Why Put Fluorite In A Telescope?
The lens surface you’re looking at isn’t made of glass!
What is Fluorite?
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 not a glass (glass 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 paired with a suitable high-dispersion glass for the negative element, fluorite can help make an objective with a very low secondary spectrum, i.e. a good apochromat with minimal chromatic aberrations.
So why don’t all lens telescopes use fluorite? The answer of course is that fluorite is expensive; it is also 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; FPL53 (and more recently FPL55) is a common example from the Japanese glass maker Ohara. These glasses have optical properties very close to those of fluorite, but still not quite 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 Takahashi doublets (and older triplets), Borg doublets, the old Vixen ‘FL’ series and modern apochromats from TEC (Telescope Engineering Company); the recent Vixen FL55SS too.
TEC still use fluorite in its bigger lenses because, I believe, ED glass blanks 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 and/or slower triplets this difference doesn’t matter much because it’s possible to get near-perfect correction with ED glass; apochromats like Takahashi’s TSA series, LZOS lenses and recent Astro-Physics models demonstrate this.
However, for large or fast triplets, and for doublets, 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 fluorite rather than just a high-fluoride ED glass? Well, the low scattering properties of fluorite mean that you can’t see a laser beam in it, whereas you can in glass: see examples below. But take care of you try this – laser scatter isn’t good for your eyes!
Fluorite Doublet or ED Doublet?
Numerous ED-glass (usually FPL-53) doublet APOs are sold today, including by Tele Vue, Sky-Watcher, Vixen, WO and others. Now you might reasonably think that all the technical hair-splitting that you just skimmed through wouldn’t mean much real difference; but you’d be wrong.
Look through (or image with) a fluorite doublet and you will realise that the level of false colour is typically lower than an equivalent ED doublet. This means that the fluorite doublet will show Venus, Jupiter, Bright stars and the Moon as white and sharp, whilst with the ED doublet they will be tinged with false colour. Images through the ED doublet may show hot blue or white stars bloated with violet rims. Whether this difference matters to you is another question, but the difference is there.
False colour shows up clearly and obviously in the Foucault test. Below are images of this test on three APOs. The first is a triplet super-APO. The second is an F8 fluorite doublet. The third is an F9 ED doublet.
Foucault tests: 1) Triplet super-APO 2) Fluorite doublet 3) ED Doublet
FS or FC?
Takahashi have made two fundamentally different designs of fluorite doublet over the years – the older FC series (lately re-launched) and the intervening FS series. This is not just a question of marketing. The FC-series (‘Fluorite Corrected’) uses a different lens design: a Steinheil doublet which puts the positive convex fluorite element at the back (‘normal’ doublets like the ‘FS’ series have the positive lens at the front). Vixen once used the same arrangement in their FL-series.
Why did they do this? The reason Takahashi originally chose a Steinheil design was simply because at the time the FC-series was introduced (the 1980s, I think) it wasn’t possible to coat fluorite. Fluorite is fragile so they put it at the back out of the way of owners’ lens cloths. Then later, when coating technology caught up, they changed to a traditional Fraunhofer lens design that put the fluorite at the front – the ‘FS’ (for Front Surface) series. Only recently have they reverted to Steinheils for the new FC series or 76mm and 100mm refractors.
So which is better, FC or FS? Well urban myth would have it that the Steinheil FCs are better, but I doubt it. Here’s why:
1) Steinheil lenses in general have steeper curves (possibly more difficult to make) and are “seldom used unless necessary” (Rutten and Van Venrooij).
2) In a conventional doublet, the front-surface fluorite confers a further advantage in transmissivity because the fluorite is the first thing the incoming light hits and fluorite transmits more light than glass … any glass.
3) Takahashi’s FOA-60 is supposedly their best corrected refractor ever and it uses a front-surface fluorite doublet lens.
I’ll leave the subject with a quote from the Takahashi manual for the FS series:
“When Takahashi first designed the fluorite apochromat refractor, they realised that the optimum design placed the fluorite element in front. Coating technology of the time did not permit this to be done.”
A Borg front-surface fluorite doublet showing laser test: fluorite only scatters the laser beam on entry and exit.
Takahashi FC-76 (new version) Steinheil fluorite doublet with the fluorite at the back.
Is Fluorite Fragile?
In theory Fluorite is a fragile material. It is also fairly soft, with a hardness of 4 on the Mohr scale: harder than your finger nail, softer than a nail.
The idea that fluorite degrades with dew and moisture seems to be an urban legend, though (and one I bought into at one time). Per Corning, who market fluorite optical components:
“Exposure to 100% relative humidity at room temperatures does not fog polished surfaces [of fluorite] even after one month.”
I owned a 20 year old FS-102 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.
I suspect this legend arose because some early Takahashi fluorite objectives (especially the FC-65 for some reason) are prone to develop haze. However, it turns out that this is due to deterioration or the flint mating element, not the fluorite!
Takahashi likely used a flint of the KzFS group – a ‘Special Short Flint’ – some of which, such as KZFS4, are sensitive to hazing in contact with water vapour (poor ‘climatic resistance’ in Schott’s terminology – KZFS4 is 3.0 on a scale of 1 to 4, whereas its eco-glass replacement is 1.0). Better coatings or a change in flint glass type means modern objectives don’t seem to suffer from this problem.
In fact, ED crown glasses with high fluoride content can be soft and fragile too, so if you own a fluorite-containing objective just treat it with the usual care.
The material used as the crown element – whether fluorite or ED glass - in your telescope (or binoculars or camera) is just one of many factors which affect the lens’ performance. The choice of mating flint element and the design, figure and assembly of the lens all count just as much or even more.
But fluorite does still seem to offer some advantages, especially for doublets from the likes of Borg, Takahashi and Vixen, which typically perform better than their ED-glass equivalents.
Meanwhile, the idea that fluorite is prone to degradation seems to be a myth, though it is softer than many glasses and needs care when wiping or cleaning.
Apochromats with fluorite lenses will often perform better than those with ED glass for a given aperture and f-ratio - as well as all but the best triplets, but lighter in weight and quicker to cool.