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The thread on the AstraIIIb's split mirror was very helpful, but did not
thoroughly review the subject of sextant optics, and raised more questions
in my mind.  We have not yet thoroughly described issues of depth of field,
field of view, magnification, and glass and lens manufacture with respect to
modern sextants.

I have been prowling around the Internet looking for online information
regarding sextant optics to fill in the gaps.  My guess is that these are
closely guarded industrial secrets, or that there is information buried in
specialized print publication that has not made its way to the Internet yet.
And we do not have optics specialists on this list.  I am not in a
geographic position that allows me to explore the print literature on this
subject.

Meanwhile I did turn up this distracting web page, where Celestaire appealed
to the US Government to redefine sextant tariff law for the AstraIIIb:
http://www.law.emory.edu/fedcircuit/july97/97-1005.html
"Celestaire ... appeals the order of the United States Court of
International Trade (defining) marine sextants as ?optical navigational
instruments? ... dutiable at 5.6% ... rather than as ?other non-optical
navigational instruments? ... which are not subject to a tariff."  It
demonstrates how lawyers can argue the definition of a sextant, including
whether they are metal instruments or not.  The central argument by
Celestaire seemed to be that sextant optics do not primarily aid human
vision, but are subsidiary to the purpose of measuring angles.  "It is
uncontested that sextants are navigational instruments; we are only asked to
determine whether they are optical ones or not."  The court concluded that
"the immediate purpose of the sextant is to allow the user to see the sun
and the horizon at the same time ? an act which the user could not otherwise
do. The intermediate purpose is to accurately measure angles which can only
be estimated by the naked eye. It is only the ultimate purpose, to determine
navigational position, that does not have to do with enhancing human vision
...".  The Court  found in favouring of continuing the tariff, "Because a
sextant aids or enhances human vision ... through the use of its
non-subsidiary split-image mirror...".

So the court reaffirms that split mirror optics indeed aid human vision, but
... precisely how?

Jim Thompson
jim2@jimthompson.net
www.jimthompson.net
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