What is Fluorite?
So what is fluorite and why is it used in telescope optics? We know fluoride is good for teeth, but why is it good for telescopes? Crystalline calcium fluoride (aka Fluorite) is a naturally occurring crystalline mineral with the simple chemical formula CaF2. For optical use, though, the crystal has to be grown in controlled conditions – natural fluorite has too many flaws, inclusions and staining. So Fluorite is a crystal, which means it’s not a glass (glass is like a supercooled fluid and has no regular crystal structure)!
The good thing about fluorite, from an optical standpoint, is that it has a very low dispersion, lower than any glass. Dispersion is the property of an optical material that measures how much it spreads light of different wavelengths to create a spectrum. So, when used as the positive lens in a doublet and paired with a suitable glass for the negative element, fluorite allows you to make a doublet with a very low secondary spectrum, i.e. a good apochromat with minimal chromatic aberration.
So why don’t all lens telescopes use fluorite? The answer of course is that fluorite is expensive; it is also soft, fragile and difficult to work. So, in recent years, glasses that approach the optical properties of fluorite have been developed. Such glasses contain a high proportion of fluorides, in place of the usual oxides. These are described as “ED” or “SD” glasses; FPL 53 is a common example from the Japanese glass maker Ohara. These glasses have optical properties close to those of fluorite, but crucially they are still not as good.
In many cases, telescopes advertised as using fluorite (or bearing a designation like “FL”) actually use high-fluoride glasses and not crystalline fluorite. Exceptions are the older Takahashi APOs and TEC APOs on sale today. Interestingly, Takahashi stopped using fluorite because the mating element glass contained a lot of heavy metals (and because the fluorite blanks became so expensive). TEC still use fluorite in it’s bigger lenses because, I believe, ED glasses just aren’t available in large sizes.
Is Fluorite Better?
As the makers of modern APOs using ED glass will tell you, there is only a small difference between a premium ED glass like FPL53 and Fluorite, but the remaining difference does mean that for a given size and focal ratio a fluorite refractor can have a lower level of chromatic aberration than an ED one. In smaller triplets this difference doesn’t matter much because it’s possible to get near-perfect correction for chromatic aberration with ED glass and APOs like Takahashi’s TOA and TSA series, LZOS refractors and recent Astro-Physics models demonstrate this. However, for larger APOs and for doublets, the fluorite does have a real advantage over ED glass. What’s more, fluorite scatters light less than glass, something I think you can see when you compare a Takahashi FS doublet to just about any other refractor.
How can you tell if your scope uses fluoride rather than just a high-fluoride ED glass? Well, I have read that the low scattering properties of fluorite mean that you can’t see a laser beam in it, whereas you can in glass. BUT, when I tried this at home with a laser pointer, all I achieved was a dangerous level of laser scatter!
Is Fluorite Fragile?
In theory Fluorite is a fragile mineral with a tendency to crack under thermal stress and it also tends to degrade in contact with water – not ideal properties for a lens, you’d think. This was one of the reasons people used as an excuse for taking a pop at the Takahashi FS series; the reasoning was that putting the fluorite in front was a recipe for problems. But know this: I used to own a 20 year old FS102 which had seen a great deal of use (including – obviously from the smears - dew and subsequent wiping) but was optically superb and showed no degradation of the fluorite whatever. Neither have there have been problems with the other FS102 I owned, nor with the FS128 or FS78.
In fact, ED glasses with a high fluoride content can be soft and fragile too, so if you own a fluorite-containing objective just treat it with the usual care.
In truth the optical materials that make up the lens in your telescope (or binoculars or camera) are just one part of several factors which affect the lens’ performance. The design, figure and assembly of the lens all count for just about as much. In my experience, all the hype about fluorite, FPL53, OK4 or whatever becomes irrelevant when you use the telescope for imaging or viewing. Either the designer and fabricator has done a good job, chosen suitable materials and production methods to produce wonderful views and images... or not.
Rather than worrying about the materials used, just buy a quality instrument from a quality manufacturer and enjoy the view.