Re: [ATM] 4" objective !

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

Saqib Sajjad wrote:

>
>
>2nd question,should the lens be made from flint glass?
>
>
The short answer is no.  At f/10, I would guess that one of your
worst problems will be color error.

The long answer is that you might consider an achromatic doublet.
If your vendor can only make "plano-" lenses, a somewhat decent
achromatic doublet can be constructed from three of them, with
the flint lens being concave instead of convex.

If I assume "crown" is something like B270 (nd = 1.523, vd = 58.6)
and "flint" is something like F2 (nd = 1.620, vd = 36.4), then an
example setup could use a 1300 mm f.l. crown, a 1000 mm f.l. crown,
and a flint that mates with the steeper crown in curvature (the f.l.
for the flint ends up being about -846 mm).  The net result is
an achromat with a f.l. of about 1650 m.  The two crowns are placed
flat-to-flat, with the steeper one toward the user.  The flint is
nearer the user, facing its mate, but spaced about 7.3 mm from it:

    R            t            g
 676.6          7.5          B270
  inf            0
  inf           7.5          B270
-520.5          7.3
-520.5           7           F2
  inf         1618.071

All of this depends a lot on "crown" and "flint".  You need
to at least get the index and Abbe numbers for them to get
the most out of them.
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