Re: [ATM] Finder

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

Richard wrote:
> Hi Richard,
>
> Sunday, January 7, 2007, 9:06:55 AM, you wrote:
>
> RFLRS> With a more vignetting (40% throughput at the field
> RFLRS> edge) and less magnification (7 mm exit pupil),
> RFLRS> however:
>
> I love an academic exercise as much as the next man Richard, but do you think,
> as 10 element finders go, it is starting to push the price/performance
> boundary a bit :)
I was starting to think that, but consider it; there are actually two
items here: the scope and the finder.  Yes, there are some cheap finders
out there.  The additional two lenses in the scope itself gives somewhat
reasonable (VS > 0.5 performance out to 4 degrees).  Then, even with
the cheap finder, you'll need an eyepiece.  If you want the full
part of the scope, R&vV recommends (wide field) an exit pupil of 5 to
7 mm -- with a 48.8 mm objective, that puts the magnification between
7 and 10.  That puts the AFOV between 56 and 70 degrees (to a first
order).  The simple lens combinations like those given in the ATM series
didn't seem to cut it; that pretty much seemed to put it into the range of
an Erfle type of eyepiece (or even more complex).  With five lenses
(and the other restrictions I put on it), I couldn't find a reasonable
solution.  While the triple doublet form also looked promising, it was
hard to come up with one that had a nice geometry.  This form seemed
the way to go, since 4 or 5 of the ten lenses could now be OTS.

Still... it'll nickel and dime (or is it farthing and pence) you to death;
but every step here seemed entirely realistic at the time.

>
> I'm grinning ear to ear because this is exactly the sort of thing I
> spend my time doing.
>
>   
 :-)    :-)    :-)
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