Calcium fluoride vs Fluorite vs Fluoro crown

[Kunama]

There seems to be enormous conjecture about the the naturally occuring crystalline mineral 'Fluorite' or 'Fluorspar' which is in its laboratory grown version known as CaF2 or calcium fluoride.

Then the telescope optical world refers to it by its natural mineral form 'fluorite'.

The real confusion arises when people marketing scopes with objectives made of FK (fluoro-crown) glass and S-FPL glasses with some fluorine component latch onto the bandwagon of CaF2 and its wonderfully low dispersion properties to sell their scopes as 'Fluorite'.

Where are the "Dirt People" when we need them.

As far as I can see Takahashi F series (FS,FC,FCT) are the real deal using laboratory grown CaF2 supplied by Canon-Optron. As for the rest who knows

but

Ohara S-FPL53 and 51, FK61 et al are not CaF2.

Dirt people please chime in to confirm ..........

[alocky]

Dirt person here. Fluorite, the mineral, is CaF2, brittle and with a hardness of 4 (softer than quartz) which is why it scratches easily. In natural specimens it is purple, pink, even yellow due to impurities in the crystal structure. A naturally occurring crystal suitable for a telescope objective is unheard of.
Not sure what else to add, but perhaps if it is placed in a powder coated tube it can suddenly defy all known optical theory restrictions and bring three wavelengths to focus at the same point when combined with crown glass...
Cheers,
Andrew

[casstony]

The most interesting difference I've read about between fluorite crystal and FPL53 glass is that a laser is invisible as it passes through the fluorite crystal lens whereas it can be seen passing though glasses. Theoretically this means less scatter in an FS series scope compared to the newer FPL53 triplets - whether it makes a difference at the eyepiece I don't know.

[Profiler]

Yep - that's my understanding too :thumbsup: hence the big price difference etc. I think PeterHa is a good person to contact for some clarification/understanding of this same point

[MortonH]

Which is better for your teeth?

[alocky]

That would be fluorine! But as an interesting aside, one of the more common abrasives used in toothpaste is feldspar.
Cheers,
Andrew.

[Larryp]

Not fluorine-sodium fluoride

[alocky]

Thanks Larry! Dentists, geologists, this thread's got everything!
:)
Cheers,
Andrew.

[LightningNZ]

Flourine would set your teeth on fire. Fluorine reactions are exciting and dangerous!

A crystal (CaF2) has a regular (crystalline) structure that repeats. It is a very different thing to a glass which is an amorphous solid. How this has any impact on refracting elements in a telescope I have no idea!
-Cam

[julianh72]

https://www.youtube.com/watch?v=rDpM9_G3Giw

Watch at 1:25 in particular for what would happen if fluorine (gas) should come into contact with your aluminium OTA!

[Renato1]

I don't know how true it is, but my friend who has an original Fluorite Vixen refractor from decades ago, says that in his researching of the topic, manufacturers had to move away from using that Flourite because, despite their best efforts, too many workers were dying (or that it all just cost way too much to set things up to stop workers dying).
Regards,
Renato

[PeterHA]

The use of CalciumFluorite crystal in optics is based on the fact that caracteristics like refractive index (<1.435), low disperiosn (~95.24) and light transmission of this material are at extrem ends which cannot be achived with commercial available modern abnormal index glass compositions.
Whilst fluoro crown glass like FK 54 (Schott), FCD 10 (Hoya) oder FPL 53 (Ohara) has properties which are closer to Calciumfluorite, it is still falling short.

To call lenses made of fluoro crown glass fluorite lenses is misleading and is a marketing attempt to sell on the back of the outstanding performance/ reputation of telescopes which used Calciumfluorit elements such as the Takahashi FS-102, FS-128, FS-150, the Vixen 102 and Calciumfluorit doublets of current production.

Doublets with one Calciumfluorite lens and a glass will still (if properly deisgned) outperform glass doublets based on the best commercial available glas compositions.

Calciumfluoride is still widely used in photographic, microscopic, lithograpy and other high performance optics, including latest HD TV camera lenses as well as in some current telescope designs. Latest might be the new 90/500 Borg which is said to have a CaF2 lens in the doublet (a new Sky90 type of scope?).

To my knowledge the biggest pieces of commercially available Calciumfluorite manufactured right now are about 400mm diameter.
There are quite a number of suppliers worldwide, amongst others the Canon subsidiary Canon Optron in Japan, Hellma in Germany and SCHOTT LITHOTEC in Jena/ Germany.

It must be said that on laboratory scale companys like Schott, Hoya, Ohara and Corning have glass based compositions which approach Calciumfluorite caracteristics, but these are not available on the market today due to prohibitive cost.

[bratislav]

Actually, there is no inherent advantage of CaF2 over say FPL53 in astronomical instruments. Fluorite's main advantage (transparency deep into UV spectrum) is completely negated by a practical mating element which is from 'main sequence' and as such will be completely opaque in UV. It is very useful in modern silicon chip manufacture where UV is used to "print" semiconductor elements smaller than a wavelength of green light (e.g. 30 nanometer technology, these days pushing 22nm). But for telescopes its advantages are moot.

Designing a doublet is far from rocket science; it is actually very easy. All you need is a mating glass of same partial dispersion (P f,e) and as wide as possible difference in Vd. If we look at today's available glass we can see that CaF2 line offers a few nearly perfect mating choices. But so does FPL53. In fact there are BETTER practical choices for FPL53 glass; ZKN7 is a well known (and used, notably by AstroPhysics) match and it offers marginally HIGHER polychromatic Strehl than say CaF2 + K5 combination which is often suggested. See

http://www.telescope-optics.net/semiapo_and_apo_examples.htm

and compare entries 15 and 16 (which are best doublet designs).

Calcium Fluorite (being a perfect crystalline structure) does scatter less than any fluorocrown glass (which is amorphous), but difference will be invisible in real life. First, there's that mating element, and second, quality of polish and coating will dominate the final at-the-eyepiece result. Although still a tricky material, it is far easier to work with FPL glass (and get that nearly perfect polish) than it is with CaF2. And multicoating of fluorite was not even done until recently (early Vixen and Takahashi doublets had its CaF2 elements uncoated).

[bratislav]

In fact, if we pick CaF2 best possible practical match (K7), over the critical visual range (C to F line) we have the absolutely identical chromatic focal shift as FPL53-ZKN7 combo.
It isn't until we pick the widest theoretical possible choice (LaKN14 which is not practical for other reasons) that we get any meaningful improvement. But then we could use better theoretical match for FPL53 ...

(top left FPL53 - ZKN7 combination; top right CaF2 - K7;
bottom left CaF2 - LaKN14, bottom right FPL53 - LaKL21)

Bottom line, in practice there is no meaningful difference between FPL53 based doublet and CaF2 one.

[PeterHA]

Very good data Bratislav, interesting and educational, great link, thanks.

Based on this I would expect, at least for visual use, more FLP-53 doublets to perform on par with the few CaF2 based doublets around (old Vixen and old and new Tak) but that seems not happen to often. I wonder why.

[blink138]

thanks bratislav that was very educational!
i have always known for example why a quality glass lens sunglass seems much sharper to look through than your standard CR39 plastic lenses
the plastic lens cannot be polished to the same degree as a hard surfaced glass lens so the softer the material the more uneven the surface is
pat

[bratislav]

The reason might be that most FPL53 doublets are not made to the same standard as CaF2 ones. AFAIK no "premium" APOs (AP, TEC, LOMO/LZOS etc) are doublets these days, which leaves 'cheap' (but still quite good) ones made in Taiwan/China (Orion, Skywatcher etc) to compete with Takahashi. Not a fair comparison I'd say.

But we have many premium triplets on the market and I'm yet to hear about Takahashi CaF2 consistently outperforming FPL/OK4 based AP, LOMO, LZOS or TEC. Unit to unit variations might make one scope better than the other one sometimes but not always. Otherwise Takahashi would be crowned world's best APO, and many people would vigorously contest that!

PS - Takahashi is predictably vague in their explanation on why is CaF2 better than ED glass :
"While ED and fluoro-crown lenses can achieve Abbe-coefficients approaching fluorite, they tend to absorb more light in the visible spectrum. This means that fluorite yields a brighter, higher contrast image."

Transmission of FPL53 :

340nm to 380nm (UV region) is > 90%
380nm to 420nm (violet region) is 99.6%
420nm to 700nm (extended visible region) is 99.7% (average)
700nm to 2000nm (IR region) is 99.6%
in the visual peak at 550 to 600nm is 99.9%

we have to ask who will actually detect that 0.1% ?

Again, quality of manufacture (*) will far outweigh the choice of glass in a modern APO. To me at least Tak is using CaF2 as a marketing tool; which is fine, as long as we do understand the whole story

(*) not to say Tak is lacking in any way; they are in fact always top notch. But so are the others on the "premuim" list

[PeterHA]

Hi Bratislav,
More good data, and I agree 0.1% transmission should not be visible.
Also true, none of the other premium brands (TEC, AP, LZOS) do doublets, the only one I looked through was a AP Star12ED from >20 years ago.
Is it feasable that Takahashi sticks to the marketing power of CaF2 and accepts the higher cost of material and processing of the CaF2 over fluoro crown? We will probably never find out.

The good thing is that competition not only keeps prices at bay, it also increases quality at the more afordable end of the market, the Skywatcher 120mm doublet scopes are a good proof of that.

Thanks for contributing all that detail.

[Marios]

Which is safer to clean with the regular alcohol options? I have read some fluoride type glass will fog over if touched with alcohol.

[blink138]

fluorite dissolves in the presence of air!, so an alcohol based cleaner or even water on the element i believe will reduce it to mush
pat

[Hans Tucker]

I had, maybe still have, concerns over the decal that was put on an APOMAX I purchased a few years ago.

[anj026]

Here is an LZOS OK4 doublet.
http://www.apm-telescopes.de/en/Telescopes-/Refracting-Telescopes-OTA/Apochromates/APM-LZOS-Telescope-Apo-Refractor-152/1200-3ZTA-2-lens-Super-ED-Apo.html

And here are some nice big fluorite doublets for when I win the Lotto!
http://www.apm-telescopes.de/index.php?page=categorie&cat=25&filter_id=1191&sorting=

[LewisM]

So what is in my Vixen FL102S? I know it's a doublet, but what is the Fluorite/Fl crown status of these fantastic scopes (with NIL chromatic aberration)

[Kunama]

I believe the FL102S fluorite optics were supplied by Canon-Optron, I also read somewhere the FL102S has the fluorite elements behind unlike the FS series Taks which have it in front. I'll try to find the reference.

[Hans Tucker]

The FC Series Takahashi Refractors also had the fluorite element at the rear. I believe the optics for the Takahashi refractors are supplied by Canon.

[LewisM]

Thanks Matt and Hans. I seem to recall that too, and there were 2 distinctly different FL102S's also - not only differing in focal ratio, but also quality (the slightly longer focal ratios being inferior optically)

[PeterHA]

Dear All,
Calcium fluorite is for all intents and purposes stable for use in telescopes when stored in the usual dry conditions.
Occasional dew is not a problem and it can be cleaned with Isopropyl Alcohol like other high performance optics.
I personally like no dew on my optical surfaces as it makes dust stick better, I use dew heaters to keep them dry.
The Vixen FL are doublets with a Calcium Fluorite element.

[Kunama]

The new FCs also have the CaF2 at the rear of the doublet.

[Kunama]

I think the FL102 came first @ 902mm, followed by the FL102S @ 920mm.

[LewisM]

Yes, I have it bass akwards. The 920's were slightly better than the less encountered 902's