Re: [ATM] Special Short Flint APO -- Some History

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

Recently, a friend and I had acquired some quite ancient pieces of glass.
Fortunately, the glass was marked with the values 517645 and 621362 (or
sometimes 620362).  These were recognized as being the icodes for a
borosilicate crown and a dense flint made by Bausch and Lomb.  They are
quite similar to the common BK7 (517642) and F2 (620364). I used the
information on Bob May's web site to get a set of Cauchy coefficients for
each of the glasses.  I could then manipulate them with my choice of
optics design tool (ZEMAX).

These pieces of glass were not cylindrical blanks, but already ground
into achromat shapes -- the biconvex crown and the plano-concave flint.
A quick estimate from measurements was 125 mm f/4.  The astronomical
uses, as is, were therefore somewhat limited.

We poked at these glasses in design for quite some time, but an achromat
is an achromat -- normal glass is normal glass.  Even if the huge amount
of glass were removed to slow it down to f/16, it would still be subject
to an achromat's secondary spectrum.

I finally happed to try adding a piece of the special short flint,
N-KzFS4.  It showed some promise, so I aimed for something on the order
of f/12.  By itself, the three glasses did somewhat better, but not
really what you would call outstanding.  But by adding an additional
air-spaced crown element -- Good Stuff!

The separate crown element turned out to be quite close to plano-convex.
It might be possible to make two such elements from one of the original
crown lenses.  That might be difficult, but it would also mismatch the
pieces -- it took 1-1/2 crown elements to mate with each flint element.
Fortunately, my friend also owned a number of Hoya BSC2 glass that
happened to match the diameter and had sufficient thickness to be the
plano-convex lens. Glass BSC2 is also on the ancient side; OSLO lists
BSC2 as obsolete, but still has the Cauchy coefficients -- it is also
similar to the Bausch and Lomb crown glass.  There was little performance
difference when BSC2 was used in lieu of the Bausch and Lomb crown.

The final prescription is:

    http://users.rcn.com/rflrs/quad1-16.atm
    http://users.rcn.com/rflrs/quad1-16.len

These prescriptions include the -0.001 adjustment to the flint refractive
index.  I believe this change from the catalog value may be the result of
pressing, as the icode was impressed into some of the glass pieces I
have.  I have no direct knowledge, though, that this would be the effect
of the pressing operation.

The design could probably be sped up some.  That would tend to more
closely match the original crown radii.  But, given the fixed central
thickness of the existing dense flint pieces, the edge thickness of the
flint element would become bothersome before it got to f/10.
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