Re: [ATM] Binocular prism glass

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

Mark Holm wrote:
> I was reading claims in the Orion catalog and got to wondering about one 
> of them.  They claim that "high index" BAK-4 prisms are superior in 
> binoculars to BK-7 prisms.  Does anybody know if this should be true.  
> Seems to me, there might be good arguments for low index and the lower 
> dispersion that usually goes with it.

The higher index of refraction of BaK4 allows for a wider range of
cone angles of TIR.  You can go to a faster f# than with BK7.
However, what I have not been able to figure out is that BaK4 is
said to have a poorer bubble quality thant BK7 -- with the amount
of glass that is used in a prism, I wonder if this can become a
problem.

> 
> Mark Holm
> mdholm@telerama.com


-- 

Rick S.

http://users.rcn.com/rflrs

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