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    Re: Artificial horizon
    From: Gary LaPook
    Date: 2005 Feb 21, 23:52 -0800

    Look on ebay for a gunner's quadrant, M2. You can use it to set
    something level to 1/10th of a mil. which is 20.25 arc seconds or .3375
    minutes of arc.
    Gary LaPook
    George Huxtable wrote:
    >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
    >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.
    >contact George Huxtable by email at george@huxtable.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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