Re: [ATM] Glass type

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

Thomas Janstrom wrote:

>Hi Richard Et Al,
>
>I've just a quick Q regarding this thread. I have been unable to find
>any real numbers for BSC2 (of which I have two 4" X 3/8" blanks) so I
>was wondering how it would work with NF11 of which I have one 4.5" by
>5/8" blank (one side is concave...)?
>
>I have most of the melt data on the NF11, but next to nothing on the
>BSC2. I know that the two blanks are very thin, but how about doing two
>halves of the one lens with the half way line being a flat surface
>between the two BSC2 blanks? Sort of like this:
>
>/||\ \\
>|||| || <-this is the flint
>\||/ //
>
>^ ^ Two halves of the crown element.
>
>Would this work?
>
>Cheers Thomas.
>
You don't really have to force those two into a single lens.

First, are you sure the flint is NF11 and not FN11?  I have
information on neither NF11 nor N-F11.

Anyway, it appears I cannot browse the OSLO obsolete catalog, so
I worked around it.  I generated wavelengths for i,h,g,F,e,d,C,r,s and
t wavelengths and then asked OSLO to generate the refractive indices:

     t 1.507060
     s 1.509531
     r 1.512600
     C 1.514024
     d 1.516496
     e 1.518416
     F 1.522067
     g 1.526367
     h 1.529911
     i 1.535926

I then put these into the Glass Coefficient calculator (using
Chance BSC517642 as a baseline) and got:

     A0   2.27115088
     A2  -9.97731395E-3
     Am2  1.03588749E-2
     Am4  2.97221651E-4
     Am6 -2.20826509E-5
     Am8  1.28948423E-6

The maximum error was (F-line) 1.2E-7 with an RMS error of 6E-8.

I plugged these into the triplet design aid, using a balance of
-1.4 (the BSC2 and FN11 lenses will have opposite signs) with
the second and third lenses bound.  By tweaking the second
radius and reoptimizing, I was able to come up with a fairly
good 70% vs 100% correction.  I then put the design into OSLO
and optimized the focus and field curvature.  The net is:

 SRF      RADIUS      THICKNESS   APERTURE RADIUS    GLASS
 
 AST   1.2490e+03      8.100000     50.800000 A     H_BSC2
  2   -3.3285e+03      2.795290     50.800000          AIR

  3    775.280000      9.200000     50.800000       H_BSC2
  4   -1.7317e+03      7.400000     50.800000         FN11
  5    1.2073e+03        --         50.800000          AIR

 IMS  -526.337942 V  1.4750e+03 V   26.182807 S

Of course you can still just use the doublet design tool and come
up with a solution -- the curves are different.  You have to allow
for enough thickness for both your lenses -- about 28% of the
semi-diameter, IIRC.

BTW: The new chemistry replacement for F2, N-F2, is quite similar
     to FN11 in anomalous refractive index behavior.  Just an
     aside.

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