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    From: Fred Hebard
    Date: 2006 Feb 28, 13:30 -0500

    Mirrors have to be very precisely leveled.  There has been much
    discussion of this on the list, which is in the archives.  Another
    alternative is to use a liquid and let gravity level it.  Unless you
    are up against the shore near tall mountains, such as along the South
    American coast, gravity gives a very fine level surface to liquids;
    the same shortcoming would also apply to bubble levels used to level
    a mirror.  I use a Davis-brand artificial horizon, and fill it with
    dark Caro-brand corn syrup overlain with a thin layer of vegetable
    oil.  I'm not sure that Polaris will be easy to see in the horizon,
    but bright stars such as Sirius, Vega, etc are easy to see, plus the
    planets, sun and moon.  Typically, I can get positions within
    0.2-0.3' of arc, and sometimes under 0.1' of arc.  That is pretty
    close to the limits of resolution of the sextant (0.05' of arc with
    an artificial horizon.  For objects moving fairly rapidly across the
    sky, timing the sight accurately is an important component of overall
    accuracy.  This would be less true for Polaris, should you be able to
    see it in the horizon.  Mounting the sextant on a tripod would also
    facilitate precise observations.
    Yours Truly,
    Fred Hebard
    On Feb 28, 2006, at 12:44 PM, cfi@licfi.com wrote:
    > Good Day to the List!
    > My name is Greg & I have an interest in historical navigation. In
    > addition
    > to enjoying
    > sailing I belong to a local Rev. War group, where I have the
    > persona of a
    > surveyor.
    > Since surveyors also had to know how to determine lat/Lon I have
    > started to
    > teach myself the Basics. At this point I have hit a small snag. It
    > should
    > be pretty straight forward to shoot Polaris But living inland I
    > need to use
    > a artificial horizon a "well leveled mirror" as the books put it,
    > and I
    > can't seem to come any closer than 3-4 NM. The sextant I am using is a
    > MAC(" Measure All Co. " - Japan ) that I bought off of  a retiring
    > blue
    > water boater on eBay; it seems to be in quite good condition, and I
    > think
    > $250 was a fair price. All I can think is that there is something
    > wrong
    > with the way I am setting it up. May I ask for some guidance please:
    >         Why does the star look "smeared" through the mirror & 'OK'
    > during
    >         a normal horizon sight?
    >         How optically flat does the mirror really need to be?
    >         How close (or far away) from the mirror should you need to be?
    >         Is a good quality carpenter?s level accurate enough?
    > Many thanks to the list in advance!
    >                                 -Greg
    > --------------------------------------------------------------------
    > mail2web - Check your email from the web at
    > http://mail2web.com/ .

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