RE: [ATM] F5 Achromat Design Help

["Jerry" <wa4guu@verizon.net>]

You can see why f15 is the ratio of choice. Even longer would make a better
lens but the mechanical length becomes impractical. However my 4" f16 BK7/F4
performs better than you would think looking at spot and LA diagrams. I
designed mine with ATM book 3 Wyld method of cooking an existing design to
my glass and f ratio and using the path difference method to adjust R4 to
correct color. Doing the G-sum to get a design and then the path difference
came to virtually the same design. I did not have OSLO then. 


I don't think just adding a third glass to BK7 and F2 or F4 will get you
far. I have had no luck finding one to give much improvement, but I don't
very well know what I am doing. Maybe someone else can. For a triplet I
would discard one of the glasses, probably the flint and try to find two
glasses to go with the BK7. Or I would just try to find three glasses to
make a good triplet.

But after much time trying to find some good glass choices I have found that
taking one of Richard's designs might be the best route for me when I start
a triplet. 

A triplet probably would not be a wise choice for a first time refractor
lens project. On the other hand a doublet is not much more difficult than a
mirror, if you have made a mirror. I wouldn't recommend a lens for a first
project. Since thickness matters you need to do a proper job on the grinding
and polishing, getting the curves to the right radius and the glass to the
right thickness the first time. You will have to redesign if you get one or
both elements too thin. And worse you may not have enough glass even for
redesigning a curve or two. At least one mirror to learn the grinding and
polishing techniques before trying a lens seems a wise course to me.

Jerry 




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