Aerospace Scientists Develop Fabrication Method for High-Precision Mirrors

Geena Ferrelli, member of the technical staff, Materials Science Department, holds up a laboratory coupon-sized replicated mirror. (Photo: Joseph Severino)

Large, lightweight, high-precision mirrors are a critical enabling technology for space-based imagery satellite systems.

Observations from space require increasingly larger optical collecting areas (i.e., bigger mirrors) for high-resolution imagery from long distances. However, large glass mirrors are time-consuming to manufacture, taking from many months to years to finish, and are very heavy. Larger mirrors that are lightweight and have high-stiffness would enable higher resolution space telescopes that can be easily sent into space. Aerospace’s Air Force and intelligence community customers are interested in lightweight mirror technology.

In recent years, composite replicated mirrors have gained increasing attention over traditional polished glass mirrors because of their potential for significant improvements in manufacturing lead-time, aerial density, and lower cost, and for their tailorable mechanical and thermal properties.

Working to advance the technology, scientists and engineers in Aerospace’s Materials Science Department, led by Dr. Rafael Zaldivar, have been experimenting with a series of unique in-house processing protocols, overcoming many critical issues with composite mirrors. “The process of utilizing a defect-free, molecularly thin, organic, monolayer release agent has been developed in the Aerospace labs to produce near-net replicated surfaces without the need for post polishing,” said Hyun Kim, a research scientist in the Surface Science and Engineering Department.

Composite replicated mirrors are manufactured by sandwiching an uncured resin between a high-quality mandrel and a preassembled composite substrate. The resin then cures and replicates the surface of the mandrel while bonding to the composite surface.

Figure 1. A schematic of a replicated mirror fabrication. (a) An uncured resin is sandwiched between a mandrel and a substrate. (b) The resin is allowed to “replicate” the mandrel surface and then cure. (c) The replicated resin/substrate is released from the mandrel.

A mandrel, used here in the figure, is a high-quality glass mirror that serves as the master copy. One analogy is the way LP (long-playing) records or CD’s (compact discs) are fashioned. One master copy or negative is created, and multiple replica are made from the master. However, nanometer-scale precision is the key to success of replicated optics.

The resulting replica resin surface is then removed from the mandrel and coated with a reflective metal to make a high-quality mirror. This process can be repeated many times using the same mandrel, allowing for finished manufacturing within weeks, rather than months or years. Conventional glass mirrors require an extensive secondary polishing process to achieve acceptable flat surfaces, hence they take much longer to manufacture.

Numerous processing challenges remain before composite replicated mirrors will require little to no secondary polishing — this has limited the quality and stability of the mirrors thus far. The effects of humidity and temperature on these mirrors also remain of significant concern.

Additionally, dimensional stability remains one of the most critical issues for these resin-based, high-precision optics, and the Aerospace team has developed a series of processing protocols for high-optical quality polymer mirrors that can be manufactured with durability and environmental dimensional stability using an ultraviolet curable replication resin system. The mirrors have been tested under a wide range of humidity, thermal, and radiation exposures.

“We are exploring how the materials and optical processing correlate. We are focused on stability in many different environments,” said Geena Ferrelli, a member of the technical staff in the Materials Science Department. The team is exploring how the composite mirrors perform under, and are affected by, an array of environments, leading to characterization, checks for crosslinking, and effects of the absorption of water, as well as exposures to thermal and radiation conditions, and discovery of where these effects may lead to breakdowns in functionality. In other words, the team is testing in the relevant environments these mirrors will undergo during their life cycles, from manufacturing to testing, storage, and in-space operation.

The team has applied for two patents with the United States Patent and Trademark Office and is in the process of filing for a third. Patents applied for include “Deposition Assembly and Methods for Depositing Model Release Layers on Substrate” and “Fabrication Assembly and Methods for Fabricating Composite Mirror Objects.” The third patent being prepped by Aerospace Legal to file with the USPTO is “A Process to Improve Dimensional Stability of Replicated Composite Mirrors.”

The team’s work on replicated composite mirrors has also been recognized as a finalist in the CAMX 2017 Awards for Composites Excellence (ACE) competition for the most creative application. According to the CAMX website, the award “recognizes cutting-edge innovations and innovators that are shaping the future of composites and advanced materials in the marketplace.”

—Nancy Profera