Re: spotting scope question-specific

Alan French wrote:
"Jerry Avins" <jya@xxxxxxxx> wrote in message
news:vZ6dneOZZqpbzIXZnZ2dnUVZ_v6dnZ2d@xxxxxxxxxx

The new low-dispersion glasses come close to good classic apochromatic
(apo) designs, but don't quite match them. 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.
The cost is in the shaping and polishing. Lenses with low-dispersion
glass command a higher price because of their better performance, not
because of higher per-unit manufacturing cost. We are still in the time
of retiring development costs and milking the market for the just
rewards or entrepreneurship, but I expect LD prices to fall eventually
to where they drive crown-flint achromats off the market. Have you
noticed that nowadays most lenses are multi-coated as a matter of
course? That was a high-end treatment not long ago.


Jerry,

Actually, one of the new low dispersion glasses, or fluorite, is the heart
of virtually any modern apochromat. ED glass is more expensive than the
vanilla crowns and flints, especially in the larger sizes used in high end
astronomical telescopes where the highest uniformity of indices is required.

Making and annealing optical glass is an energy intensive process. Folks
who know more that I about ED glasses claim prices are unlikely to go down
significantly, but I guess time will tell.

Clear skies, Alan

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.
--
Engineering is the art of making what you want from things you can get.
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