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io posso incollare qui quello che scrissi Yuri (il costruttore dei tec spaziati in olio)a suo tempo ,è in inglese ma è a mio avviso interessantissimo.
ixio
Once again about an old question, or oil vs. air.
Each design has its own positive and negative sides, but being not so much
different in most discussed area - "color correction", the last one could
be an easy suject for speculation by companies that usually do not involve
in any aspects of optical production, and being this thiw way can easy
announce for their airspaced design super correction of "1/100 of the
conventional apochromats telescope, it has reached the aberration quantity
which is much smaller than the catadioptric system..."; or the other good
sample: "The color correction on the XXX 203mm f/7 will be the best possible
of any existing apochromat design on the market" - we can only smile and
remind that optics on the computer screen has no limitations:
- no streas or bubbles, blanks are uniformly annealed from center to edge,
catalog refractive indexes, no any surface impefections, all surfaces perfectly
polished, etc. - what a hard work of the computer! Is that same easy easy in
real life? Lets see.
Both oiled and airspaced designs could be done with very good or near
prefect correction and not only on the computer screen. What is the difference
in numbers for color correction? - if talking about eye most sensitive wave
length of ~ 532nm (green light), the difference is ZERO, since both designs
optimized for this wave length.
Going to polychromatic Strehl, that counts whole visual range from 430nm to
over 650nm (or slightly wider) the situation becomes a bit better for airspaced
design, but how much is that "a bit": - sorry to say, but it is only 3 - 5% (!).
Could this difference be realized, yes; anything behind - yes again - the glass
measurements and glass internal quality has to be taking in account.
Refractive indexes of the glasses given by manufacturer for standard
measurements are with in +- 3.0 x10E-5, same time each melt of glass may be
different from catalog numbers up to 10 x 10E-4, and variations in one melt
could be a serious issue when going to larger lenses (>150mm dia.)! -
Counting only these will bring to nought "theoretical" 3-5% advantage of
airspaced design.
After being "surprised" for a few times with melt data given by manufacturers,
we do our own tests of refractive indexes of each glass on more precision
level: <1x10E-5, as a result we have the MEASURED correction of
the final objective be very close or identical to design. That not easy but
we have to do this for each new melt and if different - we have to do new
sets of test plates, tooling re-setup, etc. - All of these usually ignored
by most manufacturers, since requires time, special equipment and test
methods. For sure that is ignored by all mass production manufacturers (Asia).
Other aspect in question - coatings. The math is not needed to show
difference in cost of coating of 2 outer surfaces of oiled objective vs. all
6 surfaces of airspased objective.
No math also needed to show the winner in light transmission; light loss on
reflections from 6 surfaces of airspaced design is ~3% in the middle of visual
range and 6% and more on the edges of it - and this is in case if we have
perfect coatings; if it is an average only - the light loss on the on 6 surfaces
on
the blue end of spectrum may exceed >10-20% (deep blue looking coatings),
that will shift color balance and make background yellowish. In the oiled
design the light loss on reflections is only 0.5-0.7 %.
What is next? - Scattering light, on six surfaces three times more than on two -
where is the higher contrast to be? How about accumulated errors on 6
surfaces vs. 2 - for sure 6 surfaces has to be done on more precision level to
reach quality of two outer surfaces of oiled design - that takes more time and
increases the cost of production. Will accumulated errors affect Strehl - why
not - in best case same 3-5% or even more of polychromatic Strehl will be lost.
BTW, the third order spherical aberration of 1/4 PV resulting dramatic Strehl
lowering to 80%! That is the area of attention - the optical surfaces must be
done on highest level possible!
You may see know why we choose and stay with an oiled design in all of our
APOs.
About glass choices, or in the other words, any advantages for CaF2.
Even first three APO160 F8 were done with CaF2, we had a plan to produce
APO160 with ED glass (price difference for blanks for APO140ED vs.
APO160ED is only $400) and be priced for appr. $8000, but we went to
Fluorite. Why? Our work experience and testing the ED glasses and CaF2
blanks in sizes up to 200mm and larger showed us that homogeneity of high
quality single crystals of CaF2 was better than in ED glasses, same time the
refractive index of CaF2 is constant up to 5x10E-6, that means no
measurements are needed; no refractive index variations, as in ED glass
blanks of sizes over 6" diameter. As being said before, CaF2 by being a
crystal has near zero internal light scatter and after all the Fluorite design
gave us possibility to force focal ratio to F7 without compromising color
correction in wide range including blue end - same level in practical
realization on the as in the best airspaced lenses.
Best regards, TEC
PS. And keep in mind the best ever color correction comes from 17th cenury
doublet design of I.Newton!
Doublet in this case was a primary and secondary mirror cobinatin
(-:
_________________
C8 - skywatchered80- eq6
Canon 40d mod,
magzero mz5 e molta passione.
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