Telescope mirror blank

Thermal Properties. Many commercial glass-ceramics have capitalized on thek superior thermal properties, particularly low or zero thermal expansion coupled with high thermal stabiUty and thermal shock resistance properties that are not readily achievable in glasses or ceramics. Linear thermal expansion coefficients ranging from —60 to 200 x 10 j° C can be obtained. Near-zero expansion materials are used in apphcations such as telescope mirror blanks, cookware, and stove cooktops, while high expansion frits are used for sealing metals. 

Given these special properties, this high-performance material has been successfully used for a variety of applications such as telescope mirror blanks in precision optics and infrared-transmitting range tops (Section 4.3.1). This glass-ceramic has also been successfiilly mass produced for household applications such as cookware and woodstove windows (Section 4.2). 

Product development is mainly centered around the three product ideas of cookware, range tops for kitchen stoves, and telescope mirror blanks. 

C.L. Babcock, R.A. Busdiecker, E.C. Hagedorn Method of making low expansion crystaUized glass-ceramics and telescope mirror blanks made thereby U.S. Pat. No. 4,285,728, August 25, 1981... 

During the development of Zerodur for the MPI telescope mirror blank the question was raised whether Zerodur shows any effect of thermal ageing, which is common to many glasses. Petzoldt [4.23] performed some investigations but did not observe any effect. He concluded that one reason for that could be the relatively small amount of 30 vol% of the residual glass phase. But when Zerodur was tested for other applications, a sensitivity of the length of Zerodur to thermal history was observed. 

At the end of the project, Schott had created a new material for precision optics applications, had sold some telescope mirror blanks, but had also quite an amount of unsold material of different quality in stock. It was now up to the sales people to find other applications for Zerodur than telescope mirror substrates, thereby creating new markets for this new material. Relatively soon two new applications were found which will be described in more detail in Sect. 4.6 one was mirror substrates for reflective optics in microlithography, for which castings of approximately 500 mm in diameter were needed, and the other was housings for laser gyroscopes, for which continuous bars of cross sections of about 140 x 40 mm were the best delivery... 

Applications include transparent cookware (Visions , thermal expansion = 7 x 10 V°C), optical materials (e.g., ring-laser gyroscopes), telescope mirror blanks (Zerodur ), infrared-transmitting electric range tops, wood-stove windows, fire door glazing... 

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