Refractor buffs love fluorite, but it has some nastier sides that people should be aware of.
In the past reports of the instability of fluorite/ED glass have not been helped by some rather vaguely phrased statements made in respected optics textbooks. Here is one example, taken from Rutten & Venrooj (2002) pp 58:
“Although fluorite is ideal as an optical material, it has its drawbacks. Not only is it expensive and the size of blanks severely limited , but fluorite is highly subject to weathering. Despite its drawbacks, a number of telescope manufacturers now offer fluorite objectives.”
Because fluorite is a mineral, it is significantly more water soluble (~150mg L^-1 @25C) than a true glass.
Although it is inert to organic solvents, it is reputed to dissolve slowly in nitric acid or solutions containing ammonium ions. That said, I have personally not witnessed anything untoward on the objectives of older (late 1990s) apochromatic objectives.
Nonetheless, the solubility of fluorite is quite high actually, and although there is little to worry about if care is taken to ensure that no condensation builds up on the fluorite element of an object glass, it it is not beyond the realm of possibility that years of exposure to moist climes could potentially degrade its exacting curvature, possibly requiring slight refiguring, or worse still, replacement.
Other ways forward:
A folded, standard achromatic design in apertures of 6-inches or above seems a very straightforward way to circumvent the traditional size limitations of classical crown & flint object glasses, especially in relative apertures of f/15 or greater. High quality flats are now available to more than meet the requirements of these hypothetical instruments. Moreover, it is my belief that the excellent thermal properties of these glasses ( with thermal expansion coefficients about 2.5 times lower then fluorite), executed in a folded array, would make for a very powerful – and cost effective – large aperture telescope. There is a inspiring history of superbly functioning folded refractors already widely disseminated in the astronomical literature that testifies to the proven merits of this stratagem.
Another way forward is to review the glass types employed by ED refractors. My online review of the Meade 127ED refractor was designed to raise awareness of the possibility that economically priced ED doublets could be made with pretty good colour correction (read considerably better than the standard crown flint achromatic doublet), without breaking the bank. Vladimir Sacek, founder of Telescope Optics, was kind enough to share his thoughts on the ‘problem’ of choosing the right kinds of glass to make such a venture worthwhile:
The choices are limited by the availability of matching glasses. If you look at the relative partial dispersion (RPD) diagram (below), you see that so called “normal glasses” pretty much conform to a diagonal line. Since the amount of secondary spectrum – defined as the longitudinal separation of the green focus from the common red/blue focus – is proportional to a RPD vs. Abbe differential for the two glasses (it can also be envisioned as a height vs. base side of the right-angle triangle formed by connecting two glasses), those normal glasses can only produce ‘regular’ achromatic correction.
In addition, a practical requirement is that the two glasses differ significantly in their dispersive power. The smaller this power differential, the stronger the inner curvatures of a doublet have to be in order to bring the two wavelengths to a common focus. Excessively strong curvatures results in too much higher-order spherical aberration, which makes good correction impossible. In general, the Abbe differential for the two glasses should not be significantly smaller than about 30.
Only a small group of glasses to the left on the diagram can combine with normal glasses from the bottom portion of the glasses’ diagonal, to produce and good colour corrected objective. BK7 is too close to FPL51 for example, and that combination would produce too much higher-order spherical aberration. KF3 makes a good match for an f/9 system. KZFN2 is, obviously, a somewhat better match, but the gain is nothing to write home about (little over 10%). KZFN1 is a bit worse, and so is KZFS1. Nearly ideal matches for a medium-to-slow ED ‘scope would be KZFSN2 and N-KZFS2 and these are (currently) about ten times the price of BK7.
This reasoning, in general, also applies to triplet objectives. Colour correction changes little in a triplet vs. doublet for a given pair of glasses (adding a third glass also makes little difference in this respect). The main advantage of the triplet objective is in significantly less curved inner surfaces required.
That is the reality of it. If you look at the height vs. base ratio for the standard doublet achromat. (BK7/F2), a system with significantly better correction would have to have it about twofold – or more – smaller and, at the same time, to have the two glasses separated by not much less than three horizontal spaces.
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