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FIELD OF VIEW
It's quite understandable that the small mirrors on the Navy Mark II
might lead to assuming it has a narrow field of view.  But this is not
the case.  The Mark II has a 9d FOV compared to most other sextants
that have something like a 4d FOV.  And the shades on many sextants
reduce their FOV even further.

The FOV has nothing to do with the objective lens diameter.  Rather
it's a question of practical optical engineering, which I know nothing
about.  The Mark II scope itself has a 9d FOV which is not obstructed
at all by the small mirrors.  So the sextant as a whole has the same
9d FOV.  The explanation for this is the "apex" of the FOV cone lies
near the objective lens, which is very close to the mirrors because
the scope is so long.  So its FOV is unobstructed by the mirrors.

Thus it's not true that other sextants, such as the Mark III or the
Plath Navistar, are easier to use at sea.  The exact opposite is true,
with the Mark II having more than twice the FOV of any sextant I have
seen. (Some prism scopes do have a FOV approaching 6d.)  For those
that haven't experienced the whole-horizon wide-FOV using a
traditional horizon mirror, it makes taking a sight so much more
enjoyable and esthetically beautiful.

Nonetheless, I'm not necessarily recommending the Mark II over other
sextants.  Its small aperture is a disadvantage,  making stars and the
twilight horizons noticeably dimmer than sextants with much larger
apertures.

WHOLE-HORIZON VIEWS with TRADITIONAL MIRRORS
Simple ray tracing (as I discuss in my book) shows why a lens using a
focal plane can image the entire field even though half of the
objective is blocked off.  This analysis is confirmed with all scopes
that I have seen that use an internal focal plane.

But using the same simple ray tracing, I can't exactly see why the
Galilean scope (no eternal focal plane) doesn't display the whole
field also.  I think it's related to vignetting, depth of field, and
practical optical engineering, not something explained by simple ray
tracing.  Nonetheless, experimentally, every Galilean scope I've seen
shows only half the FOV when the other half is blocked off at the
objective.

So I understand why prism scopes show the entire field, but not why
Galilean scopes don't.  In my book I explain the former and ignore the
latter -- you don't expect an author to confess his ignorance, do you?

John


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