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    Re: Children's land-locked "Sextant"
    From: Geoffrey Kolbe
    Date: 2007 Nov 29, 06:24 +0000

    Alex wrote:
    At 04:34 29/11/2007, you wrote:
    >On Wed, 28 Nov 2007, Robert Eno wrote:
    > > Consider a black glass artificial horizon. Frieberger
    >I've seen one of those and tested it in Freiberger's
    >office (in Freiberg, Germany).
    >The price was something like 900 Euro, but they were very
    >nice to me and offered for only 700 (if I pay cash on
    >the spot:-)
    >To try, we put it on the wood table in their office
    >(on the second floor of a brick building in a quiet
    >street in a small quiet city) I asked everyone to sit
    >down and stop breathing for a while. And aimed my sextant
    >at the Sun reflection (SNO-T, 6x scope). It was shaking
    >like crazy (5-10 minutes amplitude by eye estimate).
    >So I decided not to spend 700 Euro for a piece of glass
    >in a drame with 3 screws and a pair of levels.
    >Actually I believe that a handyman with simple tools can
    >make such thing himself. Optical quality glass seems to
    >be available on the Internet for a reasonable price.
    >Besides, 30" ADVERTISED accuracy did not seem sufficient
    >for the sort of observations I am doing.
    I bought a Freiberger artificial horizon about 15 years ago and though it
    was expensive then, it was considerably less than 700 euros or I certainly
    would not have bought it! I have to say, it is a very nice piece of kit -
    though I had to grind the feet of the levels flat so they would sit
    properly on the glass without rocking.
    As Alex says, the levels have a sensitivity of 30" per division. But of
    course, by carefully centering the bubble, you can do much better than
    that. I think that with care it is possible to get the plate level to a
    tenth of this figure, that is 3" of arc,
    As for the vibration Alex experienced, that is not something that has
    bothered me - but then, the secluded valley in the Borders of Scotland in
    which I live is so remote that mobile phones don't work. The skies (when
    they are clear) are about as dark as you will get in the UK and amateur
    astronomers bring their telescopes to this area to get good seeing
    conditions. Vibration is not a problem here :-)
    Geoffrey Kolbe
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