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    Re: Artificial Horizon
    From: Geoffrey Kolbe
    Date: 2002 Apr 5, 17:40 +0100

    At 08:11 05/04/02 -0500, you wrote:
    >Has anyone tried to use a mirror and bubble level as Richard Hubbard
    >described in his book "Boater's Bowditch"?  He describes using a thick glass
    >mirror with a plywood back and thumb screws for leveling.  What is the down
    >side of this and what kind of errors would you expect?  Is the error all in
    >the level or are there others?  Thanks.
    >Jim Foley
    I have a Freiberger artificial horizon mirror. It is 5" diameter and made
    of polished black glass. It has three leveling screws placed at 120 degree
    intervals around the mirror frame. Also supplied were two spirit levels
    with 30" graduations.
    The levels are placed on the mirror itself to level it, so a silvered
    surface would be easily damaged. Front surface reflection is pretty much a
    must as, particularly at low altitude angles, the ghost reflections from
    the front of a back silvered mirror could be confusing. The compromise is a
    mirror of black glass, which is pretty rugged.
    With some fussing, I managed to adjust the levels so that when the bubble
    is centred, the levels are level to better than 10". This would be the
    order of error added into the Hs introduced by this artificial horizon.
    This is pretty much compensated for by the reduction in error due to the
    sextant arc due to the halving of the angle.
    Levels with 30" graduations are reasonably priced, are reasonably small and
    quite readily available. Engineers levels usually have a sensitivity of 10"
    per graduation. But they are very expensive and usually a bit large to fit
    on a 5" mirror. Improvements made using an engineers mirror would be a
    process of diminishing returns. Firstly, for a number or reasons it would
    be hard work to get the mirror horizontal to better than 10" and secondly,
    the errors in the sextant arc would start to dominate.
    It is not an easy thing to use. Being only 5" in diameter you have to get
    as close as possible to get a reasonable field of view to find the star of
    interest. It is best to place it on a pedestal of some sort so that you can
    walk around it to line up on the star of interest. But I have used it with
    some success non-the-less.
    Yours aye,
    Geoffrey Kolbe

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