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 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; FPL53 (and more recently FPL55) 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 Takahashi doublets (and older triplets), Borg doublets, the Vixen ‘FL’ series and modern apochromats from TEC (Telescope Engineering Company).
Takahashi was a pioneer of using Fluorite for telescopes, but stopped for a while because the mating element glass contained the heavy metal Lanthanum (so wasn’t an ‘Eco’ glass). They have started using it again in recent years – currently only in their smaller doublets - as new mating elements have become available.
TEC still use fluorite in its bigger lenses because, I believe, ED glasses just aren’t available in large sizes and in order to push the envelope with some models – the 110FL has a very fast focal ratio for its aperture (F5.6).
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.
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 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).
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. Given that fluorite is fragile and hygroscopic (water-absorbing) it seemed prudent to put it at the back out of the way of dew and 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 mineral with a tendency to crack under thermal stress. 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 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. Neither have there have been problems with the other FS-102 I owned, nor with the FS-128 or FS-78. I have carefully cleaned an FS objective with no ill effects.
In fact, ED 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.
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 too.
However, if you are in the market for a doublet APO – particularly if you want to use it for high powers or critical imaging – understand that for a given size and F-ratio fluorite lenses will perform better in terms of false colour. In fact, a fluorite doublet will typically perform as well as all but the best triplets and will be lighter in weight and quicker to cool.