Composite optics advance stellar imaging: composite materials are making it possible to add capabilities to astronomical observatories without massive changes to the existing infrastructure.
Upgrading existing facilities
The U.S. Navy's prototype optical interferometer (NPOI) consists of a movable set of telescopes and siderostats. Siderostats are simply movable flat mirrors that redirect light to a central location where beams from separate mirrors may be combined. Each of the siderostats can be placed in various locations along a Y-shaped set of 350-m-long beam paths, and light from up to six of the 50-cm-diameter assemblies can be combined at the central location. A given separation will be sensitive to spatial frequencies over a relatively restricted range so obtaining observations at different locations can fully reconstruct the object features.
In order to move beyond NPOI's already impressive resume of accomplishments, the Naval Research Lab (NRL) plans to replace the current siderostats and telescopes with larger diameter telescopes. Larger apertures collect more light, providing more signal, thereby increasing the accuracy and richness of the measurements. However, larger mirrors and telescope support structures become very heavy, and retaining the reconfigurable system would mean adding massive new facilities to move and place the telescopes, unless some way could be found to increase the aperture of the telescopes without making them too heavy.
Lay it up, lay them out
Sergio Restaino is the NRL scientist responsible for upgrading the NPOI. He needed a lightweight, thermally and environmentally stable, cost-effective material with which to build six 1.4-m telescopes, and he chose carbon fiber reinforced polymer (CFRP). CFRP can be used for the mirrors as well as its support structures, which minimizes thermal stress. CFRP mirrors are produced by replication, so cost savings can be realized for multiple identical items such as the NPOI telescopes. In addition, CFRP is lightweight.
Composite Mirror Applications (CMA), Tucson, Ariz., manufactures CFRP mirrors. CMA has already demonstrated primary mirror areal densities from 7 kg/[m.sup.2] all the way down to 0.7 kg/[m.sup.2]. Numbers like these grabbed the attention of the NPOI team, and CMA designed a 16" CFRP brassboard telescope that could be scaled to a 1.4 m diameter.
CFRP mirrors are constructed by embedding carbon fibers in a resin. Layers of carbon fiber are "laid up" in different directions to optimize the material properties for a given application. The type of resin, the ratio of resin to fiber, and the direction and weave of the carbon fibers can all be varied. Once the fiber-embedding is complete, the resin is cured and it solidifies into an environmentally stable, vacuum-compatible material. The nature of the manufacturing process makes it possible to fabricate CFRP parts in arbitrary shapes. In particular, if the material is laid up against a mandrel formed into a high-quality convex optical surface, the resin cures as a high-quality concave optical surface. This leads directly to cost savings for multiple items. "One of the advantages of using this type of technology is that once you have a mandrel of very high quality you can quickly pop out the mirrors," notes Restaino. The cost reduction of replicates is seven to 10 times lower than the initial item.
Robert Romeo founded CMA in 1991, and he has participated in many of the milestone accomplishments of CFRP technology. "Up to the mid-1990s," says Romeo, "nobody was producing mirrors better than a few waves rms in the visible. CMA produced 1/4 wave p-v optics at 150-mm-dia at that time and has been able to scale the size to 400 and 500 mm." CFRP mirrors can now be routinely manufactured with surface roughness less than 10 nm rms and optical figure errors less than [lambda]/20 over 80% of the aperture.
In the past few years, CMA, with the Univ. of Kansas, Lawrence, Dartmouth College, Hanover, N.H., and San Diego State Univ., Calif., has been developing a 0.9-m-dia telescope for the ultra lightweight telescope for research in astronomy (ULTRA) program. That program, along with the NPOI development project and others, has contributed to the rapid maturation of the CFRP technology.
Although CFRP is dimensionally stable in a wide range of environments, there have been a couple of traditional obstacles to producing high quality surfaces for visible imaging. First, composite materials absorb water from humidity in the atmosphere. At its worst, the water taken up can cause delamination of the composite material. Restaino acknowledged this concern, but also noted that this is an area where the technology has matured, and the issue can be addressed by careful attention to the resin components and the layup ratio.
Fiber print-through, variations in the surface following the contour of the embedded fibers, is the second problem. "The presence of fiber print-through will reduce image contrast if it is allowed to exist as a 10 nm or larger surface feature," says Romeo. CMA has reduced fiber print-through, again by careful process design and special materials selection. The proof is in the pictures, and CMA mirrors are currently being used for astronomical imaging.
In addition to the manufacturing technology itself, test methodology has also had to evolve. Fiber layup creates an intrinsically anisotropic material with properties that vary according to the direction of the layup, the thickness, and the layup pattern. "To characterize glass you need four measurements," says Restaino. "To characterize carbon fiber you need eleven." Mathematical models provide a starting point, but even so, he says, "we have to build little samples of different manufactured materials and test them for the different properties." Materials with specific characteristics are selected for different structural elements and optical surfaces.
All that development effort has a huge payoff. The final 1.4-m-aperture NPOI telescopes, including both structure and optics, are projected to weigh less than 120 kg. A conventional glass-and-steel telescope can easily be ten times that massive. The ramifications of the lower mass extend beyond the obvious. Of course the handling system required to move a less massive telescope can be lighter, but the pointing control motors can also be lighter duty. Smaller solid-state drive motors provide all the torque needed to position the less massive telescopes with speed, accuracy, and low power consumption.
Although composite mirrors are not yet off-the-shelf commercial items, they do have another distinguishing characteristic that will make them attractive to many system designers. Because they pop off the mandrels needing little final figuring, they can be very rapidly manufactured. Romeo notes that it takes "only a week's time to produce a 1-m parabolic primary mirror." He believes this will be another factor making CFRP optical systems attractive for airborne and spaceborne applications, although he noted that the technology is appropriate for "any weight driven and/or high thermal stability application."
These features certainly made CFRP attractive to Restaino as his team designed the upgrade to NPOI. Larger aperture telescopes will dramatically increase the sensitivity of stellar interferometry at NPOI, and, thanks to the integrated CFRP design, it will all be done with minimal changes to the existing infrastructure.
For more information: NPOI: www.hofs.navy.mil/projects/npoi/ CMA: www.compositemirrors.com/
Founder and Chief Engineer
Mountain Optical Systems Technology, www.mountainoptical.com