Re: spotting scope question-specific

I hope this discussion of optics annoys no one as out of place here and enlightens at least one person. I'm not instructing Alan, but schmoozing with him.

Alan French wrote:

...

Jerry,

A nice summary.

Thanks.

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.

As a practical matter, yes. Three glasses permit a cubic, but not every combination has three real roots. Some useless combinations allow three isofocal wavelengths, but the curve between and beyond them may depart excessively from the axis (not be flat enough). To a first order, the focal length of an achromat is a parabolic function of wavelength, with the peak in the green (for visual correction). (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.

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>.)

I have a mounted 3" f/15 airspaced doublet that I retired long ago. It supports a 1/2" eyepiece with good clarity and only very moderate color. The field is flat enough for 1.25" Erfle eyepiece. You can have it if you want to put it to use.

Jerry

Sidebar: In a cemented doublet, the rear surface's curvature of the front (usually crown) element must match the front surface's curvature of the rear element. Cementing eliminates two air-glass interfaces (thereby reducing internal reflections) and allows the doublet to be mounted as a single unit. Airspacing the elements allows the curvatures to differ and spacing other than zero, conferring two additional degrees of freedom that can be exploited refine the design. In effect, it allows a free third element made of air. The advent of anti-reflection coatings made air-spaced designs more attractive.
--
Engineering is the art of making what you want from things you can get.
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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)
  • so called new approach to design doublet APOs. ;>)
    ... Optical Details on the Burgess 92mm Fluorite Apo ... All current doublet ED and fluorite doublets that are on the ... the correct definition of what a true apochromat is on my web ... cracked fluorite element has the design, which is essentialy the same, ...
    (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: New AP scope and more
    ... It's called Fluorite and when coupled to a Schott N-KzFs2, ... you want - pretty well color free doublet. ... Fluorite costs about $1000 per kg in raw form. ...
    (sci.astro.amateur)