PROGRESS IN THE DEVELOPMENT OF A DURABLE SILVER-BASED HIGH-REFLECTANCE COATING FOR ASTRONOMICAL TELESCOPES.
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PublisherThe University of Arizona.
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AbstractInfrared astronomical observations have been severely limited by the radiation emitted by the aluminum coated surfaces of telescope mirrors. Hence, the reduction of emissivity with retention of coating durability for telescope optics is now an important area of study essential for the improvement of infrared astronomical observations. To avoid the considerable effort that a search for a new material superior to aluminum would demand with uncertain outcome, only existing materials were investigated and silver with a protective dielectric layer was thought to be a possible solution. Experiments at Kitt Peak National Observatory showed that the optical performance of a silver coating is better than aluminum in the infrared and visible regions, but even with a sapphire overcoating the silver proved to have such poor environmental resistence that an experimental mirror had to be recoated after just ten months in service. Thus, the improvement of the durability of a silver-based high reflectance coating becomes a key issue that has to be solved for such a coating to be selected for infrared astronomy. Furthermore, most telescopes are used also in the ultraviolet and so any successful coating would need to reflect well in this region. Silver is poor also in this respect. In this dissertation, we describe the development of a silver-based high reflectance coating that can withstand the humidity and pollution common in the open air and which has good ultraviolet, visible, and infrared performance. The successful design incorporates a silver reflective layer with a copper underlayer and a stack of dielectric overlayers. Prototypes of the candidate coating have been deposited on two-inch glass slides and tested in a controlled environmental chamber and under true operating conditions on Kitt Peak in Arizona. The improved durability, partly due to the copper underlayer, has been investigated with analytical techniques, including Rutherford backscattering. The results showed that the protection of the silver is due not only to the dielectric stack and the copper beneath the layer, but also to a small amount of copper that appears on the outer surface of the silver layer. This surface copper may result from diffusion through the silver layer after the coating deposition, or may be a consequence of a process during film growth. Whatever the reason, the candidate coatings have better optical performance and improved durability so they are recommended for use on large telescope mirrors for astronomical observations in the near ultraviolet, visible, and infrared spectral regions.
Degree ProgramOptical Sciences