Re: [ATM] RE:Refractors

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

Richard F.L.R. Snashall wrote:

> Richard F.L.R. Snashall wrote:
>
>>
> But, in view of Steve's response, at a lower scale (simple
> achromat), you could try (200 mm CA at roughly f/14):

A glance at Roger Ceragioli's refractor design page yields
another Littrow (THAT's the name I was looking for!) design
using SK11 and BaSF2.  As his article mentions, though,
those glasses are not known for their robustness.

Looking for more, I generated:

      R           t           G
   1210          20           Schott N-BK7/BK7       -or-
                              Xinhu/Chengdu H-K9L    -or-
                              Hoya BSC7              -or-
                              Hikari E-BK7           -or-
                              Ohara S-BSL7           -or-
                              Sumita BK7
  -1210          12.5         Schott F5/F5HT         -or-
                              Xinhu/Chengdu F1/H-F1  -or-
                              Hikari E-F5/F5         -or-
                              Ohara S-TIM5           -or-
                              Sumita F5
    inf

Looking at a Schott glass selection of this, this design
isn't quite as good, nominally, as the previous design.
Schott lists the nominal tolerances as +/-0.001 for nd
and +/-0.3 for vd.  For an nd change of +0.0005 to +0.001
or a vd change of -0.05 to -0.3, the performance of this
design equals the previous design.  It begs the question
as to whether it would be possible to mix-n-match the
glass selection to come up with an excellent implementation.

BTW: Based on the Schott glass properties, both of these
glasses are really cheap and robust.  Wonder why we don't
see more of this combination instead of the F4/F2 mating
(other than the fact that the F4/F2 mating requires
shallower curves, of course).
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