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Jim Thompson asked-

>How does the body appear across the entire circular view of the telescopes
>when using a split mirror?

I don't know about the Astra IIIb, but here, I hope, is a general answer to
Jim's question.

The reflection from the silvering is getting on for 100%. But even an
unsilvered glass surface reflects light to some extent, just as you can see
in a window-pane. Light can be reflected in this way from both surfaces of
the unsilvered part of the horizon mirror. So you still see an image of the
Sun in that part of the glass, but a significantly dimmer one than in the
silvering.

Some old sextants would take advantage of this, by arranging to reduce the
brightness of the ray from the Sun, by moving the telescope exactly
parallel with itself so as to look only at the image in the unsilvered
part. Indeed, this is the only purpose I can see in having that unsilvered
area of glass at all. Otherwise, in a split-horizon sextant the horizon
mirror could just as well be truncated at the edge of the silvered part.

On the BBC reenactment, in the Endeavour replica, of Cook's passage North
of Australia, the navigators, who were observing lunars for longitude, had
modern sextants, both split-horizon type and whole-horizon type. They
commented, when trying to locate the Moon high up in a bright daytime sky,
that it was very difficult in a whole-horizon sextant, but the greater
contrast of a split-horizon instrument allowed the Moon to be picked up
much more readily.

George.

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