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    Re: Octant as dipmeter
    From: George Huxtable
    Date: 2004 Nov 22, 00:23 +0000

    Alex asked->
    >By the way,
    >can you post or send me
    >a picture of your self-made dipmeter?
    I don't have a photo, but have made a rough sketch, nearly life-sized, in
    .GIF encoding, which I hope you can decode, at
    >And one more question: which model of plastic sextant
    >do you use and what sort of precision you obtain with it,
    >say for altiutude sights in perfect conditions?
    It's an Ebbco micrometer sextant, made by the East Berks. Boat Co., in
    Henley, Oxfordshire, England, about 20 miles from here, but more recently
    I've seen it marketed under the French Plastimo label.
    They claim that the rack is machined, not moulded, in the polycarbonate
    body, and maintains a precision of 1 arc-minute. I've had no reason to
    doubt that, but have made no determined attempts to check it. Under good
    conditions, I expect to be within about 3', which is all I would expect on
    my little 26-footer from even the best sextant. It meets my needs
    I've had this one for about 8 years now, since I broke its predecessor,
    another Ebbco. Usually, if you drop them, they are so light they just
    bounce back, which is a great advantage on a small boat where everything
    gets dropped. On that occasion, I lost my balance, dropped it, and trod on
    it, which was more than it could resist. Its replacement cost me then about
    ?50, say $90.
    It has a 7-inch radius arc. My sketch omits the two horizon shades, off the
    left of the picture..
    The Ebbco has weaknesses. There's quite a lot of unwanted stray light gets
    into the x3 telescope, but the worst aspect is the laminated shades, which
    deteriorate over time. The mounting of the horizon mirror is poorly
    designed in my view: worse than in the earlier model. It has three screws,
    which act against three stainless springs, but one of those screws isn't
    opposite its corresponding spring, so there's a small bending moment on the
    glass. Worse, one of the screws presses against the silvering, and damages
    it locally. I always check index error before and after measurements, and
    it appears to be sufficiently stable.
    The sketch shows where the back-facing periscope fits. It's supported on a
    plastic bracket which is fixed behind the body of the sextant by a couple
    of screws, and is easily removable. It doesn't need precise location. I
    have to remove one of the three shades in the index path to provide room to
    fit it.
    I bought the periscope from an optical dealer for about ?5 ($10, say).
    Perhaps it was salvaged from some sort of military sight. The body is
    aluminium, anodised black, 40mm wide, with three holes 34mm dia. drilled
    through to provide a light-path. It has nice front-silvered mirrors, about
    40 x 50 mm.
    If you have further questions, just ask.
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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