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My experience with modern metal sextants is somewhat limited, so the
contributions by Alex and by Bill Morris ("Engineer") are of interest to me.

Bill has recently been kind enough to send me photos of his ingenious
calibration setup, and the sextant that was then under test looked unusual,
so I asked Bill about it. He explained that it was a Russian SNO-T, a
sextant that's quite unfamiliar to me, excepy via Alex's postings.

What struck me was its rather strange (to me) construction, in that the
index arm swings on the "wrong" side of the frame; the right-hand face (when
the sextant is in use and upright). That means that the index mirror,
instead of being fixed directly to the arm, is instead at the other end of
the pivot pin, which passes through the bearing on the frame. I wonder if
the Freiburger, which is said to have many features in common with the SNO,
is the same in that respect. In previous postings, there has been no mention
of that aspect of the SNO design.

I don't see any great advantages, or snags for that matter, in putting the
index arm on that "other" side of the frame. One consequence is that the
sextant's handle, to avoid clanging on the arm, has to be mounted on a
bridge piece, spanning the back of the sextant, so that the arm can swing
beneath it.

Is that the only sextant to have been designed that way? If so, I wonder
why.

Another unusual feature is that there is a magnifier, to ease the reading of
the micrometer drum, and also there's no Vernier with that drum ( which,
otherwise, would be used to interpolate the minutes to tenths). I suspect
these aspects are connected: if there was a Vernier, then the magnifier
would have to be able to swing, to read it over its span of 10 divisions or
so, around the drum's arc. So doing without the Vernier allows the magnifier
to be fixed in place in a simple housing. The choice made by the designer,
then,  was either a simple magnifier or a Vernier, but not both. I suggest
that any observer worth his salt should be quite able to estimate to tenths
of a minute without a Vernier to aid him, especially with a magnified image.

One virtue in providing a magnifier could be in allowing the diameter of the
read-off scale on the drum to be kept small. Otherwise, if a drum is made
large enough to be read with ease directly, then the shaft joining it to its
worm must be made long enough to avoid that large drum clanging on the arc,
which increases the bulk of the complete read-out assembly. This is a bit
speculative, as I haven't seen one of these sextants, just Bill's pics.

Finally, I ask those familiar with this instrument whether they see any
advantage in the straight-through, inverting, 6x telescope, compared with an
equivalent, non-inverting, prismatic ocular of similar power and
light-grasp? Presumably, the prismatic would be a bit heavier; are there
other differences?

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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