing platforms. These concepts have been realized in SIRII (Space-based IR-imaging interferometer), a new light-weight, compact SWIR and MWIR imaging interferometer designed for space-based surveillance. The sensor design is configured as a six-element Fizeau interferometer; it is scalable, light-weight, and uses structural components and main optics made of carbon fiber replicated polymer (CFRP) that are easy to fabricate and inexpensive. A three-element prototype of the SIRII imager has been constructed. The optics, detectors, and interferometric signal processing principles draw on experience developed in ground-based astronomical applications designed to yield the highest sensitivity and resolution with cost-effective optical solutions. SIRII is being designed for technical intelligence from geo-stationary orbit. It has an instantaneous 6 x 6 mrad FOV and the ability to rapidly scan a 6x6 deg FOV, with a minimal SNR. The interferometric design can be scaled to larger equivalent filled aperture, while minimizing weight and costs when compared to a filled aperture telescope with equivalent resolution. This scalability in SIRII allows it address a range of IR-imaging scenarios.
Radiation effects are well know to cause significant degradation in polymer materials. Low earth orbit (LEO) radiation exposures cause ionization potentials that can undermine mechanical properties of polymers. In particular, small scale degradations can undermine carbon / polymer composite mirrors used in imaging applications. High specularity surface finish is required for optical mirrors and that surface is vulnerable to radiation ionization degradation thereby undermining the optical performance of the mirror in that environment. Experiments involving radiation ionization and its effects on replicated carbon/polymer composite mirrors will be will be discussed; 6 replicated carbon/polymer composite mirrors on the Materials on the International Space Station Experiment, MISSE 7 and MISSE 8, the replicated RICH mirror the Alpha Magnetic Spectrometer (AMS-02) and testing on the RICH 1 replicated mirrors in the LHCb experiment. Results are favorable for optically coated composite mirrors in terms of mirror figure, reflectivity and surface finish, but no so on uncoated polymer mirrors.
The Adaptive Optics Center of Excellence for National Security at the Naval Postgraduate School has implemented a technology testing platform and array of facilities for next-generation space-based telescopes and imaging system development. The Segmented Mirror Telescope is a 3-meter, 6 segment telescope with actuators on its mirrors for system optical correction. Currently, investigation is being conducted in the use of lightweight carbon fiber reinforced polymer structures for large monolithic optics. Advantages of this material include lower manufacturing costs, very low weight, and high durability and survivability compared to its glass counterparts. Design and testing has begun on a 1-meter, optical quality CFRP parabolic mirror for the purpose of injecting collimated laser light through the SMT primary and secondary mirrors as well as the following aft optics that include wavefront sensors and deformable mirrors. This paper will present the design, testing, and usage of this CFRP parabolic mirror and the current path moving forward with this ever-evolving technology.
For the passed several years, the Naval Research Laboratory (NRL) has
been investigating the use of Carbon Fiber Reinforced Polymer (CFRP)
material in the construction of a telescope assembly including the optical
components. The NRL, Sandia National Laboratories (SNL), and
Composite Mirror Applications, Inc. (CMA) have jointly assembled a
prototype telescope and achieved “first light” images with a CFRP 0.4 m
aperture telescope. CFRP offers several advantages over traditional
materials such as creating structures that are lightweight and low
coefficient of thermal expansion and conductivity. The telescope’s
primary and secondary mirrors are not made from glass, but CFRP, as
well. The entire telescope weighs approximately 10 kg while a typical
telescope of this size would weigh quite a bit more. We present the
achievement of “first light” with this telescope demonstrating the imaging
capabilities of this prototype and the optical surface quality of the mirrors
with images taken during a day’s quiescent periods.
We report on the optical performance of carbon fiber reinforced polymer composite (CFRP) mirrors after 1.49 years exposure onboard the Materials International Space Station Experiment (MISSE 7). Mirror samples were placed on the MISSE 7A tray, outside the ISS from October 2009 and retrieved September 2011. The environment was an extreme environment exposure test, which is considered “worst case” for survivability of composite mirrors for imaging applications in low-earth orbit (LEO) The results from testing the returned flight samples show degradation in two of the mirror’s aluminum coatings. However, the surface figure of one of the coated mirrors remained largely unchanged after the long-duration experiment. Test results will be compared against the original, pre-flight mirror performance for each of the 3 samples.
An active reflective component can change its focal length by physically deforming its reflecting surface. Such elements exist at small apertures, but have yet to be fully realized at larger apertures. This paper presents the design and initial results of a large-aperture active mirror constructed of a composite material called carbon fiber reinforced polymer (CFRP). The active CFRP mirror uses a novel actuation method to change radius of curvature, where actuators press against two annular rings placed on the mirror’s back. This method enables the radius of curvature to increase from 2000mm to 2010mm. Closed-loop control maintains good optical performance of 1.05 waves peak-to-valley (with respect to a HeNe laser) when the active CFRP mirror is used in conjunction with a commercial deformable mirror.
We describe the construction and application of innovative deformable mirrors for adaptive optics (AO) being developed
at the University of Arizona's Center for Astronomical Adaptive Optics. The mirrors are up to 1 m in diameter, with high
actuator stroke, and are optically powered. Scientific motivations for the work include the detection of earthlike planets
around other nearby stars, as well as non-astronomical applications such as directed energy and horizontal imaging for
defense and security. We describe how high resolution imaging is delivered over an unusually wide field of view by
ground-layer AO. This technique employs multiple laser guide stars to sense the instantaneous three-dimensional
distribution of atmospheric turbulence. Imaging with high signal-to-noise ratio in the thermal infrared is enabled by
embedding the deformable mirror directly in the telescope. We also describe recent work to develop a new generation of
these mirrors with lighter weight and improved robustness by use of replicated composite materials which shows
promise for greatly reducing the cost of AO and broadening its appeal, particularly for non-astronomical applications as
well as for a new generation of extremely large ground-based telescopes of 30 m diameter now under construction.
The Naval Research Laboratory and Sandia National Laboratories have been actively researching
the use of carbon fiber reinforced polymer material as optical elements in many optical systems.
Active optical elements can be used to build an optical system capable of changing is optical
zoom. We have developed a two-element optical system that uses a large diameter, thin-shelled
carbon fiber reinforced polymer mirror, actuated with micro-positioning motors, and a high
actuator density micro-electro-mechanical deformable mirror. Combined with a Shack-Hartmann
wavefront sensor, we have optimized this actuated carbon fiber reinforced polymer deformable
mirror's surface for use with a forthcoming reflective adaptive optical zoom system. In this paper,
we present the preliminary results of the carbon fiber reinforced polymer deformable mirror's
surface quality and the development of the actuation of it.
Thin-shelled composite mirrors have been recently proposed as both deformable mirrors
for aberration correction and as variable radius of curvature mirrors for phase diversity,
auto focus, and adaptive optical zoom. The requirements of actuation of a composite
mirror far surpass those for MEMS deformable mirrors. This paper will discuss the
development of a finite element model for a 0.2 meter carbon fiber reinforced polymer
mirror for use as a variable radius of curvature mirror in conjunction with a MEMS
deformable mirror for aberration correction.
Carbon-fiber reinforced polymer (CFRP) composite is an attractive material for fabrication of optics due to its high
stiffness-to-weight ratio, robustness, zero coefficient of thermal expansion (CTE), and the ability to replicate multiple
optics from the same mandrel. We use 8 and 17 cm prototype CFRP thin-shell deformable mirrors to show that residual
CTE variation may be addressed with mounted actuators for a variety of mirror sizes. We present measurements of
surface quality at a range of temperatures characteristic of mountaintop observatories. For the 8 cm piece, the figure
error of the Al-coated reflective surface under best actuator correction is ~43 nm RMS. The 8 cm mirror has a low
surface error internal to the outer ring of actuators (17 nm RMS at 20°C and 33 nm RMS at -5°C). Surface roughness is
low (< 3 nm P-V) at a variety of temperatures. We present new figure quality measurements of the larger 17 cm mirror,
showing that the intra-actuator figure error internal to the outer ring of actuators (38 nm RMS surface with one-third the
actuator density of the 8 cm mirror) does not scale sharply with mirror diameter.
Thin-shelled composite mirrors have been recently proposed as both deformable
mirrors for aberration correction and as variable radius-of-curvature mirrors for
adaptive optical zoom. The requirements on actuation far surpass those for other
MEMS or micro-machined deformable mirrors. We will discuss recent progress
on developing the actuation for these mirrors, as well as potential applications.
Carbon fiber reinforced polymer (CFRP) composites provide several advantages as a substrate for thin-shell adaptive
secondary mirrors, including high stiffness-to-weight ratio and low coefficient of thermal expansion (CTE). We have
addressed some of these concerns using a prototype CFRP mirror under actuation. Using 4D and Newton interferometry,
we present measurements of surface quality at a range of temperatures. Under actuator relaxation at room temperature,
its surface error is low (92 nm RMS) and dominated by edge curvature. This error is reduced further under best actuator
correction to 43 nm RMS, placing it into consideration for use in near-IR astronomy. The low surface error internal to
the outer ring of actuators - 17 nm RMS at 60°F and 33 nm RMS at 20°F - suggests that larger mirrors will have a
similar figure quality under actuator correction on ground-based AO systems. Furthermore, the actuator forces required
to correct the figure are small compared to the dynamic range of voice coil actuators (~0.1 N). In addition, surface
roughness is characterized to address the effects of high spatial frequency errors.
The Naval Research Laboratory (NRL) has been exploring the use of meter class telescopes using Carbon Fiber Reinforced Polymer (CFRP) material for support structure and optics, resulting in over an order of magnitude reduction in weight over traditional steel and glass telescopes. In conjunction with Composite Mirror Applications (CMA), for the past three years this program has proceeded from conceptual phase to prototype development. In this paper we will review the various stages of this program. We will also present the status of our 0.4 meter and 1.4 meter telescopes. Experimental results from these developments and testing will be shown.
The Naval Research Laboratory (NRL) has been exploring Carbon Fiber Reinforced
Polymer (CFRP) material for telescope construction, including all support structures and
optics, resulting in over an order of magnitude reduction in weight over traditional steel
and glass telescopes. This has allowed the exploration of novel micro-positioning motors
for telescope pointing. A typical inertial drive telescope pointing system relies on the
large mass of the optics and telescope assembly. A prototype 0.4 meter telescope has
been developed with a total mass less than 8 kilograms necessitating the investigation of
non-traditional drive systems. This paper reports on many of the prototype 0.4 meter
telescope specifications, including optics, structure and drive hardware. Initial testing is
of the open-loop drive system is also reported.
Presented are unique concepts for space telescopes and optics, based on carbon fiber reinforced polymer (CFRP) thin-shelled
mirror technology. Thin-shell CFRP mirrors have been proven for IR and longer wavelengths and to a large
extent, visible wavelength optics. The unique structural/mechanical and lightweight characteristics of thin shells open
the design possibilities for advanced space telescopes with active/adaptive mirrors. Low weight and general ease of
manufacturing of CFRP structures can result in reduced part-count and inexpensive lightweight telescopes for space
applications. Three advanced mirror concepts will be presented in this paper, 1) Advanced stowage of thin-shell mirrors
for segmented telescopes, 2) advanced deformable mirror concepts, and 3) simple and inexpensive fabrication concepts
using simplified molding tools for space telescope mirrors. Also presented will be empirical data of CFRP thin-shell
mirrors and composite structures produced supporting their use for space telescope applications.
Robert Martin, Robert Romeo, Geoff Barber, Andre Braem, Nick Brook, Bill Cameron, Carmelo D'Ambrosio, Neville Harnew, Kenneth Lessnoff, Fabio Metlica, Dave Websdale
The design, manufacture and testing of lightweight Carbon Fiber Reinforced Polymer (CFRP) converging spherical
mirrors for the RICH-1 Cherenkov detector of LHCb are described. The mirrors have low areal density to minimize the
amount of material in the path of traversing particles and need to be fluorocarbon compatible to avoid degradation in the
C4F10 radiator gas. The total area is about 2m2 and high reflectivity (typically 90%) over the wavelength range 200-600nm is required.
CFRP (carbon fiber reinforced plastic) is an attractive material for fabrication of optical systems. The stiffness-to-weight
ratio of CFRP is high, the coefficient of thermal expansion is low, the structure thermalizes rapidly, and many of
the structural properties can be tailored to the application. We have used CFRP and CFRP-aluminum sandwich panels
extensively in the structures of astronomical, optical telescopes up to 1.5m diameter aperture. In designing the optical
structures, we have chosen some key fabrication techniques with CFRP that take advantage of the mechanical properties.
This paper discusses the design and fabrication of 2 major telescope projects. The 1m ULTRA telescope has both the
optics and the OTA fabricated from CFRP. This telescope has been recently installed at the SDSU Mt. Laguna
Observatory near San Diego. A 1.4m telescope for the Naval Research Lab is being designed and fabricated at CMA.
The optics, OTA, and the mount are all being fabricated using CFRP composites. The result is a lightweight structure
which can be moved or deployed as necessary.
The use of composite materials in the fabrication of optical telescope mirrors offers many advantages over conventional
methods, including lightweight, portability and the potential for lower manufacturing costs. In the construction of the
substrate for these mirrors, sandwich construction offers the advantage of even lower weight and higher stiffness.
Generally, an aluminum or Nomex honeycomb core is used in composite applications requiring sandwich construction.
However, the use of a composite core offers the potential for increased stiffness and strength, low thermal distortion
compatible with that of the facesheets, the absence of galvanic corrosion and the ability to readily modify the core
properties. In order to design, analyze and optimize these mirrors, knowledge of the mechanical properties of the core is
essential. In this paper, the mechanical properties of a composite triangular cell core (often referred to as isogrid) are
determined using finite element analysis of a representative unit cell. The core studied offers many advantages over
conventional cores including increased thermal and dimensional stability, as well as low weight. Results are provided
for the engineering elastic moduli of cores made of high stiffness composite material as a function of the ply layup and
cell size. Finally, in order to illustrate the use of these properties in a typical application, a 1.4-m diameter composite
mirror is analyzed using the finite element method, and the resulting stiffness and natural frequencies are presented.
Presented is a poster presentation relating to the final assembly and testing of the Ultra Lightweight Telescope for
Research in Astronomy, ULTRA [1]. The program is a 3-year Major Research Instrumentation (MRI) program funded
by NSF. Objectives are to demonstrate carbon fiber reinforced plastic (CFRP) composite mirrors for ground-based
optical telescopes. Presented will be final assembly of the telescope including the unique features of the system
including the 27 kg primary mirror, hexapod secondary mirror control, motorized iris for the primary mirror cover. Also
presented are results of the optical testing of the 0.4m mirrors used as developmental optics in the program.
Deployment costs of large aperture systems in space or near-space are directly related to the weight of the system. In
order to minimize the weight of conventional primary mirrors and simultaneously achieve an agile system that is capable
of a wider field-of-view (FOV) and true optical zoom without macroscopic moving parts, we are proposing a
revolutionary alternative to conventional zoom systems where moving lenses/mirrors and gimbals are replaced with
lightweight carbon fiber reinforced polymer (CFRP) variable radius-of-curvature mirrors (VRMs) and MEMS
deformable mirrors (DMs). CFRP and MEMS DMs can provide a variable effective focal length, generating the
flexibility in system magnification that is normally accomplished with mechanical motion. By adjusting the actuation of
the CFRP VRM and MEMS DM in concert, the focal lengths of these adjustable elements, and thus the magnification of
the whole system, can be changed without macroscopic moving parts on a millisecond time scale. In addition, adding
optical tilt and higher order aberration correction will allow us to image off-axis, providing additional flexibility.
Sandia National Laboratories, the Naval Research Laboratory, Narrascape, Inc., and Composite Mirror Applications,
Inc. are at the forefront of active optics research, leading the development of active systems for foveated imaging, active
optical zoom, phase diversity, and actively enhanced multi-spectral imaging. Integrating active elements into an
imaging system can simultaneously reduce the size and weight of the system, while increasing capability and flexibility.
In this paper, we present recent progress in developing active optical (aka nonmechanical) zoom and MEMS based
foveated imaging for active imaging with a focus on the operationally responsive space application.
The development of sensors that are compact, lighter weight, and adaptive is critical for the success of future military initiatives. Space-based systems need the flexibility of a wide FOV for surveillance while simultaneously maintaining high-resolution for threat identification and tracking from a single, nonmechanical imaging system. In order to meet these stringent requirements, the military needs revolutionary alternatives to conventional imaging systems.
We will present recent progress in active optical (aka nonmechanical) zoom for space applications. Active optical zoom uses multiple active optics elements to change the magnification of the imaging system. In order to optically vary the magnification of an imaging system, continuous mechanical zoom systems require multiple optical elements and use fine mechanical motion to precisely adjust the separations between individual or groups of elements. By incorporating active elements into the optical design, we have designed, demonstrated, and patented imaging systems that are capable of variable optical magnification with no macroscopic moving parts.
The portability of meter-class telescopes has been limited by the weight of the mirror, tube assembly and the mount required to provide pointing and tracking. The novel lightweight carbon fiber reinforced polymer telescopes being developed for array population at the Naval Prototype Optical Interferometer are orders of magnitude lighter than traditional telescopes. When combined with a lightweight carbon fiber mount, these telescopes will be easily transportable from one telescope station to another to change the interferometer baseline. The mount for a lightweight telescope is currently under development at Composite Mirror Applications, Inc. This paper reports on the design constraints of the mount, the scalability to larger aperture telescopes and the integration of sensors to measure the performance characteristics of this system during operation.
Adaptive optics systems are commonly added onto conventional astronomical telescopes to improve the wavefront quality in the presence of atmospheric turbulence. Recent successes in the development of carbon fiber reinforced polymer telescopes have significantly reduced the weight of meter class telescopes making them portable, however, most adaptive optics systems continue to be constructed on large optical benches. The Navy Prototype Optical Interferometer is developing several 1.4 m portable telescope with internal wavefront correction. As part of this upgrade, a prototype 0.4 m aperture telescope has been constructed and a light weight, compact adaptive optics system is being developed. We present in this paper the design of an adaptive optics system for the lightweight telescope. The key to this system is the incorporation of a compact wavefront correction device and a novel collimation optic within the base of the telescope.
We present the technical status of the Ultra Lightweight Telescope for Research in Astronomy (ULTRA) program. The program is a 3-year Major Research Instrumentation (MRI) program funded by NSF. The MRI is a collaborative effort involving Composite Mirror Applications, Inc. (CMA), University of Kansas, San Diego State University and Dartmouth College. Objectives are to demonstrate the feasibility of carbon fiber reinforced plastic (CFRP) composite mirror technology for ground-based optical telescopes. CMA is spearheading the development of surface replication techniques to produce the optics, fabricating the 1m glass mandrel, and constructing the optical tube assembly (OTA). Presented will be an overview and status of the 1-m mandrel fabrication, optics development, telescope design and CFRP telescope fabrication by CMA for the ULTRA Telescope.
Under contract from the Cornell-Caltech Atacama Telescope Project (CCAT), Composite Mirror Applications, Inc.
(CMA) has undertaken a feasibility design study for the use of Carbon Fiber Reinforced Plastic (CFRP) panels in
forming the primary mirror surface. We review some of the past projects using CFRP panel technology for
millimeter and submillimeter wavelength radio astronomy telescopes. Pros and cons of the technology are
discussed. A particular panel configuration was proposed and computer modeled with finite element analysis
(FEA). The technology of replicated CFRP panels for short wavelength radio astronomical telescopes is mature and
cost effective. For shorter wavelengths into the IR and visible, it is becoming a very attractive alternative to
traditional, heavy glass or metal technologies.
The focus of the ULTRA Project is to develop and test Ultra-Lightweight Technology for Research applications in
Astronomy. The ULTRA project is a collaborative effort involving the private firm Composite Mirror Applications, Inc
(CMA) and 3 universities: University of Kansas, San Diego State University, and Dartmouth College. Funding for
ULTRA is predominately from a NSF three year MRI program grant to CMA and KU with additional support from
CMA, KU and SDSU. The goal of the ULTRA program is to demonstrate that a viable alternative exists to traditional
glass mirror and steel telescope technology by designing, fabricating and testing a research telescope constructed from
carbon fiber reinforced plastic (CFRP) materials. In particular, a 1m diameter, Cassegrain telescope optics set and
optical tube assembly (OTA) are being designed and fabricated by CMA. The completed telescope will be deployed at
SDSU's Mt Laguna Observatory in a refurbished structure (new dome and mount provided via KU and SDSU). We
expect that a successful completion and testing of this project will lead to future use of CFRP technology in larger
telescopes and segmented telescopes. This paper describes the OTA (optical tube assembly) that has been developed
for the ULTRA project. The mirror technology is described in another paper in this conference. A poster describes the
ULTRA project overview in more detail.
Presented are results of continuing optical mirror development program for the NSF ULTRA Telescope. Development
of a 16-inch f/4.0 parabolic mirror has been undertaken to adequately define scale-up fabrication procedures for 1m and
1.4m mirrors. 16-inch mirrors have been produced to λ/15 rms ( λ=633nm) in the wavefront. These mirrors have been
used to produce astronomical images in a Newtonian telescope and yielded quality optical images. Presented will be
results of the fabrication of the 1m, f/3 parabolic primary mirror mandrel for the 1m ULTRA Telescope. Also presented
will be lab test data and astronomical mages produced under the 16-inch program as well as test data from the
replications from the f/3 1m parabola.
Presented are results of a fabrication program to produce the Ring Imaging Cherenkov, RICH, mirror for the Alpha Magnetic Spectrometer, AMS-02, which is to be placed on the International Space Station. Composite Mirror Applications, Inc., CMA, in Tucson AZ was contracted by Carlo Gavazzi Space, CGS, to produce a conical mirror 1.3m diameter 0.5m in height, from high modulus carbon fiber, flight qualified composite materials, having an optical surface on the inside of the cone. The flight model mirror was completed to specification, yielding nearly 2m2 of replicated optical surface area and weighs 8 kg. CMA measured the surface roughness and slope errors and the mirror dimensions were measured using a CMM at The University of Arizona's Instrument Shop. The results show the mirror meets conformance to the required specifications. The RICH mirror is currently undergoing flight testing and integration.
In this paper we present some preliminary results of an ultra-light weight telescope manufactured entirely with Carbon Fiber Reinforced Polymer (CFRP), including the optics, coupled with a light weight Adaptive Optics (AO) system. This research has many scopes, ranging from long baseline interferometry to laser communications. In this paper we will examine some of the mechanical properties of the telescope and describe the testing that the system is undergoing.
AMiBA consists of a 90 GHz interferometric array telescope with dishes ranging in size from 0.3 to 2.4 meter in diameter, mounted on a 6-meter fully steerable platform. The dishes are attached to the receivers, which are mounted on a platform controlled by a six degree of freedom hexapod mount. The hexapod mount is a parallel connection manipulator also called Stewart Platform. The basic reference for this mechanism is a paper by Stewart. The Stewart Platform is a unique kinematically constrained work platform. It can be manipulated through the six degrees of freedom. The hexapod also provides better accuracy, rigidity, load to weight ratio and load distribution than a serial manipulator or traditional manipulator. The advantages of the hexapod shows that it is a great choice for the AMiBA project. Vertex Antennentechnik GmbH fabricates the hexapod. Testing has started in Germany. The telescope will be delivered in the summer of 2004. The 6m in diameter hexagonal platform is made of carbon fiber reinforced plastics (CFRP) and consists of seven pieces of three different unique types. The platform can be disassembled and fits in a container for transportation. The mounting plane flatness is an important issue for the platform assembly. The deflection angle of the mounting plane relative to any other mounting position must be less than 20 arcsec. Meanwhile, the platform must endure a loading of 3 tons. The platform has been built by Composite Mirror Applications, Inc. (CMA) in Tucson, and mounted on the Hexapod in Germany. This report describes the design and testing of platform and mount for the AMiBA telescope.
We report progress in composite mirror technology made since the previous Backaskog ELT workshop. Significant achievements include the fabrication of extremely lightweight mirrors with areal density as low as 1 kg/m2, diffraction limited optical performance at visible wavelengths, meter class mirrors, a portable telescope with 0.5 m mirror, large thin deformable mirrors for adaptive optics, 1m x 2m mirrors, and a six meter telescope platform.
New, very large telescopes with apertures of 30, 50, and 100 meters are being proposed by the astronomical community. Superpolished or ultrapolished mirrors with low scattered light levels and the use of adaptive optics for near-diffraction-limited performance would make such large telescopes a turning point in astronomy. The secondary mirror for the Euro50 will be a four meter adaptive optic made of a low expansion graphite-filled cyanate ester resin composite produced using a replica transfer technique. We have made three 1/3rd meter diameter prototype composite adaptive optic mirrors of this cyanate ester composite material. Because of the embedded graphite fibers, the composite material has a measured expansion coefficient in the 10-8 range, as has Zerodur or ULE glass. It is very much lighter, more rugged and more economical than Zerodur or ULE, and can be fabricated in weeks, not months. The Zerodur mandrels upon which these replica transfer mirrors are made are superpolished using centrifugal elutriation, so the replica surface has an rms roughness of 0.6 to 0.8 nm. It thus scatters about an order of magnitude less light than typical conventionally polished astronomical mirrors. In adaptive optic mirrors with sub-mm thick faceplates the number of plies used is insufficient to produce an isotropic surface. For mirrors 2 mm thick, with more plies, the surfaces are isotropic, and the slight astigmatism sometimes resulting from the mesh in the ply can be corrected by actuators to make them attractive mirrors. They must be supported to maintain a good optical figure over a meter diameter mirror. The support requirement may be met by using a new type of mechanical/piezoelectric actuator adjustable to a fraction of a wavelength. The mechanical actuators have a coarse adjust of over an mm and a fine adjust of less than a wavelength of light. They can be used in series with a novel type of piezoelectric actuator for final static adjustment. The low voltage, up to 2.5 kHz frequency piezoelectric actuators have a displacement of approximately one μm per volt, 82 times greater than conventional piezoelectric actuators, and a throw of ±30 μm or more. Compliant faceplates can be adaptive as well as active. Calculations indicate that for actuator spacings of about 4 cm the effective mirror stiffness equals that of a solid Zerodur mirror with a conventional 6:1 diameter to thickness ratio. The effect of gravitational sag for composite mirrors is calculated to be negligible. They are thus a good choice for the secondary mirror for the Euro50 as well as for the primary or secondary mirrors for other giant telescopes.
We suggest new approaches towards direct detection, in the near term, of Earth-like planets around Sun-like stars. Optical detection of such 'exo-planets' at visual wavelengths requires telescopes mirrors >1m diameter with very accurate figures, extremely smooth surfaces, and highly unusual shapes. The availability of such mirrors, with reasonable fabrication times and at affordable cost, is an issue of major concern. We describe how composite mirror technology is being developed to meet these very challenging requirements. A number of non-circular mirrors of modest (0.5 to 1.2m) aperture have already been made. We present data on the development and status of a low cost process that can make mirrors of corornagraphic quality with arbitrary shapes, very highly smooth surface, and ultra-low areal density.
KEYWORDS: Mirrors, Composites, Space mirrors, Space operations, Interferometers, Monochromatic aberrations, Spherical lenses, Space telescopes, Optical fabrication, Control systems
During the course of performing space flight qualification testing of composite mirrors at NASA GSFC, a serendipitious event was observed which, in retrospect, should have been obvious. Investigation of this phenomenon leads to a promising avenue towards the fabrication of large aperture precision spherical mirrors with very long radius of curvature (>f/100). Such mirrors are required for future missions such as the Stellar Imager. We report on the observation and analysis of the event, optical measurements, and the development of associated active figure control systems.
Presented is a lightweight mirror compatible with the extremely large telescopes being proposed. The mirror exhibits an areal density of less than 12 kg/m2, which is the same relative areal density as the primary mirrors on the 100-meter Robert Byrd Radio Telescope. The panels can be fabricated in 2.5-meters diameter with traceability to 4-meters. Presented are preliminary results of a set of large demonstrator mirrors, their characteristics and traceability to producing 4-meter low-cost and easily fabricated mirror segments for large telescopes.
New very large telescopes with apertures as large as 100 meters are being proposed. They will be made up of mirror segments only a meter or two in diameter and phased together. The diffraction-limited resolution of a mirror is directly proportional to its diameter, and the light-gathering-power is proportional to the square of the diameter. Near-diffraction-limited performance using adaptive optics would make such large mirrors very exciting. We have built two small prototype composite adaptive optic mirrors of graphite fiber impregnated cyanate ester resin driven by actuators spaced 4 cm apart and with a faceplate influence function of 5 cm. The second mirror assembly also makes possible a 2 cm actuator spacing. The overall figure is not yet as good as desired, but we believe that much of this problem can be corrected by mechanical adjustment of the actuator rest positions and use of low expansion mandrels. This mirror concept, when realized in primary mirror segments a meter or more in diameter, should make correction possible for atmospheric turbulence under almost any observatory seeing conditions. The composite optical faceplate in the most recent prototype had a roughness of 0.6 to 0.8 nm. Two centrifugal elutriation super-polishers, each over 1.2 meters in diameter, are in place to produce superpolished mandrels on which to form superpolished faceplates over a meter in diameter. Scattered light from such a mirror surface will be reduced by as much as a factor of ten, as compared to conventional fresh feed polishing. The name "transfer mirrors" rather than the widely recognized but poorer quality "replica mirrors" is given to such faceplates. They have an expansion coefficient comparable to ULE quartz or Zerodur, and are lightweight with 10-20, an aerial density of 17 kg/m2 for the mirror with a 4 cm actuator spacing or 34 kg/m2 for the mirror with 2 cm actuator spacing. In both cases the effect of gravitational sag is minimized. A 60 volt potential results in actuator displacement of 5 mm as measured inter
KEYWORDS: Mirrors, Composites, Monochromatic aberrations, Curium, Global system for mobile communications, Space operations, Diffraction, Optical fabrication, Space telescopes, Space mirrors
We report on the development of composite mirror technology under the NASA Gossamer Spacecraft Initiative program. The objectives are to produce moderate aperture, extremely low areal density mirrors with smooth surfaces and good optical figure.
The need for extremely large aperture telescopes drives the requirement for new materials and novel approaches to mirror production. Many lightweight mirror concepts are currently being persued, some with promise for extending their ability to facilitate 100-meter and larger space telescope primaries. These concepts include some rather unorthodox materials in unique configurations. Past experience in producing extremely thin CFRP composite mirrors, using unidirectional CFRP prepreg tape, has led us to a more novel CFPR material, which could further reduce the mass and cost of their predecessors. We present a carbon-based, ultra-lightweight fleece material, which have been shown to exhibit high specularity and extremely low areal density, 200 grams/m2, at 2-plies, in contrast to more typical unidirectional CFPR material.
The aperture of monolithic space telescope primary mirrors placed on orbit is limited to payload faring diameters, the largest being about 4-meters. This requires a novel stowage approach for monoliths larger than 4-meters. Very large aperture telescopes, 50 to 100-meter diameters, planned for deployment in the next 10 to 20 years will also require very large mirror segments in an effort to manage the phasing of the entire surface. The larger the mirror panels the fewer that will be required for such apertures. If the mirrors can be made thin enough to be deformed into a cylinder or undeformed but closely nested, enough surface area can be placed on orbit to facilitate large aperture telescope mirrors. 8-meter monolithic mirrors can be rolled into a 2.5-meter diameter cylinder with the secondary support structure stowed in the cylinder to maximize the payload faring volume. Hyper-thin mirrors can be closely nested in order to maximize volume as well. Presented is a design and engineering model of a 0.9-meter diameter hyper-thin, ultra- lightweight spherical composite mirror and methods, which led to the fabrication of the mirror.
We report on the successful development of a new ultra lightweight optics technology. Intended applications include telescopes in space, on the Moon, and adaptive optics. The technology employs a novel process of optical replication using standard industrial grade graphite fiber composite materials. Composite replication makes possible telescope mirrors that combine extremely low areal density, large aperture, high surface smoothness, and high optical quality. Fabrication times and costs have been demonstrated to be far below that of competing lightweight optics technologies. The very low areal density achieved, ranging from 1 to 5 kg/m2, makes possible multi-meter telescopes in space and on the ground. We present data on moisture absorption and outgassing, thermal expansion, thermal hysteresis, and improvement in optical figures. Applications to date include submillimeter telescopes and large optical arrays.
We report progress in the development of very lightweight uv-vis-ir mirrors for space- and ground-based applications. The mirrors are made by replication using spaceflight qualified graphite cyanate ester composite materials. We have developed a process that successfully overcomes the problems of fiber print-through, vacuum instability, and appearance of bond lines on the surface. These problems have thwarted previous attempts in the development of composite optics. We describe our process and present some recent results. These include the fabrication of mirrors with highly smooth surfaces, low mid-frequency ripple, and areal density 2 kg/m2 at 60 cm aperture. We also present data on bond lines and active optical figure control.
The use of continuous fiber reinforced plastic, CFRP, composite materials is introduced here as a viable material for optical telescopes. The thermal characteristics of CFRPs make them attractive as dimensionally stable materials for all-composite telescope structures and mirrors. Composite mirrors have only recently shown promise as replacements for heavier and more fragile glass mirrors. The areal density of a CFRP mirror can be as much as 10 times less than that of a glass mirror. Optical test results show CFRP composite mirrors can be fabricated with an average surface roughness of less than 10 angstroms. Concept models of scope and CFRP optics with associated figure and roughness data are presented.
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