High-Performance Durable Coatings for Large Astronomical Optics

University Of California-Santa Cruz, Santa Cruz CA

Investigators

Abstract

With modern semiconductor detector arrays approaching near-ideal performance, the throughput of telescopes and their associated instrumentation is becoming limited by the coatings applied to the optical components. Mirror coatings must have the highest reflectivity attainable, be able to withstand extremes in temperature and humidity, be cleanable, and be as durable as possible. Transmissive optics (lenses), by contrast, require extremely low-reflectivity surfaces, yet the coatings to achieve these low light losses must also be essentially impervious to contaminants and periodic cleaning. Today's best coatings leave much to be desired in these regards. Aluminum is still the standard telescope mirror coating, despite losses as high as 10% in the optical red spectral region, while the best antireflection coating is soluble in water and therefore difficult to maintain. Astronomy is not the only discipline that would gain from improvements to optical coatings. Solar arrays and solar concentrators, for example, also benefit directly from better throughput, and the potential economic impact of a 10% gain in electrical conversion efficiency would be astounding, when considered on the worldwide market. Improvements in optical coatings require not only new and better formulations but also more uniform and reliable application techniques. The coatings lab at the University of California Observatories (Santa Cruz), headed by Dr. Andrew Phillips, has been engaged in efforts to improve both reflective and anti-reflection coatings for astronomical optics for a number of years. Starting with improvements to the infrastructure of their coating facilities, Dr. Phillips seeks to pursue two goals: a definitive comparison of the efficiency and uniformity of depositing coatings through the relatively new technique called "ion-assisted deposition" vs. the more traditional approach of "sputtering". The improvements will also extend their coating capabilities to include the reactive deposition of nitrides, which are critical to the highly reflective coating considered to be the current state of the art. A novel moving stage inside the vacuum chamber will also allow application onto large substrate areas with improved thickness and process uniformity. This same new equipment will be used to develop both high-performance protected-silver coatings for mirrors as well as multi-layer sol-gel anti-reflection coatings for large lenses. Funding for this work is being provided by NSF's Division of Astronomical Sciences through its Advanced Technologies and Instrumentation program.