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Ein Vorschlag zur Erzielung hochreflektierender Teleskop‐Optik
Author(s) -
SchmidtKaler T. H.
Publication year - 1994
Publication title -
astronomische nachrichten
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.394
H-Index - 63
eISSN - 1521-3994
pISSN - 0004-6337
DOI - 10.1002/asna.2103150410
Subject(s) - layer (electronics) , substrate (aquarium) , optical coating , materials science , dissolution , optics , optoelectronics , nanotechnology , coating , physics , chemistry , oceanography , geology
Since 60 years the common method generally used to coat astronomical mirrors is vacuum deposition of pure aluminium by evaporation. Such mirror surfaces degrade very fast resulting in a considerably reduced optical throughput. Protecting layers (oxides like SiO 2 , fluorides like CaF 2 ) are not applied because their removal, necessary before realuminization, involves damage to the optical surface of the mirror substrate which would necessitate a costly refiguring of that surface. The reduction of the throughput exceeds typically 30 % for a Cassegrain system; it precludes the introduction of excellent optical design solutions in astronomy based on Schwarzschild's theory using four‐reflection optics. In addition, the cost of the conventional procedure of aluminizing and its risks increase tremendously with the size of the mirrors of the very large telescopes now planned or under construction. Here I propose a three‐layer composion in two forms (Fig. 1): a) stopping layer ‐‐ reflecting layer ‐ protecting layer, or b) carrier layer ‐ protecting layer. The stopping layer is supposed to be completely insensitive to the chemical agents dissolving or detaching the protecting (and the reflecting) layers. On the other hand, in modification b), the carrier layer is supposed to be chemically removable from the substrate, thereby detaching also the protecting (and reflecting) layer. As an example for a layer system of type a) experiments are under way with the system gold/silver/quartz on various mirror substrate materials. Quartz can be easily dissolved by fluorid acid without any damage to the gold stopping layer. Dense. pore free Al 2 O 3 +SiO 2 layers without defects can be expected to protect the silver reflectance R ⩾ 0.98 under real observatory conditions effectively for more than 10‐20 years. In addition this protecting layer permits frequent and effective cleaning of the mirror from dust etc. The reflectance of silver exceeds that of fresh conventionally aluminized mirrors with R ≅ 0.87 for all wavelengths λ ⩾ 3370 Å. In the blue the silver mirror realizes with R = 0.995 ideal conditions for Schwarzschild optics. In the V‐IR region it is with ≅ 0.99 by the factor 1.2 better than present‐day mirrors.