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.
Summary
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.
