Re: [ATM] N-KZFS-4

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

Ellen Mackenzie wrote:

>Has anyone on the list have a piece of N-KZFS-4, 30 mm thick and 135 mm
>dia or a plate that I can cut same out of. Shott has a minimum purchase of
>any glass at $1000 USD. I am desperate to finish a refractor
>superapochromat objective. I know this is just a shot in the dark as it is
>listed as 11 times the cost of BK7. KZFS-4 will not do with the glass I
>have.
>
Before there is too much confusion from the public messages...

Ellen has some Bausch & Lomb glass, BSC517645 and pressed DF621362
with the pressing marked with a slightly different value (620362).
In its present state, it is rough ground and looks to be what might
be needed for something like a 500 mm f.l. f/4 achromat. A design
was needed that utilized that glass, but with better performance
than it would have with its current use.  We have been working
some designs back and forth and came up with something that looked
somewhat reasonable.  The BAK4 design is quite similar to the
following design for a 125 mm f/12 refractor:

Surf      Radius  Thickness       Glass      Diameter
 OBJ    Infinity   Infinity                         0
   1    658.4885   11.52753      N-BAK4           130
   2   -6780.276   23.12584                       130
 STO    Infinity   6.881435                  121.6813
   4   -272.3951   14.15397          F2           130
   5   -143.8506          8     N-KZFS4           130
   6    449.4114   19.88205       N-BK7           130
   7   -248.3615   1501.053                       130
 IMA   -947.4752                             26.02613

The stop is placed coincident with the front mounting of the
triplet; its diameter calculated to provide the 125 mm effective
aperture.

While this glass selection may not be optimal, it is one aimed
to achieve minimal cost near-apo visual performance; especially
given that, in the final design, F2 would be replaced by DF621362
and N-BK7 by BSC517645, both of which were in hand. The field
here is shown as that for a 1-1/4" eyepiece; the performance
should for allow a 2" eyepiece, though, if desired.

The use of Hoya BSC2 instead of N-BAK4 is also a selection of
in-hand glass.  While not up to the visual performance of the
BAK4 design, it is still quite reasonable.  It also has the
advantage that a design can be found that allows the swap of the
front lens among Hoya BSC2, Bausch & Lomb BSC517645, Ohara S-BSL7,
and Schott N-BK7 (or pretty much any of this glass flavor), with
a common triplet configuration. These have a front lens that is
also really close to plano-convex -- they could probably be coerced
into it.

---

Richard (not to be confused with Richard or, for
that matter, Richard)
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