Re: [ATM] very long focus objective

["Richard F.L.R. Snashall" <rflrs@rcn.com>]

Peter Abrahams wrote:
 > It is difficult to follow the sub-thread I'm commenting on........
 >
 >
 > 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.

Each line represents an optical surface; these are described as
having a radius of curvature (R), a thickness to the next surface (t),
a glass type to the next surface (G), and  diameter (D).  The
first lens here is made out of BaK1 glass and is 27 inches in
diameter and 2 inches thick, with the two ROCs of 159.01 inches
(convex) and -2533.6 (also convex, but on the second side, so it
is negative in value).

 >

This is actually a legitimate (?) prescription for
a 5 inch f/240 (100 foot focal length) lens.  However,
the "silly" part is the length is over 6 feet forcing
the outer lenses to be huge (beyond silly, actually).

While Bob May is correct in saying that a doublet would
suffice, the prescription

        R           t          G        D
       inf        -0.004                5    STOP
      745.05       0.35       BK7       5.6
     -425.9        0.26       F4        5.6
    -1649       1199.886
     -443.05                          120    IMAGE

would provide a plus/minus three degree tolerance on alignment,
on the order that Michael gave for the singlet.  The issue here
is who is going to mount the lens on top of the pole?  Will
that individual be able to set it up within the tolerance?
The fifty foot image plane of that "silly" lens allows a plus/minus
fourteen degree tolerance.  On second thought, though, perhaps the
image plane should actually be curved to match the lens to image
plane distance -- in that way, if the lens is mis-aligned, the
distance to the image would not change -- the eyepiece would not
need to move in any direction (Bob is right, though, a f/240 the
depth of field is quite large).

Less silly, but still a tad cumbersome, while allowing a plus/minus
12.5 degree tolerance.

         R           T            G         D
      105.3          0.6         KF3        8.6
      846.04         5.534
     -364.7          0.3         BaFN10     5.6
      169.44         0.1
        inf         11.083                  4.871  STOP
     -209.61         0.6         BaK4      11
     -115.49      1188.751
    -1200                                 520      IMAGE

 > But my comment is:
 > Jim Hysom of AE Optics in England told me that he made a set of very

That is the intent of the large fields.

But, as Ken pointed out, none of these is very "historical", and
may be disqualified in that respect.

-- 

Rick S.

http://users.rcn.com/rflrs


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