Re: [ATM] lensmaker for a very long focus objective

[Woodchuck <djv@bedford.net>]

For those who are wondering what the historical precedents for this
long-focus, single-element objective telescope is, there's a brief
discussion and some engravings of such telescopes by Hevelius at

http://es.rice.edu/ES/humsoc/Galileo/Things/telescope.html

These are from the last quarter of the 17th century, hence are exact
contemporaries of the immortal NEWTON, whose contributions to our
hobby are more widely known, and a great deal handier in the field.

I'm a bit negative on the Adler's attempt to re-create one of these
hundred foot plus telescopes, it would seem to be the same sort of
project as re-creating one of the sillier early attempts at making
an airplane.  (I have in mind the one with a half dozen wings, with
the heavy multiple engines mounted atop the stack.  There's a fun
movie of its sole "flight", it simply pitch over in to the ground.
Somebody forgot his moment calculation :-).

It would be MUCH more fun for the Adler and for the children targeted
by the project to actually create something that can be looked
*through* rather than looked *at*, say a 2" telescope of perhaps
10-15 feet focus with a Huyghenian eyepiece in the 4" fl range, (or
a Galilean for terrestial views) this could even be hung from a
stout pole or plinth (a sonotube used as originally intendend),
with a block and tackle mounting (although this might prove a hazard
to the slow of foot or mind).  To improve the educational content,
I would say to treat the chromatic aberrations with benign neglect,
perhaps even using a flint glass to emphasize them.

The purpose of the exhibit is to show the difficulties of the
ancients, not their follies. (The latter is all too often what
educational exhibits try to do, a sort of "boosterism" for "progress",
thus fueling the demonic engine of hubris).  And part of the exhibit
should include some nice short focus reflectors of NEWTON's design
(say a 4" instrument of 2 feet focus and 1" eyepiece, in righteous
mahogany tube, including the very clever bowling ball and flower
pot style mounting), showing that the ancients were neither fools
nor morons, and could overcome their difficulties through the
confluence of intelligence and diligence.

It might help to point out that the long focus refractors were
*quite* practical for one of the main astronomical tasks of their
time (and all others), namely mapping the heavens, and that quite
precise meridian and other instruments were constructed.

Maybe with enough interest, the Adler could open a sort of workshop
in their basement where the interested might experiment with optical
construction, heh heh.  (I remember the Adler's programs of the
1950's).

Dave

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