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    Re: Artificial horizon
    From: George Huxtable
    Date: 2005 Feb 20, 23:25 +0000

    Some more information about artificial horizons.
    
    The best text and pictures I know are to be found in that delightful book
    by list-member Peter Ifland, "Taking the Stars". He devotes twenty-odd
    pages to such reflectors and art. horizon devices, with many pictures of
    various liquid troughs and one of a black-glass reflector.
    
    Bruce Stark refers to a block of "talc" that went with Lewis and Clark. I
    remember reading that this refers to what we would now call mica, which can
    be split off in thin strips of quite high optical quality, if the original
    material is good enough, to make transparent windows for the cloche that
    covers a reflecting trough. However, I can't quote a reference to support
    that memory. If there remains any net refraction in the light path through
    this pair of windows, it can be allowed for by reversing the cloche, part
    way through the series of observations.
    
    Alex has written-
    
    "On another suggestion, to use black tea,
    it should have the same problems as water I used:
    too liquid, too much disturbed by vibration.
    A more viscous liquid should be more appropriate
    (unless you have a really solid foundation like a concrete
    slab:-)"
    
    I think Alec's problem here is that he is observing from a verandah or
    balcony. No matter how solid this might seem, it's FAR more sensitive to
    vibration than something that's firmly planted outdoors, on solid ground.
    In his special circumstances, a more viscous liquid would be better, but a
    firm foundation would be better still.
    
    As for the mirror-type artificial horizon, no doubt it can be made to work
    well enough, but there are many more ways of getting an inaccurate answer,
    compared with a self-levelling fluid.
    
    There seems no serious problem to me in taking a thick piece of float-glass
    or perhaps even plexiglass and gluing to it three adjusting screws with
    cone-feet. You should be able to check that the upper and lower surfaces
    are parallel by examining the reflected image of a distant object (a star)
    using a telescope from a sextant. As long as the reflected images from
    front and back surfaces coincide exactly, then the two surfaces are
    parallel, in which case there's no call for black-glass to eliminate the
    lower reflection. I'm not sure how you ensure flatness, though.
    
    You need a VERY sensitive and small and light spirit level, such as is
    available for incorporation into a theodolite. A friend recently bought a
    small level with a sensitivity of 20 arc-seconds per marked division (if I
    remember correctly). Of course you use such a level in both x and y
    directions across the surface, reversing it to ensure there's no error due
    to the level itself.
    
    But the glass plate, on its screw feet, must be set on a completely firm
    foundation. The weight of the spirit level being put on and taken off must
    not give rise to any flexure anywhere. The tripod or other footing that it
    sits on must not tilt AT ALL as you approach it and shift your weight
    about. I can imagine that accurate setting of such a plate might be a very
    fiddly business, but I have never tried to do it.
    
    On the other hand, with a fluid level, you just pour the stuff in, and
    there it is. Can't go wrong. I know which I would prefer. Especially if the
    alternative costs $900.
    
    George.
    
    ================================================================
    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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