Re: Fluoritkristall Apos vs. FPL-53 Fluor ED Ap
Hallo allerseits,
hier ein Zitat aus einer Diskussion auf Astromart vom 21. Oktober 2005. Roland Christen schreibt:
Beginn des Zitates:
"Ohara FPL53 ED glass is made largely from Fluorite. It is a remelt of the material with other ingredients added to make a homogeneous glass which can be molded to a lens shape and which has no crystal planes. It takes a high polish and can be coated with a hard multi-layer.
Fluorite itself is a soft crystal which is 3 times harder to polish than FPL53, and in the highest grade necessary for precision lenses is 2 - 3 times more expensive. It is very difficult to coat real crystalline fluorite because the crystal planes break through the surface and cause the coatings to be weak at the crystal boundaries. Fluorite has no advantage whatsoever for amateur optics. It is neither easier to make, nor is it less expensive. The only advantage that it has is increased transmission in the UV, something that is negated by the need for a mating element in any case. Even if the mate could pass UV light, nobody views deep sky objects in the UV anyway.
There are a number of companies who use the generic term fluorite to market what is really an ED optic. Both Zeiss and Leitz in Germany make spotting scopes which use ED but are marketed as Fluorite. I suppose that anyone using Ohara FPL53 could say that it was a fluorite optic since the main ingredient in that glass is Fluorite, but I prefer to only designate that label to any optic that uses real crystalline fluorite material.
It is possible that despite these drawbacks, some company will produce a real fluorite lens. At times there are large quantities of fluorite windows made for certain applications, and there will be surplus available (this has recently happened for another glass which I am interested in). In such cases, some of the material may not have the UV transmission required for the original project, but will be fine for an amateur optic.
In normal situations, fluorite is way too expensive for a typical amateur application. I have here a sample 100mm dia x 20mm thk fluorite disc supplied by a Chinese glass company who specializes in fluorite manufacture and lens fabrication. The raw blank was $400 and a fully polished single fluorite element with multilayer coating was quoted at $1000. That does not include the mating element or any cell. A raw 100mm FPL53 lens blank from Ohara will cost approximately $100 - $150 depending on thickness."
Ende des Zitates
Daraus kann man ersehen, dass die Frage zur Berechtigung von Fluorit durch sinnvoll ist. Allerdings ist ebenso klar, dass auch FPL53 ein sehr teures Material ist.
CS
Andreas
