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    Soviet and American dip-meters
    From: Alexandre Eremenko
    Date: 2012 Mar 29, 19:05 -0400

    After some prelinimary studies of the Gavrisheff patent and the picutre,
    and of the Soviet N-3 pictures, I want to share my observations:
    1. Gavrishev does not mention in his patent that some mean to regulate
    the light, at least from one side is needed.
    The brightness of the horizon images must be equal for good observation.
    And one usually has glare under Sun.
    This is added in Shufeldt's
    photo: something that
    looks like sextant index shades (protruding downwards in the photo).
    Also two handles are visible. Apparently when the observer turns 180d,
    the device it turned upside down. In one of the two positions, the drum
    will be above and this is inconvenient.
    The Soviets have a diaphragm for the same purpose, which seems to be a
    better solution. This probably also implies that they use a Kepler
    2. I don't like Gavrishev's mechanism connecting the screw 46 with the
    lever 39 through a ball nut. This nut will inevitably have some backlash,
    and there is no provision against this. On my (non-profesional)
    opinion, some spring is necessary, pressing the lever to the screw,
    downwards or upwards, to avoid this backlash.
    3. Gavrisheff proposes two shapes of the main prism. A simpler one will
    show the horizon vertical. Of course this is preferable, because
    otherwise you need 2 extra reflections, with some loss of light, of
    Soviet instrument shows the horizon vertical.
    4. The Soviet instrument looks simpler, and you see
    from the photos that it is smaller.
    Let me try a "reverse engineering" exercise:-)
    It probably has only two prizms, one fixed (for the ray coming from
    the left), another rotating about the axis of the telescope.
    Or about some parallel axis, in which case one needs a cogwheel.
    (The adjusting screw and the drum are co-axial with the telescope.)
    There is no screw and worm connection, no backslash.
    The scale is large enough to graduate to 1/5 of a minute, and can be read
    easily with a naked eye to 1/10.
    (I still have to think whether an arrangment with two prizms is possible,
    perhaps one needs 3).
    The difficulty of making this in home workshop may be in making
    and graduating the ring with divisions. (For the Gavrisheff arrangement
    I was contemplating using some old SNO drum and worm:-).
    5. A study of Gavrisheff's patent shows that the thing is more comlicated
    than I thought, and perhaps beyond the possibilities of a home workshop.
    (Certainly not for me:-)
    6. Shufeldt says that Gavrisheff's dipmeter (with a powerful telescope)
    has accuracy 0'1, and the Soviet textbook says that N-5 has accuracy
    0'3 (with a very small telescope, probably like the standard SNO
    inverting). Of course 0'3 is enough for normal Cel Nav.

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