Flourite use in apo's?

Started by MZUKO, 03/14/2004 02:36PM
Posted 03/14/2004 02:36PM Opening Post
Hi,


Can anyone provide some of the advantages for the use of flourite
in apo's?
Assuming two scopes of identical aperature, one a flourite with a
slightly faster focal length and the other an ED lens(both meet abbe
criteria), would one see a differnce at the eyepiece?
Is flourite less durable over time? More fragile?

Thanks,

Marc Zukoff
Posted 03/14/2004 05:10PM #1
Depends on the designs (and the particular type of ED) in question.

Calcium Fluorite ("U" before "O") is a crystal which has optical properties which are condusive and appropriate to apochromatic design.

The efficacy of any particular "ED" glass (and there are many types which fall under the generic "ED" category) will depend on its characteristics *and* the objective design into which it's integrated (i.e., the characteristics of the mating element(s)).

Yes, there are a couple/few "synthetic fluorite" glasses which are, essentially, optically equivalent to Calcium Fluorite. However, there are other factors which must be considered when a lens designer/manufacturer makes those decisions (i.e. Calcium Fluorite or ED/SD/UD glass) -- namely, cost & supply at that particular given time.

As far as apochromatic performance goes...
There are objective designs utilizing Calcium Fluorite which *can* be "bettered" by other designs utilizing ED/SD/UD glass. Likewise, there are designs utilizing ED/SD/UD glass which can be "bettered" by other designs utilizing Calcium Fluorite. Regardless of this fact: for most folks in most scenarios, differences to the eye from one excellent apochromat to another will likely be extremely miniscule (if not invisible).

I wish there was a "cleaner" answer, but lens design (the "recipe," if you will) *is* more important than a given element (or "ingredient").

Best wishes.
-Dan
Posted 03/14/2004 08:34PM #2
Mark
I fortunately have two fluorite scopes that are triplets and the performance is excellent. I am sure the best synthetic fluorite(FLP53 etc) glasses have similar performance.

For purely visual use I am sure a doublet with fluorite or the best synthetic fluorite can deliver virtually everything that a triplet can but for film photographic uses the triplet may be superior.

I have an old 80 mm semi apo doublet with short flint glass that I have compared with more modern ED doublets and it shows a little more false colour but possibly superior planetary views.

I am sure the quality of manufacture and choice of glass in a doublet can lead to outstanding performance. Longer focus achromats if well made can also deliver excellent performance, one only needs to look through a Zeiss Telementor to realise this.

I believe that the notion Fluorite is not all that durable is more of a fear than reality. Most Fluorite scopes have the fluorite element at the rear of a doublet or sandwiched in a triplet. ED glasses can also be very fragile.

Also Tak FS series scopes have hard coated fluorite lenses which resist the elements.
Best wishes

Kevin