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    Re: Bubble horizon
    From: Geoffrey Kolbe
    Date: 2002 Dec 19, 10:26 +0000

    At 13:18 18/12/02 -0800, you wrote:
    >"Dr. Geoffrey Kolbe" wrote:
    >> To answer George Huxtable's question on the sort of accuracy one should
    >> expect from a bubble sextant. I have a Link A-12 aircraft sextant (still
    >> available from Celestaire, last time I looked,) and I would expect an
    >> altitude measurement to be within +/- one minute. I am disappointed if I am
    >> two minutes out and I cannot remember the last time I was more than two
    >> minutes out.
    >That is impressive. I have never done a serious accuracy test with
    >mine, but I doubt I could match your performance. What are you
    >shooting? Sun? Stars?
    That is on both sun and stars. Yes, the little A-12 is a pretty neat
    sextant. The scale vernier is only graduated in 2 minute intervals, but it
    is relatively easy to interpolate to one minute accuracy.
    The aim it to get the celestial object in the centre of the bubble, which
    forms a ring around the object. This sighting picture is very similar to
    target rifle shooting, where the aim is the centre the round aiming mark
    inside a ring foresight. Since I have shot at international level, and have
    consequently spent many, many hours practicing the art of centering a round
    aiming mark in the foresight ring, I do, perhaps, have an edge over most
    people in using a bubble sextant.
    It is worth noting that the "bull" in international shooting is around one
    minute in size. A good shot, using just "iron" sights with no
    magnification, can usually hit the bull (or ten ring) 9 out of 10 times. A
    good shot can achieve a group half-width of about half a minute, which is
    usually considered to be the limits of resolution of the human eye.
    However, the "vernier acuity" of the eye, the ability to align one line or
    mark up with another, is about a tenth of a minute. Centering an aiming
    mark inside a ring - even a fuzzy aiming mark - owes more to the "vernier
    acuity of the eye/brain combination than base resolution of the eye.
    Contrary to intuitive thinking, making the foresight ring just slight
    larger than the aiming mark leads to poor accuracy and large group size.
    Target rifle shooters always have foresight rings which leave a generous
    annulus between the aiming mark and the inside of the foresight ring. I
    would suggest that if the bubble size in a bubble sextant can be adjusted,
    it is left at least twice the size of the sun. This size of bubble is no
    impediment to getting good results with stars as well.
    >Are you able to get batteries to fit the night illuminator? The ones
    >I've tried are a shade too fat to go into the nose cap that holds the
    >bulb. It seems C cell diameter has increased slightly since the 1940s,
    >when the sextant was made.
    Yes, I had that problem too, and agree that the diameter of the C cell must
    have increased by about half a millimeter over the years. Such batteries
    used to have cardboard outer coverings, which could be removed to allow the
    battery to fit into the cap. But that is not the case these days. I just
    put the cap in a lathe and machined out sufficient to allow a modern
    battery to enter the cap.
    Geoffrey Kolbe.

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