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

["David Harbour" <scarab2@cox.net>]

This is the easiest thing in the world to accomplish. One may purchase a
lens of extremely long focal length, made in glass, of about three inches'
aperture, from any vending optician. The longest focal length lens one can
buy is .25 diopters; this will make an extremely long aerial telescope. I
have already made some that perform wonderfully well. He would have to stop
down the four or five inch lens in any case; he would not have to stop down
a standard glass ophthalmic blank. Extra bonus is that it will be corrected
for spherical aberration. With a color filter, it could perform stunningly.
A .25 diopter lens yields a focal length of four meters; longer than he
wants; next option is to ask for a .50 diopter ophthalmic blank. I have very
smoothly fitting, "telescoping" tubes that these ophthalmic blanks (about
$35 for glass, which he wants) will fit. I know the perfect brand name to
buy, they are almost taylor made for an telescope with ophthalmic blanks
(though I note that he wants to dispense with a tube altogether)

A perfect choice for an aerial telescope would be the .25 diopter ophthalmic
blank, yielding a four meter focal length. Better than the originals, no
need to generate headaches over trying to grind one one's self. What an easy
project. Glad to be of help. If you go to a dispensing optician (eyeglass
seller) and order either a .25 or a .50 opthalmic blank, specify that it
have the minimum "reflex" possible in its design (tell him that you want the
"flattest" or least "bent" (curved backwards, like a meniscus) .25 blank
available, and in glass only.

David
Great Plains Instruments
http://www.atmsite.org/contrib/Harbour/index.html
scarab@2cox.net


-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net]On Behalf Of
Peter Abrahams
Sent: Thursday, March 25, 2004 11:09 PM
To: atm@atmlist.net; bstephenson@adlernet.org
Subject: [ATM] lensmaker for a very long focus objective

The Adler Planetarium in Chicago is looking for a craftsman to fabricate an
objective lens, about 100 feet in focal length.  It would be a singlet,
aperture perhaps 4 or 5 inches.
They have a small budget, but if they could afford a professional, they
would have hired one.
There is one unusual benefit to the craftsman to this project; this work &
your name will likely survive as long as western civilization does.

Director Bruce Stephenson  bstephenson@adlernet.org   has been
communicating with a historical telescope email list.  Here are excerpts
from his emails:
----
As part of a new permanent exhibition at Chicago's Adler Planetarium, we
want to construct an aerial telescope for museum visitors to look through.
....museum visitors today (especially young museum visitors) want to do
more than look at things in cases.
The aerial telescope idea is an extension of this thought. It's an
attention-getter, an "attractor" in museum-speak, something that will bring
people over to see what it is.
For practical reasons it's unlikely to be moveable. We just want to show
people that it was possible to make a telescope without a tube that works,
and that they can look through to see a magnified view of, say, the Sears
Tower.
If it is feasible to make one of these, we would probably want to use it in
the daytime to look at a fixed target, fairly low in the sky since that's
where all the fixed targets are.
We will attach the objective to a flagpole or something. An existing pole
is 100' from a likely viewing area, so I suggested an objective with 100'
focal length.
......the eyepiece removable (when no facilitator is available to show
people how to use it).
One of our points will be that this kind of instrument suffered from many
deficiencies, which is why reflectors took over in the 18th century.

I'm about ready .....commissioning an ultra-long focal length objective
lens. (If we can afford it, of course. I have no idea what the cost would
be.)
Given all the uncertainties, perhaps the best approach would be to ask
anyone who might be interested to contact me directly.    - Bruce
----
I hope the ATM list has room for any discussion on how to make such a lens;
but proposals should be emailed directly to Bruce.
--Peter Abrahams


=============================
Peter Abrahams   telscope@europa.com
The history of the telescope and the binocular:
     http://home.europa.com/~telscope/binotele.htm


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