Re: Holy S----! An 8" Mak-Cass for $999 Canadian?!

From: John Savard (jsavard_at_excxn.aNOSPAMb.cdn.invalid)
Date: 12/10/04 

Date: Fri, 10 Dec 2004 22:09:12 GMT

On 10 Dec 2004 00:11:37 GMT, chaslx200@aol.com (CHASLX200) wrote, in
part:

>I dont know of any type of glass that can replace flourite, and cost lots less!

No, there isn't. The glasses that can come close to replacing flourite
cost perhaps a little less, and are perhaps a little less fragile, but
not by a whole lot.

There are plenty of glasses, or other transparent substances, that cost
a lot less than fluorite, and can make a real apochromatic lens.

You can even use *plastic*.

Using a hard crown glass, a very, very dense flint, and a very light
flint can also allow you to make an apochromatic lens, because the
glasses in the traditional "just add lead" part of the refractive index
and dispersion chart do have _some_ variation in secondary spectrum.
(This variation does, of course, mean that on the secondary spectrum
chart, they are off the other "glass line". Just not by much.)

But it's still true that you wouldn't "replace fluorite" that way. The
difference in secondary spectrum is smaller, and the index of refraction
isn't as high as in fluorite. Thus, while an apochromatic objective
could be made, for a given focal ratio, the curvatures would be larger.
Thus, other aberrations would mean you couldn't get as short a
telescope.

Two-element telescopes using a fluorite-like glass, even if it might be
questioned whether or not they are real apochromats, can be made in
shorter lengths.

Basically, then, you can't replace fluorite, except by the exotic
special fluor crowns... because fluorite has more to offer than just the
possibility of apochromatism.

John Savard
http://home.ecn.ab.ca/~jsavard/index.html

Relevant Pages

  • Re: spotting scope question-specific
    ... Takahashi has used fluorite, the original ... If you want to make an apo using three glasses, ... Any three glasses will not make an APO. ... (ED glass can make a very flat parabola, so that even though the quality of the chromatic aberration is that of a doublet, the degree of color can rival a apo. ...
    (rec.birds)
  • Re: spotting scope question-specific
    ... Fluorite is a mineral, calcium fluoride. ... clear enough for telescope objectives of commerce. ... The new low-dispersion glasses ... ED doublet is clearly cheaper to produce than an apo triplet. ...
    (rec.birds)