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    Re: accurate sextant
    From: Gary LaPook
    Date: 2008 Feb 22, 18:20 -0700

    Gary LaPook writes:
    
    The magnifier pivots to two positions, one over the micrometer drum and
    the other over the limb which allows reading each of these scales
    illuminated by the luminescent paint around the magnifier.
    
    gl
    George Huxtable wrote:
    
    >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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