so called new approach to design doublet APOs. ;>)

From: ValeryD (aries_at_mercury.kherson.ua)
Date: 11/20/04 

Date: 20 Nov 2004 00:16:28 -0800

From: "tmboptical" <TMBoptical@a...>
Date: Thu Nov 18, 2004 10:11 am
Subject: Optical Details on the Burgess 92mm Fluorite Apo

With Tom Davis' excellent review of the Burgess 92mm fluorite
apochromat, and for a doublet, its color free performance, I
thought it is the right time to explain why this telescope
is so special.

All current doublet ED and fluorite doublets that are on the
market today are not true apochromats. You may want to read
the correct definition of what a true apochromat is on my web
site at:

http://www.tmboptical.com/itemsGrid.asp?cat_id=32

The ED and fluorite doublets refractors (even the highest
quality), do not have three color crossings, where three
widely spaced colors come to the same focus. They also all
have a significant amount of out of focus violet. This may
not be apparent at first, but when you compare, say a Tak
doublet fluorite to a true triplet apochromat (TMB, AP),
or a Newtonian, on a bright object, you then can easily
notice the yellowish coloration, and the violet halo
around stars like Vega or a planet (in a dark sky) like
Venus, at high power.

This is because all ED and fluorite designs use a matching
flint glass, such as KF-3, KzF-2, and other flints that are
at the same general point in the partial dispersion area on
the glass charts. The partial dispersions of these flints do
not match up well in the violet and very deep red, to the ED
or fluorite elements, and the result is visible color.

Now you might ask, how is it that the Burgess fluorite
doublet can have so much better color correction, so much
so that there is no color in focus at all, and only the
smallest trace of out of focus color on a star like Vega,
and that assumes that you have good color vision, and know
how to look for very small color errors.

The secret is that the Burgess fluorite doublet uses a
special crown element with the fluorite. Again, on the
partial dispersion glass chart, you can see that this
special crown glass has almost a perfect partial match
to fluorite, and thus, the one element cancels the color
of the other element to an exceedingly high degree. So much
that it has the color correction of a triplet lens, and
like a high quality triplet apo, has three color crossings
too.

This is the ultimate way to make a production fluorite
doublet. I have designs on my computer using the same
fluorite/crown glass at larger apertures, that have the
same or better color correction, at only a small increase
in focal ratio. Or, in other words, this lens scales up
nicely, and can give the same color free performance in
larger apertures.

This is truly a new level of performance in the fluorite
and ED doublet market. We are very proud that we can bring
this product out to the marketplace, and at a price that is
more than competitive.

Thomas Back
TMB Optical

=================================================================

Dear Mr. Back,

Sure, your self-confidence has no limits. And as one, well known
in optical design and manufacturing, person said, someday your head
will explode due to your unlimited self-confidence.

I should mention specially for your instance, that ARIES 7" F/8
Fluorite
objective you saw at one StarParty (AstroFest?) which was with
cracked fluorite element has the design, which is essentialy the same,
as you called "truly a new level of performance in the fluorite
and ED doublet market". Also, I should to note, that these ARIES
objectives were designed and made about 7 years ago. The largest one
was 12" F/9. And right now we making tubes for a serie of 6 such 7"
F/8
Fluorite doublets.
It will be useful to note also, that these doublets has color
correction,
that in 430nm-656nm range, is same good as LZOS made TMB 6" F/8 Super
SD TRIPLET. And, of course, such objectives has better contrast, that
these
triplets (include 175mm ones) because they have two surfaces less,
shorter
lightpass in the glasses. These doublets also have significantly
shorter
colldown time. Objectives have lesser mass and a telescope has better
balance.

Of course, I am not a first person, who know this approach to APO
doublets
design and manufacturing. So, this approach to design can't be called
as
"truly new level". Please, note this specially for yourself.

VD

Relevant Pages

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