Zitat von Sven_Wienstein:
...nämlich gegen ein Vakuum verformt...
Zitat von UwePilz:
So hat es Schmidt mal erfunden. Hergestellt wird es schon längst nicht mehr so. Es werden spezielle Polierlappen benutzt, die sehen ähnlich as wie große Kleeblätter. Die Polierzeit wurde mal erprobt und wird dann für jede Platte so verwendet. Die Genauigkeit hängt also von der Prozessstabilität ab.
Zitat von Sven_Wienstein:
interessant, wo gibt es Info darüber?
Ich ... meine ... Celestron selbst hätte vor Jahren mal den "klassischen" Prozess beschrieben ...
Hallo allerseits,
es gibt zwar zahlreiche Beschreibungen von Herstellungsmethoden für Schmidt Korrektorplatten, dazu gehören insbesondere auch die bekannten Artikel aus "Amateur Telescope Making" über die originale Vakuummethode von Bernhard Schmidt, wie auch alternative Poliermethoden wie die erwähnte Kleeblattmethode.
Von Celestron und Meade gibt es aber verständlicherweise keinerlei offizielle Information über ihre Herstellungsmethoden, und das betrifft natürlich insbesondere das Verfahren zur Korrektorherstellung.
Ich war Mitte der Siebziger Jahre mal zu Besuch bei Celestron in Torrance, CA, um Details für eine von mir bestellte 14" Schmidt Camera zu besprechen. Bei der Gelegenheit wurde ich zwar vom Chef Alan Hale jovial empfangen und durfte auch die Produktionsstätten besichtigen, aber gewisse Bereiche ware "off limits" und dazu gehörten natürlich auch die Korrektoren. Diesbezügliche Fragen wurden zwar höflich aber immer ausweichend beantwortet. Vermutlich ist es beim Konkurrenten Meade ähnlich. Es lässt sich natürlich auf Dauer nicht vermeiden, dass solche Betriebsgeheimnisse auf inoffiziellen Kanälen z.B. über ehemalige Mitarbeiter durchsickern und daraus entwickeln sich dann teilweise wilde Spekulationen.
Übrigens hat
Bernhard Schmidt in seiner einzigen kurzen Publikation der Sternwarte Bergedorf in Hamburg mit dem Titel
Ein lichtstarkes komafreies Spiegelsystem auch nichts Konkretes zur Herstellungsmethode erwähnt. Er war in dieser Hinsicht ausgesprochen verschlossen. Was wir darüber wissen, stammt überwiegend von seinem damaligen Mentor Walter Baade, dem späteren Direktor von Mt. Wilson und Palomar Observatories, und von Richard Schorr, dem damaligen Direktor der Sternwarte Bergedorf. In dem Artikel von Paul Hodges über "Bernhard Schmidt and His Reflector Camera" in 'Amateur Telescope Making' Band 3, S. 365-375, findet man u.a. dazu:
From March to June, 1929,Schmidt accompanied Baade on an ecclipse expedition to the Phillipines, and one evening when they were in the Indian Ocean told him that he had solved the problem of designing a coma-free reflector that would have good light-gathering power and wide field. He sketched the basic design and then the details of the construction, possible forms of the correcting plate, size of field, residual color, vignetting, etc., and it was evident that the whole thing had been thought out some time earlier - perhaps even before leaving Hamburg. Baade urged that Schmidt plan to build such a reflector as soon as possible after returning to Bergedorf but the reply was that Schmidt must first think up an "elegant" way to grind the aspherical surface of the plate or in the vernacular of optics to "figur" it. To do it with small grinding tools Schmidt thought would be a sloppy job and would produce a plate that was full of zones.
After the expedition had returned to Hamburg, Schorr in his energetic fashion joined Baade in urging Schmidt to get on with the work but to no avail ... Finally in the winter of 1929-1930 he remarked casually that he had at last devised an elegant solution for the figuring of the plate but for some of the details needed to consult a treatise on elasticity. Baade suggested the recently acquired "Handbuch der Physik" and after more than a week of library work Schmidt announced that he had found what he needed and was ready to assemble his grinding equipment.
There was a heavy metal pan of carefully calculated diameter and with its upper edge ground at a precise angle of bevel based on the coefficient of elasticity of the particular glass plate that was being used and of the minute dimensions of the toroidal curve he wished to produce. The glass plate (of greater diameter than the pan) was sealed to the ground edge of the pan and then, by means of a hand pump and manometer, air was exhausted until a particular negative pressure had been developed. This caused the glass plate to warp slightly, the center being low, the glass over the edge of the pan high. Holding the plate in this position by maintaining a constant negative pressure, Schmidt ground the upper surface until it was again plane, and then when the vacuum was released the plate sprang back until its bottom surface was plane, its upper surface slightly figured.
Baade remembers visiting the basement workroom one morning. Schmidt was napping briefly after 36 hours of uninterrupted work and on waking accepted cigars but declined coffee and sandwiches because, though the plate now had the desired curve, there was still the drudgery of obtaining the necessary polish. Twelve hours later the plate was finished and, as predicted, was free of zones.
The camera went into operation early in 1930 and many dozens of stellar photographs were made in the following weeks and months. The windmill photograph (shown as Fig. 1 on page 355), which was quite incidental, was made on a very cold winter night early in 1931 when the Schmidt camera was about a year old.
Schmidt's one and only publication appeared in 1932 and would not have appeared at all except for the loyal and diplomatic urging of his friends Schorr and Baade. Finally with the aid of the inevitable cognac, coffee, cakes and cigars he was induced to dictate the paper to Baade and after rewriting it for a last polish to give final approval to its publication.
In Schmidt's original camera the diameter of the correcting plate was 14.4 inches and that of the front surfaced morror was 17.3 inches. The mirror had a radius of curvature of 49.2 inches, and the speed of the system was f/1.7.
ATM Book 3 enthält noch zahlreiche andere interessante Artikel über Schmidt Korrektoren. Dazu gehören auch detaillierte analytische Beschreibungen der Korrektorform und ihrer Herstellung mit der beschriebenen Vakuummethode, aber auch andere Methoden werden erwähnt. Interessant ist auch der folgende Hinweis auf Seite 371:
Elegant as it it is, Schmidt's partial vacuum method of figuring correcting plates has not been used to any extent. Why? This question was submitted to Chief Optician Don O. Hendrix of the Mt. Wilson and Palomar Shops, who figured the correcting plates for a number of Schmidts, culminating in the 48-inch "Big S" at Palomar Mountain. He replied:
"Schmidt's original vacuum bending method of generating the proper correcting curve was and is very successful. The only bugs are: (1) The plate will usually break if bent down enough to generate a curve for cameras f/1.5 or faster. (2) One can, by ordinary methods, make a correcting plate before he can compute the necessary curvatures and collect or make the necessary pumps and auxiliary apparatus. (3) For very fast cameras the curve obtained is not sufficiently exact and requires considerable hand correction. The reason for this is that as soon as one grinds for a while the deflection is not that for a parallel plate."
"Schmidt was a great believer in trade secrets and did not publish or otherwise divulge his method of correcting plates except to a few intimate friends, and thus almost lost priority for the method. Many others, including myself, independently worked out the same scheme of correcting Schmidt plates and it was not known until after Schmidt's death exactly how he made them."
Der erste und der dritte Punkt erscheinen etwas artifiziell angesichts der Tatsache, dass es kaum Schmidt Optiken gibt, bei denen der Kugelspiegel schneller als f/1.5 ist. Der zweite Punkt scheint eher eine gewisse persönliche Einstellung als ein objektives Argument zu sein. Bleibt eigentlich nur der Hinweis auf 'Trade Secrets' und der Eindruck, dass sich Optiker als Individualisten verstehen, die gern ihre eigenen Methoden pflegen, aber nicht darüber reden mögen ...
Mit freundlichen Grüßen,
Peter
