Apochromatisme : suite et fin (...)

La précedente discussion sur ,les doublets Taka et la définition de l'apochromatisme, m'a ammené à écrire à Thomas Back, designer des optiques TMB. Je lui ai posé la question suivante: Est-il possible d'avoir un objectif qui réponde aux critères imposés par Ernst Abbe ? Qu'en est-il des TMB ?
Réponse de Thomas Back:

>>Here is Abbe's definition of an apochromat:

"An objective corrected parfocally for three widely separated
wavelengths and corrected for spherical aberration and coma for
two widely separated wavelengths."

He was talking about microscope objectives at the time, but it
certainly can be applied to astronomical objectives. Let me
simply state that no objective on the market today meets this
definition in its strictest form. All the ED and fluorite doublets
have only two color crossings (the only exceptions are the now
canceled TMB fluorite doublet, the rare Astro-Physics 130mm
and 152mm ZKN-7/FPL-53 ED doublets, and the Aries CAF-2 doublet),
but with two or even three crossings, the correction for spherical
is not met in these objectives.

Again, using the Abbe definition in its strictest form, an
objective must not only have 3 widely spaced color crossings,
coma correction, but be free from spherochromatism. Now, even
exotic objectives that are designed for full correction of
spherochromatism, will still, at some level, have some
residual spherochromatism. What Abbe did not make clear is
at what level is spherochromatism sensibly perfect?

Optical designers and experienced planetary observers have
different opinions on what "sensibly perfect" spherochromatism
is and what the peak visual wavefront should be. The 20th
century's greatest optical scientist, James G. Baker, in
his revolutionary article "Planetary Telescopes," stated that
"sensibly perfect" is 1/4.5 wave OPD P-V spherical at 486nm,
1/40 wave OPD P-V spherical at 546nm, and 1/8 wave OPD P-V
spherical at 656nm. Notice the extremely high value at the
null wavelength. One must remember that Baker was talking
about the ultimate planetary scopes, so a figure this high
will not be found in commercially made telescopes -- by
anyone.

To answer the question about how the TMB apochromats meet
the Abbe definition, they do as well as any on the market.
The 100mm f/8 model has from 486nm to 706nm better than
1/10 wave OPD P-V spherical, which exceeds the Baker
requirement. On the computer, the null is also in excess
of 1/40 wave, but glass and figuring limitations do not
allow such astronomical wavefront figures.

Finally, all TMB triplet SD and fluorite objectives are free
from coma.

Thomas Back
TMB Optical<<

J'éspère que cela répondra à certaines de vos questions.

Merci de m'avoir lu,

T02021

[Ce message a été modifié par T02021 (Édité le 12-09-2002).]

[Ce message a été modifié par T02021 (Édité le 12-09-2002).]

Tom Back: "Let me
simply state that no objective on the market today meets this
definition in its strictest form. All the ED and fluorite doublets
have only two color crossings (the only exceptions are the now
canceled TMB fluorite doublet, the rare Astro-Physics 130mm
and 152mm ZKN-7/FPL-53 ED doublets, and the Aries CAF-2 doublet),
but with two or even three crossings, the correction for spherical
is not met in these objectives."

Traduction: "Je peux affirmer qu'aucun objectif sur le marché n'atteint les critères énoncés par cette definition (Abbe) dans sa forme la plus stricte. Tous les doublets ED ou fluorite n'ont que deux couleurs au même foyer (Les seuls exceptions sont le TMB doublet fluorite (annulé), le rare Astrophysics 130mm, le 152mm ZKN-7/FPL-53 ED et le doublet Aries Fluorite), mais avec deux ou même trois couleurs à un même foyer, la correction de l'aberration de sphéricité n'atteint pas les critères demandés avec ces objectifs."

Tom Back s'exprime. Tout est dit. Les doublets ED et Fluo, en dehors de ceux expressément cités ci-dessus ne sont que des semi-apochromat. Cela inclus Takahashi.

Il faut croire que je n'avais pas écrit trop de bêtises en parlant de "Grande Illusion" pour les doublets "APO".