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The main advantage of Kepler telescope in comparison with Galileo
is the larger field of view (for given magnification).
This makes telescopes
with larger magnification
practical.

A Kepler telescope can be made direct by inserting an extra lens.
However for the use in astronomy and navigation it does not matter
whether the image is inverted or not,
so they do not insert this extra lens.
(An extra lens absorbs some light).

The Galileo scope has one advantage: it is much shorter (for given
lens diameter). For twilight observation you need a larger lens
(to be able to see the horizon). A Kepler telescope with such large lens
would be inconvenient because of its length. One can reduce length by
making it prismatic, but this also absorbs some extra light.

In modern sextants, ideally one uses a Galileo scope for twilight
observations,
and a prismatic Kepler (like in most binoculars) for Sun observations.
The disadvantage of prismatic version of Kepler is that it is much
heavier, and also absorbs some extra light.

The only modern sextant that I know which uses a classic, inverting Kepler
scope
(without prisms or extra glass) is SNO-T.
Everyone who tried an SNO-T inverting thinks it is better than
usual sextant telescopes.
The quality of image, magnification and field of view is like in a good
prismatic, but the weight is much less.

Alex.


> Why is the inverting telescope to be prefferred in sextants, as suggested
> in the book "Wrinkles in Practical Navigation"? The author states
> "superiority over the direct one is unquestioned".  If so where can one be
> obtained, for an Astra?  Is it worth the money?
> Randy
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