Large-Field and Focusing Schlieren Methods
Schardin essentially overlooked the large-scale possibilities of the lens-and-grid schlieren technique which he and H. Maecker pioneered [2]. Sixty years later, this and other opportunities have emerged to free the schlieren technique from its traditional
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In most cases it is unfortunately impossible to studythermo-hydrodynamic flowfields at full scale, owing to the limited dimensions of the available mirrors. Hubert Schardin [1] Schardin essentially overlooked the large-scale possibilities of the lens-and-grid schlieren technique which he and H. Maecker pioneered [2]. Sixty years later, this and other opportunities have emerged to free the schlieren technique from its traditional bonds of small scale. Concurrently some of these approaches allow narrow depth-of-field as well, hence the ability to focus upon a "plane" in a 3-D schlieren field. Both possibilities are crucial to the renewed vitality of schlieren imaging. These specialized schlieren techniques are given chapter status because they are the most important and most recent developments in the field, and because they are expected to playa strong role in the future of schlieren imaging.
4.1 Large Single- and Double-Mirror Systems 4.1.1 Availability of Large Schlieren Mirrors In Chap. 3 the cost of schlieren-quality parabolic mirrors up to about 65 em diameter was discussed. We have amateur astronomy to thank for the larger examples of these. At about f/5 - to minimize telescope length - they are short for schlieren use. Nevertheless they make fine z-type systems of large aperture if only simple illumination is used, and if astigmatism is dealt with properly. One manufacturer, Glass Mountain Optics, has supplied very-successful twin Y. A 71em-diameter parabolic mirrors on lightweight substrates for schlieren use [174,175] . They have the ability to produce mirrors up to 2.5 m diameter (see App. D). Such large traditional glass schlieren mirrors can be custom-made in the 1-2.5 m diameter range, but they are shockingly expensive. The conventional rule is a
G. S. Settles, Schlieren and Shadowgraph Techniques © Springer-Verlag Berlin Heidelberg 2001
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4 Large-Field and Focusing Schlieren Methods
6:1 diameter/thickness ratio, but some are willing to try 14:1 or 15:1 [175]. The optical fabricating industry has undergone an upheaval and several of the traditional stalwarts who once made large monolithic schlieren mirrors are no longer in business. Those who are, as of this writing, are listed in App. D. At the same time some fresh opportunities may be on the horizon for large schlieren-quality mirrors. For example, new earthbound telescopes now achieve large aperture by way of segmented mirrors: Kodak fabricated 91 thin l-m hexagonal segments for the Hobby-Eberly Telescope for $16,500 each [137]. Not that any such mirrors are known to be available for schlieren imaging, but the manufacturing technique and the possibility of surplus mirrors is worth noting. The same can be said for large, light space telescope mirrors [176-178]. The 2.4-m Hubble Space Telescope mirror, for example, is a glass face-sheet with an "eggcrate" backing. Such lightweight mirrors are contemplated for meter-class amateur telescopes, so schlieren imaging cannot be far behind. Finally, stretchable-membrane mirrors are proposed by Waddell e
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