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    From: Mike Merriman
    Date: 2006 Feb 28, 13:37 -0500

    please title messages.
    
    If for any reason this bounces, please feel free to use:
    
    Mike.Merriman@alumni.tufts.edu
    ----- Original Message -----
    From: "Fred Hebard" 
    To: 
    Sent: Tuesday, February 28, 2006 1:30 PM
    
    
    > Greg,
    >
    > Welcome!
    >
    > 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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