A007 Merkmale eines Apochromaten
Von Roger Cergioli gibt es hier einen sehr umfassenden Beitrag, der sich in Chapter 4a/4b mit der geschichtlichen Entwicklung von optischen Gläsern,
von farbreinen Teleskop-Optiken, mit dem Busch Halb-Apochromaten und mit den strengen Merkmalen eines Apochromaten ausgehend von Abbe,
König-Köhler und anderen Autoren auseinandersetzt. Die für mich wichtigsten Zitate will ich im Laufe der Zeit hier darstellen.
Yet apochromats continued to be made, particulary by Carl Zeiss in Germany (Abbe's employer), which developed four styles: an early doublet "half-apochromat" built down to a speed of about f/18, called the "A" objective (designed by A König) with very good color correction; a later revision of this, also a doublet built down to a more manageable f/10, called the "AS" objective (designed by A. Sonnefeld) with reduced secondary spectrum; a triplet full apochromat built down to f/15 and modelled on Taylor's lens, called the "B" objective (also designed by A. König) with excellent color correction; and a later triplet built down to f/11, employing dense flint glasses and called the "F" objective (designed by H. Köhler & R. Conradi), which fell between the B and the AS in color correction, but whose radii of curvature were much weaker than those in either of the other types, making this lens easier to build and mount [cf. Riekher, pp. 212-216; A. Sonnefeld, 'Der Königsche Apochromat B,' Zeitschrift für Instrumentenkunde 61 (1941), pp. 261-264; A. König and H. Köhler, Die Fernrohre und Entfernungsmesser, 3rd ed. (Springer Verlag, 1959), pp. 61-62, & 128-139; J.G. Baker, "Planetary Telescopes,"
Applied Optics 2.2 (1963), pp. 111-129, especially pp. 117-118; G. Mann & A. Sonnefeld, "70 Years Astronomical Instruments Department [sic], Part I: 1897-1946," in Jenaer Rundschau 12.3 (1967), pp. 159-170; and U. Laux, Astrooptik, 2nd. ed. (1999), pp. 26-34].
Because of their expense, small apertures, and delicacy, for many years apochromats made no large impression in the amateur telescope market. But during the 1980s that changed when improved glasses and the use of index-matching oils and special tapes made it possible to construct simpler, cheaper oil-spaced triplets. At the same time, more affluent buyers could choose fluorite doublet objectives with nearly perfect color correction. This occurred because of the commercial industry to grow artificial fluorite crystals. Not only that, but Schott Glass Technologies and the Japanese glass manufacturer Ohara, building on Otto Schott's early work with abnormal dispersion crowns and his attempts to introduce fluoride compounds into the glass melts, began to market crown glasses that approached fluorite in their optical properties. These are called the "fluor-crowns" and are often designated "ED" or "extra-low dispersion" glasses, because their total dispersion is much lower than ordinary glasses and in the extreme cases approaches the extent and quality of fluorite itself.
Once ED glasses became readily available and a lucrative market was found to exist for high-end "apos," manufacturers dropped the use of short flints and redesigned their products using the more expensive fluor-crowns instead. This also occurred because the fluor-crowns' chemical stability was superior to that of short-flint glasses, and their low, abnormal dispersions made even better color correction possible, using longer radii of curvature. They thus opened the door to the shorter focal ratios which amateurs demanded. Moreover, since fluor-crowns are true glasses, not crystals, they are more shock-resistent than fluorite and capable of being fabricated with conventional glass working technologies.
Ohara is at present a major supplier of ED glasses (although other firms also melt them). They produce three varieties, called "S-FPL 51," "S-FPL 52," and "S-FPL 53." S-FPL51 is the least abnormal and delicate, while S-FPL53 (the latest to be marketed) is softer and can be broken rather easily, though it is less fragile than fluorite. Its optical properties, however, come closest to fluorite of any true glass. Thus it is widely used in triplet apochromats today. Doublet apochromats employing ED glasses are also marketed, though not as extensively.
Over the years there has been a trend toward faster apochromatic telescopes. The earliest examples had focal ratios of about f/15-f/20, just as slow as the largest visual achromats of the 19th century. H. Dennis Taylor's triplet came in at f/18. Zeiss's A-objective was built at athe B-objective at f/15. The 1980's short-flint triplets were faster, about f/12-f/10. By the 1990s with the arrival of the ED triplets, focal ratios declined to f/9. Presently, f/6-f/8 seem common speeds for 100-125mm lenses, and f/7-f/9 for 150mm and larger glasses. Field flatteners and correctors can be employed to turn these instruments into medium field astro-cameras [cf. U. Laux, Astrooptik, 2nd. ed. (Sterne und Weltraum, 1999), pp. 54-55 for an example] for general purpose imaging.