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    Perpendicularity check
    From: Alexandre Eremenko
    Date: 2004 Sep 22, 11:13 -0500

    I recently bought an SNO-T sextant.
    This is my first sextant and now I am playing with it
    day and night:-) Btw, the decision to buy this one
    came after reading some favorable
    coverage of Soviet sextants in this list.
    (For those interested in
    the Russian sextants, I add few remarks at the end of this message).
    Now I have many questions, and here is the first one:
    All sextant books and manuals describe how to check
    perpendicularity of the index mirror to the plane of the arc,
    but it seems to me that the books I read do not describe this
    test properly.
    "Place the sextant on a horizontal table,
    and put two visors on the arc at 0 and 120 degrees.
    (One can use any two objects of equal height, with straight edges,
    like dominoes; Celestaire sells
    special cylinders for this, and Russian sextants come with
    special diopters).
    Then you look at the right edge of the index mirror,
    trying to align
    the direct image of the upper edge of the visor at 0
    with the reflected image of the upper edge of the mirror at 120.
    If they are aligned, the mirror is perpendicular to the arc".
    No book specifies exactly where your eye should be with
    respect to the sextant.
    However, everyone who did this test carefully on a sextant with
    large mirrors should have noticed that the result strongly
    depends on your eye position, more precisely on the height of
    your eye over the table. And this height makes a very substantial
    To see this, just move your eye a bit up and down,
    trying to align the
    images of the visors near the upper-right corner
    of the index mirror
    and then near the lower-right corner. You will see that this is
    Simple geometry confirms this.
    The same simple geometry shows WHAT this test really checks.
    It checks the perpendicularity of the index mirror to the
    plane passing through the upper edges of both visors AND YOUR EYE.
    This plane strongly depends on your eye position, and in general,
    it is not necessarily parallel to the plane of the arc.
    A simplified version of this test, as described in Chauvenet
    and several other books does not use the visors.
    You just look on the arc itself, trying to allign its reflected
    image with the direct image. But when you do this, it is even harder
    to keep your eye in the plane of the arc!
    I suppose this problem becomes insignificant if the sextant
    is large and the mirrors are small (as I see on the pictures of old
    Can anyone give a reference on a more complete discussion of this
    Now on my SNO-T purchase.
    I confirm the opinion expressed earlier in this list that
    the Russian (tuchkan) and Ukrainian (mmely.ru, maur) dealers
    are trustworthy. They replied my e-mails promptly.
    Moreover, they specialize on marine equipment and they
    KNOW something about the merchandise they cell,
    unlike some other dealers on e-bay.
    The SNO-T I bought was made in 1990 but it was sold as "new"
    (was never used), had factory wrapping, a spare mirror, all
    and probably came from a Navy warehouse.
    It has two scopes: an inverting one (7x30) and a Galilean one,
    So far I noticed two defects:
    a) the box is very poor.
    It is made of pine, and has some iron screws
    in it! One of the two pine clamps that hold the sextant was broken
    during the transportation (I suppose some custom oficer broke it when
    trying to take the sextant out!). But due to the careful packing
    with a lot of foam, the sextant was apparently not damaged.
    As it was noticed earlier
    on this mailing list, the box is very small.
    This has evident advantages but also a disadvantage: you have to
    detach the telescopes to store the sextant.
    b) The inverting telescope (7x30) seems to be of poor quality.
    (Or maybe I don't understand some of its hidden wondeful features:-)
    As I understand, a modern 6x scope is nothing but a half of a
    regular binocular. So I compare this SNO-T scope with my binoculars
    (a very old Zeiss and an old Russian 8x30) and the binoculars
    seem infinitely better. The Galilean scope is OK and I use it for
    all my observations.
    The results of the various tests for the instrumental error
    are non-conclusive yet.
    I can report some Sun, Moon and stars measurements which I made
    from my balcony under the ideal conditions, with an artificial
    horizon. These measurements seem to show errors
    of about 0.4' (for the Sun) but I do not have an Almanach yet;
    I am using the "Complete on board Celestial Navigator" which lists
    the Sun position with only 1' precision. I hope that more precise
    Almanach data will permit me to explore the ultimate sextant
    There is also a unique feature which I like: the non-electric
    illumination system. (I was taught that "electricity and salt water
    are incompatible":-)
    A magnifying glass with luminiscent casing
    is used to take the arc and drum reading. It works for about 30
    minutes after exposure of the sextant to bright light.
    I find this very convenient for taking lunar distances from
    my balcony late at night:-)
    If there is interest in this group, I can continue with my
    reviews of the old Soviet navy equipment (I recently bought a fine
    super-precise 3-armed protractor, and a star globe, the things which
    were not used in the West in the last 50 years).

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