Re: spotting scope question-specific

"Jerry Avins" <jya@xxxxxxxx> wrote in message
news:cfWdndwBgeN0A4TZnZ2dnUVZ_tidnZ2d@xxxxxxxxxx
Clear skies to you, Alan. The moon was bright and crisp last night, and
seeing was the best we've had here in months. Naturally, the bright moon
washed out most other interesting objects.

Fluorite is a mineral, calcium fluoride. Much of it is beautifully
colored, but it is too soft for everyday jewelry. I know that it is used
in microscope objectives, but I find it hard to imagine pieces large and
clear enough for telescope objectives of commerce. there are nice
pictures of fluorite at http://www.gemhut.com/fluorite.htm.

Apochromats can in theory be made of any three different glasses, but
only one with optical characteristics similar to fluorite makes the
exercise worth while. What telescope makers call fluorite is really a
glass with fluorite-like characteristics. The new low-dispersion glasses
allow the design of doublets nearly as good as apochromatic triplets.
From http://www.skywatchertelescope.com/Pro.html: "The two-element
objective lens is fully multi-coated and utilises special, extra-low
dispersion FPL-53 fluorite glass for the virtual elimination of false
colour. Performance of these telescopes will both delight and surprise
the most critical observer, and can be genuinely compared to
Apochromatic telescopes of other brands in price brackets normally way
beyond the reach of the average user."

Kowa makes lenses with synthetic fluorite crystal as well as with ED
glass. http://www.kowa-usa.com/sporting_optics/fluorite.php is
interesting reading. including "The ED lens used in our TSN-663/664 is
made of fluorite glass. At Kowa, the name "fluorite" is only used for
true fluorite crystal.

ED glasses are relatively soft (but harder than fluorite) and therefore
harder to polish. (Sleeks plague my amateur efforts even with crown
glass.) They may be a bit more expensive (as are rare-earth glasses) but
the cost of raw materials is not the major factor in pricing optics. An
ED doublet is clearly cheaper to produce than an apo triplet. Some ED
doublets are labeled apo because their performance is nearly that good,
but paraphrasing Lincoln, calling one an apo doesn't make it an apo.

Jerry

P.S. A single lens has a different focal length for every wavelength. An
achromat uses two glasses to bring two wavelength to the same focal
length. An apochromat uses three glasses to bring three wavelengths to
the same focal length. In all cases, the extra degrees of freedom that
each additional surface provides allows greater control not only of
chromatic aberration, but of other faults, such as spherical aberration,
coma, and astigmatism.

Jerry,

A nice summary.

There is some confusion over the use of the term "fluorite," mostly by the
marketing folks. Because it has to be very uniform, telescope makers use
the synthetic fluorite. Takahashi has used fluorite, the original
Astro-Physics 90EDL "Stowaway" is a triplet using fluorite as the center
element, and TEC (Telescope Engineering Company) would be happy to make
fluorite triplet for you. Other APOs use fluor-crown glasses, and FPL53 is
a popular choice.

If you want to make an apo using three glasses, they have to be carefully
chosen. Any three glasses will not make an APO.

The additional degrees of freedom certainly do provide more opportunity to
control aberrations, as well as for better color correction. The first
prototype AP Stowaway was a fluorite doublet, but at f/5 its performance was
not quite what was wanted. If you are willing to go to a slower lens, or
restrict the aperture, a doublet can do quite well. Even a vanilla crown
and flint achromat can do quite well if it is long enough. A 3" f/15
achromat is quite good and show very little color. (But would be a bit long
for most birders <g>.)

Clear skies, Alan



.

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
    ... 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: Holy S----! An 8" Mak-Cass for $999 Canadian?!
    ... There are plenty of glasses, or other transparent substances, that cost ... But it's still true that you wouldn't "replace fluorite" that way. ... questioned whether or not they are real apochromats, ...
    (sci.astro.amateur)
  • Re: spotting scope question-specific
    ... If you find a used apo scope ... in your price range, don't hold out for something more modern. ... Not much of the cost of a good lens, let alone a scope, is in the glass. ... Fluorite is a mineral, calcium fluoride. ...
    (rec.birds)
  • Re: TEC APO 160 costs $12,000 ??
    ... >> TEC is taking orders for a 160 mm APO. ... But TEC's price structure is ... >Yuri announced this on the TEC Yahoo Group a few months ago. ... So does the extra $4000 account for the cost of the fluorite compared ...
    (sci.astro.amateur)
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