Thin menisci

Thin menisci made of Zerodur are used as mirror substrates in astronomical telescopes (see Sect. 4.6.1). For massive conventional mirror substrates, a thicknessidiameter ratio of 1 6 was a usual rule. With this ratio, the mirror blank is adequately stiff. This means that its function is not restricted by deviations in the contour caused by weight, wind pressure, temperature inhomogeneities, etc. 

This rule is no longer applicable when the following requirements must be met  

In both cases, the thickness diameter ratio is approximately 1 20 to 1 50. Such mirror substrates are called thin menisci. 

There are three essentially different techniques for the manufacturing of thin menisci from Zerodur  

In the conventional technique (see Sect. 4.2.2), thin plates are cut with a wire saw from thick, already ceramized cast pieces and are ground with diamond tools on all sides to achieve the desired contours, thicknesses, and diameters. 

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Copies of Boris Cahan s thesis were circulated among many working in the fundamentals of the field in the 1970s. The thesis gave its readers access to the details of the elegant experimental studies it reported on the interplay between meniscus shapes and the thickness of the boundary layer, i0, electrolyte conductivity, and the local heat developed in porous electrodes used in fuel cells (and which tends to dry out thin menisci). 

Zerodur can be supplied in the form of disks, rectangular blocks, prisms, rods, and cut pieces. Thick disks which are self-supporting can be produced with up to more than 4 m in diameter and with volumes of about 8 m (equivalent to 20 metric t). Thin menisci which are not self-supporting can also be produced up to masses of about 20 t the largest sizes produced so far were the 8.2 m menisci for the Very Large Telescope of the European Southern Observatory see Sect. 4.3.1. 

In some cases, tighter tolerances are required by optical figuring shops. Zerodur thin menisci or piano-spherical blanks can also be supplied in near net shape. Net shaping means linear dimension tolerances of 0.2 mm or smaller. The spherical grinding of such net shaped blanks leads to profile tolerance zones ranging from 0.05 to 0.40 mm P-V. These tolerances are achieved not only for blanks of 1.0 m in diameter but also for thin menisci of 8.2 m in diameter. 

Fig. 4.19. The most economical manufacturing techniques for thin menisci for two relative apertures (f l, f 2) (A) conventional technique, (B) slumping technique, (C) spin casting technique...

Spin Casting of Thin Menisci with Large Volumes... 

Rapid developments also took place in two main directions in the design of telescope mirrors, which became lighter and larger. Schott participated in several development programmes investigating different approaches with respect to their feasibility. The outcome of these studies revealed that the preferred solutions are thin menisci which are supported by active actuators. These developments are highlighted by the present engagement of Schott in the production of thin menisci of more than 8 m in diameter and with 29 mm thickness. 

As mentioned above the deformation of large thin menisci under their own weight is the main difference in the manufacturing steps following the casting as compared to the process for massive blanks. Tensile stress values of... 

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