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John Karl wrote-

| After collecting seven popular sextant makes and models, I've
| discovered that the Mark II has a rare design that I was blissfully
| unaware of for decades.  It has a very wide VOF terrestrial scope with
| an internal focal plane.  Just like a prism scope, even with a
| traditional split-horizon mirror, they show a complete image across
| the VOF using only half the objective, essentially just like a whole-
| horizon mirror.  (You can see this in any binocs by placing a opaque
| card over half of the objective.)  Most sextants have cheap Galilean
| scopes that don't behave this way.

I was somewhat intrigued to read this, about the field of view that remained
unobstructed even though half the objective was obscured, that applied to a
terrestrial scope and not to a Galilean. John Karl has had some interesting
things to say about sextant scopes, in his book "Celestial Navigation in the
GPS age", that I haven't read elsewhere, and they provide fuel for thought.
[I hope to report back more on that book when I have had time to study it a
bit further, unless someone else jumps in first.]

In his book, John writes on page 105  "Sextants use three types of
telescopes; the prism, the terrestrial, and the Galilean telescope". He
omits a fourth category, that of the inverting telescope. In my own Vernier
sextant, from the early 20th century, the higher-gain telescope, with x6 and
x12 eyepieces, is in that category; the other is x3, Galilean.. But it's not
only "historical" sextants that have such inverting scopes: I think Alex may
confirm that his modern Russian SNO is fitted similarly. An inverting scope
certainly takes a bit of getting used to, but once upon a time all high-gain
sextant scopes were made that way, and navigators managed to adapt to their
star-groups being upside down, and the sea being above the sky. So that
category deserves a mention.

I have compared 5 different scopes that I happen to have at home, from the
viewpoint of how they behave when a card is drawn across to obscure an
increasing part of the view, a few inches in front of the objective, to test
John's contention above, about which he gives more detail in his book.

The inverting scope from my old Vernier sextant, used with the higher-gain
(x12) eyepiece, behaves just as John describes. As you move a card across
the view, the brightness progessively dims, becoming dimmer from one side to
a slight extent, but you can't really see an image of the edge of the card.

I have compared that with two Galilean scopes with a gain of x3, one from a
plastic sextant, the other from my old Vernier sextant, and the two behave
similarly, and very differently from the x12. Just as John describes, the
card cuts off the image from one side, and you see a slightly fuzzy image of
the edge of the card, across the field of view.

And I have compared also two x6 telescopes, the inverting one from my
Vernier sextant with its x6 eyepiece, and a prismatic monocular, not from a
sextant at all. These two behave similarly, and in a way that is
intermediate between the x12 and the x3, with the card-edge apparent, but
much less sharp than with the x3.

What do I conclude from that evidence? It seems to me that this
field-of-view shadowing effect may not be related to the TYPE of scope, as
John implies, but simply to its magnification. John attributes it, instead,
to the presence or absence of an intermediate focal plane, between different
telescope types; being absent in the Galilean case. My observations don't
actually disprove that contention, but leave me unconvinced by it. I would
like some more evidence, or closer reasoning than John has provided, please.

John ends with the somewhat pejorative comment- "Most sextants have cheap
Galilean scopes that don't behave this way." Maybe a Galilean scope is
cheaper than others, but in many circumstances it's the appropriate one to
use in low-gain applications, in a small craft at sea, when anything
stronger than x3 is just an embarrassment.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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