While the first thought about telescopes is that they are instruments designed to look at objects in outer space, the truth is that they are instruments designed to gather light. As technology has advanced, the ability of telescopes to gather light more efficiently has increased—up to a point. There has not yet been a technology developed to enable the use of silver, the most reflective surface material. Most telescope mirrors today consist of thin layers of aluminum, because silver corrodes and tarnishes too easily. It requires the use of a protective barrier that prevents degradation of the silver but does not interfere with its reflective and other optical properties.
That problem might be an issue of the past, however. A collaboration between researchers at the University of California at Santa Cruz, UC Observatories (UCO), and Structured Materials Industries has led to the development of a new technology for the production of larger-scale silver telescope mirrors (https://news.ucsc.edu/2017/09/mirror-coating.html).
Following a discussion with Joseph Miller, then director of UCO, Nobuhiko Kobayashi, professor of electrical engineering in the Baskin School of Engineering at UC Santa Cruz began working with astronomers Andrew Phillips and Michael Bolte, who are also at the university. Backed by funding from the National Science Foundation and with the support of the current UCO director Claire Max, the researchers are developing an astronomical atomic layer deposition (ALD) system that can apply effective, protective coatings to the surfaces of large silver-based telescope mirrors.
Compared to more conventional physical deposition techniques, ALD, which is widely used in the electronics industry, allows the deposition of highly uniform protective coatings one molecular layer at a time. Existing ALD systems, however, are designed to apply coatings to silicon wafers and are much too small for the production of telescope mirrors. The researchers, after conducting a lab-scale study, designed a larger system that can be used for the production of telescope mirrors. They then found an equipment manufacturer—Structured Materials Industries (SMI) of Patterson, New Jersey—willing to fabricate build the device.
The system was delivered in July 2017 and can accommodate telescope mirrors up to 0.9 meters in diameter. The researchers are conducting tests with the intent of optimizing the protective coatings. Their goal is to develop protective coatings that last at least 10 years. They also believe that the instrument is scalable and should work to protect larger telescope mirrors. In particular, they are working to develop a system that will be applicable for the mirror segments to be incorporated into the Thirty Meter Telescope (TMT), a proposed astronomical observatory initially intended to be located in Hawaii. This extremely large telescope will be three times as wide, with nine times more area, than the largest currently existing visible-light telescope in the world, according to the TMT International Observatory. Its 30-meter-wide primary mirror will consist of 574 mirror segments, some of which will be 1.4 meters across.
The UC Santa Cruz researchers also would like their system to enable the recoating of existing aluminum mirrors. Compared to building ever-larger telescopes, recoating existing instruments with silver mirrors would be a cost-effective approach to improving their ability to collect light. One example is the 1.8-meter-diameter hexagonal segments used to form the 10-meter primary mirrors of the twin Keck Telescopes in Hawaii.
Currently, blank mirror segments are coated with silver and an initial protective coating using a physical deposition process. This coating is intended to protect the silver surface of the mirror segments during transfer to the ALD system. The researchers are working to develop an ALD process for deposition of the silver surface as well, thus eliminating the need for any transfer and the allowing the use of the superior ALD process for all phases of mirror production.