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    Re: Sextant accuracy (was : Plumb-line horizon vs. geocentric horizon)
    From: Alexandre Eremenko
    Date: 2005 Feb 12, 02:35 -0500

    I can confirm what Frank says:
    I have two scopes, a usual 3.5x and an inverting 7x.
    After long trials I found that it is hardly possible
    to achieve good accuracy in star-to-star distances
    with my 3.5x. In all my observations (from the land,
    I have not tried my sextant in sea yet) I find the 7x
    far superior to the usual scope.
    I cannot say it is easy with the 7x, sometimes I am 0.3-0.6 off,
    but after long
    practice I can achieve 0.2 to 0.3 accuracy most of the time,
    with my 7x scope, both for
    lunar and star-to star distances.
    Averaging a series of 5-6 such
    observations usually gives 0.1 accuracy.
    Let me share some training experience:
    a) my common mistake in the beginning was to observe
    very bright Moon. It obscures the star. One has to use
    a filter, sometimes two, on the Moon, so that the star is
    well visible when it touches.
    Same applies to a pair of stars where one is substantially brighter.
    In general, better results with star-to-star distances can be
    achieved with stars that are not very bright. Same applies to index
    check with stars: use a weak star.
    b) With lunar distances, it is hard to understand in the beginning
    how should it look when the star really "touches" the Moon.
    I used the following method with Frank's online lunar calculator:
    I measure a distance and reduce it immediately to see the error,
    then I measure it again, trying to correct my mistake,
    and reduce, and repeat
    this until I get
    a good result consistently.
    The procedure works even better with star-to-star distances:
    they change so slowly that you can preset your sextant on
    the exact distance, and then see how it looks. And then
    slightly turn the drum and try to achieve the same view again.
    > > Frank Reed a ?crit:
    > >
    > > Pierre Brial, you wrote:
    > > "Not yet. My sextant has a mechanical accuracy of 0.2', but despite
    > > the
    > > fact I have adjusted it, repeated tests on star distances show me that
    > > it is difficult to have a practical accuracy of under 0.8'. But may be
    > > this is also because of my lack of experience or inaccuracies in my
    > > eye."
    > >
    > > Do you have a good telescope on it? I have found that I can only get
    > > down to 0.2' accuracy with a 7x35 monocular on my sextant.
    > I think you got the point. I've got a 3.5x38 telescope. Recently I tried
    > a lunar distance with Saturn. The ephemeris give for Saturn a semi
    > diameter of 0.2', that is a whole diameter of 0.4'. But through the
    > scope, I see only a spot. A bright one, but nevertheless a spot. So the
    > magnification is too small to get a 0.4' accuracy.
    > > "Are these [Bowditch] notes available somewhere ? Foreign accounts on Reunion for
    > > 18th century are not well known here..."
    > >
    > > It's in the Boston Public Library in Massachusetts. I have only read
    > > summary accounts, but next time I'm out east, it's on my list of
    > > things to read. I'll let you know if there's more of interest
    > > regarding Reunion.
    > I will be very grateful if you can find informations about this. Also
    > let me know in case these note will be published.
    > Best regards
    > Pierre Brial

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