[ATM] very long focus objective

[Peter Abrahams <telscope@europa.com>]

It is difficult to follow the sub-thread I'm commenting on........

Michael Lindner wrote:
 >A 1 degree tilt of the objective would result in the focal plane moving 
about 1.5' to the side

......then added:

 >Huygen's "pole-and-rope" telescope. A rope was stretched taught between 
the pole mounted
 >objective and the eyepiece to keep them aligned and at a fixed distance.

Richard F.L.R. Snashall wrote:
 >>>  Not a problem (all dimensions in inches):
         R        t            G          D
      159.01      2.0         BaK1       27
    -2533.6      31.244
     -191.86      0.5         LLF1        7
      137.45      4.5
       inf       36.929                   4.769  STOP
      984.83      2.0         K5         28
     -203.06   1161.697
       inf                              600      !! IMAGE !!
Sorry for the silly factor;-)          Rick S.    <<<

I can't figure out these figures, and don't know whether they were meant to 
be taken seriously ('silly factor'....??).  I'd like to know what they mean.

But my comment is:
Jim Hysom of AE Optics in England told me that he made a set of very long 
focus mirrors (8 inch aperture, 300 foot focus), and they had an unusual 
quality of not requiring the eyepiece to be placed in strictly defined 
location.
The eyepiece could be placed anywhere within a circle of perhaps 5 inches 
diameter, and an image would be captured with no lack of definition.  The 
'focal point' was more like a disk.  The objective did not have to be 
precisely 'squared on'.
He felt that this resolved one problem about the usability of these aerial 
telescopes.
However, a planet or other small object would still require precise 
tracking.  At the magnifications used with these telescopes, capturing & 
tracking a planet would still be quite difficult.
Hysom recalled working with a lens designer on a very long focus lens, and 
thought that Jenkins & White, 'Fundamentals of Optics', might have some 
details.  A plano convex lens, or a 'cross convex' lens, with R2 much 
flatter than R1, has some advantages.  A planoconvex lens has an 
interesting attribute, you can aim the objective by holding a lamp next to 
your eye & catching the reflection of the lamp.
-----

Ken Hunter wrote:
 >...difficulty to overcome is the incident stray light.....At 100 feet, 
the objective is
 >a very small contributor to the light that enters the eye lens

This is  problem.  Baffle the eyepiece so that it only accepts light from 
an area 6 inches in diameter, placed 100 feet away?  This would be like 
looking into a soda straw and would require very precise alignment.
Hysom agreed that daytime use would be very difficult or perhaps impossible 
due to stray light.
I don't know of a solution to this & I expect a solution should be found 
before commencing the project.
--Peter


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


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