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    Re: Artificial Horizon
    From: JC Sutherland
    Date: 2002 Apr 5, 22:21 +0100

    Quoting "Foley, Jim" :
    > 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
    There are three problems with using a glass mirror,  which can be overcome.
    First the mirror needs to be front silvered so that there is no reflection from
    the non-silvered glass surface
    Second the glass needs to be thick enough so that there is no bending of the
    mirror by the supporting frame and
    Thirdly the spirit level needs to be sensitive enough to  detect an out of level
    of better than 1 minute of arc this means using a precise engineering level and
    these are rather expensive.
    I have used a 6 mm thick glass mirror which I epoxied to a mounting disk. The
    epoxy shrank during setting and this was enough to produce a blurred image of
    the sun on reflection. I recommend using at least 15 mm glass. Ordinary window
    glass is not flat but Float glass is nearly as good as ground and polished plate
    By the way, as the glass is to be front silvered it need not be transparant.
    Best of luck.

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