Exploring New Deposition Processes for Astronomical Coatings

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This project will improve optical coatings for astronomical mirrors. The primary purpose of astronomical telescopes is to gather as much light as possible for analysis. However, at every reflection, a certain amount of light is absorbed by the mirror surface. Currently, the standard mirror coating is aluminum, with roughly a 10% loss at each surface. Since modern telescopes usually use three or more mirrors, this results in 30% or more of the light being lost. Silver is a significantly better reflector than aluminum, losing only about 3% of the light at each reflection. Silver coated mirrors would result in significant gains in light collection at a fraction of the cost of building a new larger telescope. However, silver is prone to tarnishing and corrosion, so it must be covered by transparent protective layers. Progress towards suitable silver mirror coatings has been made in recent years, but a top protective layer that can hold up to years of use is still lacking. This project will address that missing piece by exploring two new thin film deposition techniques that are expected to extend the lifetimes of silver mirror coatings significantly. While this project is aimed specifically at coatings for telescope mirrors, it has potential use in other areas, such as increasing the efficiency of solar concentrators for energy production.This project will install specialized equipment for exploring new thin film deposition techniques in an existing vacuum chamber. The first new deposition method is High-power Impulse Magnetron Sputtering (HiPIMS). In conventional sputtering, atoms are knocked off source material by ions (charged atoms), and then land on the surface being coated. HiPIMS modifies this by creating the ions in very short pulses at high power, giving the ions more energy. Consequently, the atoms knocked out are also at higher energy, which helps compact the growing thin film. The second new technique is Filtered Cathodic Arc (FCA) deposition, in which an electric arc going directly into the source material vaporizes the material. The outflowing stream contains particles as well as atoms, so a magnetic field is used bend the stream. This filters out particles and neutral atoms, resulting in a very pure stream of high-energy ionized atoms. In this project, these two high-energy processes will be used to make specialized materials such as diamond-like carbon and certain nitrides. These materials have the properties that are needed for transparent protective layers. In addition, these techniques can deposit silver at very fast rates, which improves reflectivity. Samples of protected-silver mirror coating will be made, and then placed in a harsh environment to indicate how long they will endure on telescopes.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.