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

[Ken Hunter <atm_ken_hunter@yahoo.com>]

I believe they are trying to make a SINGLE ELEMENT
lens of long focus complete with color spectrum so
they can demonstrate the difficulties the OLD MASTERS
had to deal with to obtain useful optical systems and
also to demonstrate their facility to obtain useful
data under those circumstances. It won't be easy
now... It was harder then.

Making a 100 foot focal length lens would not be that
difficult with modern glass (I believe they wanted a 5
inch diameter approx). With a F-Ratio like that they
could setup a Fresnel lens a foot across and see an
image greatly magnified complete with color etc...
and, they could use something like a Binocular
objective of 10 inch focal length to be a bit more
reasonable (but more difficult to align) and still get
an image. 

The most difficult part of the program is going to be
finding someone to climb the pole and align the darn
thing. It would be better positioned on a short tower
(guyed) and able to be aligned remotely through
stepper or servo controls.

Another difficulty to overcome is the incident stray
light. With a 5 inch optic 100 feet away, the image is
going to have NO contrast if it's even seen. For an
example, Take a pair of Binoculars apart and try using
the objective and eye lens together without the case.
The image can be seen but it is ALL WASHED OUT by the
incident light. This is with the objective only a
couple of feet away where it portends a fairly large
part of the observable angle of the eye lens. At 100
feet, the objective is a very small contributor to the
light that enters the eye lens and it may indeed be
unnoticed.

The lens is easy, getting it to work is 98% of the
problem.

Like I said, I can make the lens (most of us here can)
but the results would probably not be worth the effort
and even if you got a PERFECT lens, there would always
be the question of it's character and worth.

Ken Hunter



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