Robert C. Romeo

PROGRESS IN VERY LIGHTWEIGHT OPTICS USING GRAPHITE FIBER COMPOSITE MATERIALS

We report progress in the fabrication of very low areal density (,5 kg/m 2 ) optical mirrors using space-flight-qualified graphite-fiber- reinforced cyanate ester composite materials. Previous attempts have been thwarted by fiber print-through at the surface. We find that the problem can be successfully overcome if the composite is processed to leave a very thin layer of resin at the surface. Results thus far include replicas with surface microroughness ,1 nm rms, midfrequency ripples ,3 nm rms, areal density 2 kg/m 2 at 42-cm aperture, and freedom from print-through after vacuum drying and ion milling. The process is being extended to the fabrication of very lightweight meter-class optics for space UV astronomy applications, x-ray optics, and other ground- and space-based applications.

Ultralightweight and hyperthin rollable primary mirror for space telescopes

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.

Advances in composite mirror and telescope technology

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.

Ultralightweight precision optics technology

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.

A novel process to fabricate mirrors with very long radius and ultrasmooth surfaces

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.

Fabrication and testing of very lightweight composite mirrors

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.

CFRP composite thin-shelled mirrors for future space telescopes

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.

Development of lightweight mirror elements for the Euro50 mirrors

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.

Fabrication and testing of ultra-lightweight Gossamer-class composite 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.

Ground‐Based Adaptive Optic Transfer Mirrors For Space Applications: I. Design and Materials

Projectors 15 meters or more in diameter and of excellent quality will be needed to successfully transmit laser beams from the earth to satellites in space. They can be made up of adaptive optic mirror segments one or two meters in diameter connected to make a phase‐continuous mirror surface. Since they must be used in poor as well as good seeing conditions, the faceplate influence function radius must be short and the actuators closely enough spaced to use with Fried coefficients of 2 to 4 cm. Under a Phase II NASA program we are developing a lightweight prototype adaptive optic mirror segment meeting these requirements. It uses a graphite impregnated composite cyanate ester faceplate. Design and materials considerations will be discussed..