Computer Science
Scientific paper
Jul 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000spie.4003..312t&link_type=abstract
Proc. SPIE Vol. 4003, p. 312-323, Optical Design, Materials, Fabrication, and Maintenance, Philippe Dierickx; Ed.
Computer Science
1
Scientific paper
Silver has the highest reflectance of all of the materials, but it tarnishes in the presence of sulfides, chlorides, and oxides in the atmosphere. Also, the silver reflectance is very low at wavelengths below 400 nm making aluminum more desirable mirror coating for the UV region. We have found a way to prevent silver tarnishing by sandwiching the silver layer between two thin layers of NiCrNx, and to extend the metal's high reflectance down to 200 nm by depositing the (thin) Ag layer on top of Al. Thus, the uv is transmitted through the thin Ag layer below 400 nm wavelength, and is reflected from the Al layer underneath. This UV-shifted durable coating provides a valuable alternative to the aluminum coating for telescope mirror coatings where high throughput and durability are important considerations. The throughput for a telescope with, say, six reflections from silver coatings is (0.97)6 equals 83% compared to (0.92)6 equals 60% for aluminum coatings, or 28% less. The use of silver coatings allows more photons to be collected by primary mirror. Aluminum also has a reflectance dip at 850 nm caused by inter-band transitions which is eliminated by placing the thin Ag layer on top. This paper describes a non-tarnishing silver coating having high reflectance down into the UV region. The average specular reflectance is 70% - 97% in the near-UV, 95% - 99% in the visible region, and >= 99% in the infrared region covering the total wavelength range 200 nm to 10,000 nm.
Thomas Norman L.
Wolfe Jesse
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