Getting over Fluorite; and other random thoughts on objective design.

Refractor buffs love fluorite, but it has some nastier sides that people should be aware of.

In the past reports of the instability of fluorite/ED glass have not been helped by some rather vaguely phrased statements made in respected optics textbooks. Here is one example, taken from Rutten & Venrooj (2002) pp 58:

“Although fluorite is ideal as an optical material, it has its drawbacks. Not only is it expensive and the size of blanks severely limited , but fluorite is highly subject to weathering. Despite its drawbacks, a number of telescope manufacturers now offer fluorite objectives.”

 Because fluorite is a mineral, it is significantly more water soluble (~150mg L^-1 @25C) than a true glass.


Although it is inert to organic solvents, it is reputed to dissolve slowly in nitric acid or solutions containing ammonium ions. That said, I have personally not witnessed anything untoward on the objectives of older (late 1990s) apochromatic objectives.

Nonetheless, the solubility of fluorite is quite high actually, and although there is little to worry about if care is taken to ensure that no condensation builds up on the fluorite element of an object glass, it it is not beyond the realm of possibility that years of exposure to moist climes could potentially degrade its exacting curvature, possibly requiring slight refiguring, or worse still, replacement.

Other ways forward:

A folded, standard achromatic design in apertures of 6-inches or above seems a very straightforward way to circumvent the traditional size limitations of classical crown & flint object glasses, especially in relative apertures of f/15 or greater.  High quality flats are now available to more than meet the requirements of these hypothetical instruments. Moreover, it is my belief that the excellent thermal properties of these glasses ( with thermal expansion coefficients about 2.5 times lower then fluorite), executed in a folded array, would make for a very powerful – and cost effective – large aperture telescope. There is a inspiring history of superbly functioning folded refractors already widely disseminated in the astronomical literature that testifies to the proven merits of this stratagem.

Another way forward is to review the glass types employed by ED refractors.  My online review of the Meade 127ED refractor was designed to raise awareness of the possibility that economically priced ED doublets could be made with pretty good colour correction (read considerably better than the standard crown flint achromatic doublet), without breaking the bank. Vladimir Sacek, founder of Telescope Optics, was kind enough to share his thoughts on the ‘problem’ of choosing the right kinds of glass to make such a venture worthwhile:

The choices are limited by the availability of matching glasses. If you look at the relative partial dispersion (RPD) diagram (below), you see that so called “normal glasses” pretty much conform to a diagonal line. Since the amount of secondary spectrum – defined as the longitudinal separation of the green focus from the common red/blue focus – is proportional to a RPD vs. Abbe differential for the two glasses (it can also be envisioned as a height vs. base side of the right-angle triangle formed by connecting two glasses), those normal glasses can only produce ‘regular’ achromatic correction.

 In addition, a practical requirement is that the two glasses differ significantly in their dispersive power. The smaller this power differential, the stronger the inner curvatures of a doublet have to be in order to bring the two wavelengths to a common focus. Excessively strong curvatures results in too much higher-order spherical aberration, which makes good correction impossible. In general, the Abbe differential for the two glasses should not be significantly smaller than about 30.

Only a small group of glasses to the left on the diagram can combine with normal glasses from the bottom portion of the glasses’ diagonal, to produce and good colour corrected objective. BK7 is too close to FPL51 for example, and that combination would produce too much higher-order spherical aberration. KF3 makes a good match for an f/9 system. KZFN2 is, obviously, a somewhat better match, but the gain is nothing to write home about (little over 10%). KZFN1 is a bit worse, and so is KZFS1. Nearly ideal matches for a medium-to-slow ED ‘scope would be KZFSN2 and N-KZFS2 and these are (currently) about ten times the price of BK7.

 This reasoning, in general, also applies to triplet objectives. Colour correction changes little in a triplet vs. doublet for a given pair of glasses (adding a third glass also makes little difference in this respect). The main advantage of the triplet objective is in significantly less curved inner surfaces required.

That is the reality of it. If you look at the height vs. base ratio for the standard doublet achromat. (BK7/F2), a system with significantly better correction would have to have it about twofold – or more – smaller and, at the same time, to have the two glasses separated by not much less than three horizontal spaces.

             Relative Partial Dispersion Diagram (courtesy Vladimir Sacek)

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4 thoughts on “Getting over Fluorite; and other random thoughts on objective design.

  1. Interesting stuff (as usual !) Neil.

    Does FPL53 glass share some / all of the weaknesses of Flourite do you think ?

    It seems to have many of the positive optical characteristics as far as I can see although those Vixen 102mm Flourites are, from all I’ve read, clearly superb performers.

    On the topic of other designs, I had an opportunity to test drive one of the TAL Apolar 125’s a while back. While the example I tried had some idiosyncrasies, I could see that the unusual (to me at least) 6 element optical design was very well corrected for SA and CA considering that it, as far as I’m aware, used no “exotic” glass types.

    It seemed to me that the Apolar optical design has great potential if it can be manufactured to a consistent quality and remains “user friendly” in terms of maintenance.

    On the folded refractor design, I have an old and dog eared copy of Gunter D Roth’s book “The Amateur Astronomer and his Telescope” (which I’m sure you are familiar with) in which there are some fine examples of folded design including a lovely 4″ coude refractor by Maurice Wachter and Roth’s own 5″ folded refractor which I think is a Schiefspiegler design or similar.

    I suppose the above scopes were manufactured the 1970’s or 1980’s but I imagine that todays optical technologies could be used to create something really superb.

    Cheers and thanks for the interesting article,


    • Thanks John, much appreciated!

      FPL-53 has a CTE just a little lower than Fluorite but still not as low as traditional crown & flint.

      Yeah, those new Apolar ‘scopes from Tal. It’s a very interesting design alright, what with a single (crown only?) element up front and retrofocally corrected with sub aperture elements; a dialytic design I suppose. John Wall – inventor of the Crayford focuser – devised a very interesting and promising system similar to this; the hypochromat.
      The only issue with these is sensitivity to proper placement but if correctly executed, deliver the readies.
      Those old folded refractors sure were cool too. The Coude is such a neat system to sit and study the night sky comfortably. These are all alive and well as ATM projects but unfortunately not as commercially available designs ( as far as I’m aware).
      Variety is the true spice of life. And it’s all good.

      Best regards,


  2. I would like to comment on the Design of the TAL Apolar 125 6 element telescope. This is an optical configuration that I have been experimenting with for over 10 years now. I have built many telescopes with stock lens components that are very similar to this telescope. I my understanding this design is not the same as the retrofocal optical layout of the hypochromat which corrects the aberrations of the single element objective with multi-elements which are located at and behind the objective focus. (The image inverts before there is any attempt to correct aberrations with additional lenses) The Apolar 125 corrects all aberrations within the single element objective focus cone and not behind it. It is basically a widely spaced triplet design very similar to the Rogers dialyte which was invented more than 100 years ago except for the adding a rear third stage that corrects Lateral chromatic aberrations.
    The designs that I have made are also modifications to the Rogers Dialyte having a third stage near but not at or behind the focus point of the objective. They do not use expensive low dispersion glass and some of them are achieving near APO performance.


  3. Hello David,

    Thanks for your message. Very interesting indeed!

    I’ve not had the pleasure of using the Tal Apolar refractor but I believe John Huntley (Somerset, UK) had a spin with one; and he was rather impressed with the design as well. It is most encouraging to see new refractor designs coming to the fore and I believe folk are ingenious enough to come up with more cost effective ways of doing the same thing.

    The Zerochromat seems to be looking up too!

    What is clear is that the current brigade of triplet apos are somewhat of an economic anomaly in the world of telescope products and anything that can be done to redress that issue would be a good thing in my opinion.



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