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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
>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@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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