TMB 100mm F/8

Tom Back wrote:

"In fact, the TMB 100mm/800mm SD triplet is the best
corrected lens on the market now, from any company."

This is not a fact. Of course this objective is too small
and too slow to be compared with other, more practical
F/# APO triplets.

But I can state and place my reputation as optical designer,
that such cheap combination as any well made 100mm F/10
achromat (say, Sinta one) in combination with Chromacor-II
(when properly collimated in a scope) will beat that 100mm F/8
SD triplet in terms of secondary color correction and will be
completely free of spherochromatism.
If to see on spot diagram, all colors from 420nm to 656nm are
INSIDE Airy disk diameter. This can't be achieved with that
100mm F/8 SD triplet.

OK, this is the case with F/10 achromat. No problem, let take
100mm F/8 and again use a Chromacor-II. Again, color correction
within 420nm to 656nm is better, than in TMB 100mm F/8 SD triplet.

Let take 120mm F/8.3 achromat and Chromacor-II. The same result -
color correction is better, than in TMB 100mm F/8 SD triplet.

Even 150mm F/8 achromat being correctly matched with Chromacor-II
show ZERO color on any object.

Of course, for many such true APO as SD triplet is more attractive
telescope design. But it is not the best in the color correction.


Valery Deryuzhin
ARIES.