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A further thought about this "Observator" sextant has struck me. Bill Morris
wrote, in [6123]-

Members can read the original patent document at
http://v3.espacenet.com/origdoc?DB=EPODOC&IDX=EP0082556&F=0&QPN=EP0082556.

Its claim to originality are that the filters are contained safely within
the viewing means and that they can be made of cheap material like
photographic film, as they do not have to have flat parallel faces, lying as
they do behind the objective lens of a Galilean telescope.

=============

Which brought this comment from me in [6129]-

It's true that the two filters do indeed "lie behind the objective", but so
far behind it that they are closely in front of the Galilean eyepiece.
There, they sit side by side, the horizon shade to the left and the
reflected-light shade to the right, controlled by separate adjusting knobs.

=============

But is Bill's comment correct, that therefore "they do not have to have flat
parallel faces"?

Imagine a thin wedge prism, being deliberately interposed into the optics,
in just one side of the split viewline, just before the light enters the
eyepiece lens. Wouldn't that displace the apparent direction of one of those
images, by the deflection-angle of the prism, as seen through the eyepiece,
and not the other?

If that's correct, then the requirement for optical quality in the shade, in
its new position, is no less than it is in a traditional sextant design. And
if so, the suggestion that the accuracy of the instrument would not be
degraded by the use of photographic film, instead of optically flat glass,
is at least questionable.

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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