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    Re: [Fwd: [Fwd: Davis Sextants]]
    From: John Brenneise
    Date: 1999 Jan 25, 21:21 EST

    I suspect that some of my error comes from ripples in the dish of oil.
    Also,
    I suspect that my location, just SW of San Jose requires some
    differentiation
    with respect to refraction.   Also, my shots that I took in the vicinity
    of noon
    had to be done with the practice bubble, since the double angle
    technique of
    using the plate of oil outstrips the number of degrees on the arc of any
    of the
    sextants that I've had (including a MK3 which I gave to my nephew, hence
    I can't
    do the analysis on it...).  Not only is the practice bubble not
    advertised to be all
    that great, but I mixed some virtual apples with my virtual oranges by
    combining
    sights taken with the two differing methods.
    What I need to do, evidently, is find a fairly smog-free location with a
    real horizon
    to practice in...
    > -----Original Message-----
    > From:	Bob Trenkamp [SMTP:trenkamp{at}XXX.XXX]
    > Sent:	Monday, January 25, 1999 6:00 PM
    > To:	John Brenneise
    > Cc:	navigation{at}XXX.XXX
    > Subject:	RE: [Fwd: [Fwd: [Nml] Davis Sextants]]
    >
    > At 05:08 PM 1/25/99 -0800, John Brenneise wrote:
    > >Has anyone subjected any of the sextants to which this thread of
    > >discussion refers to a test of  thermal expansion issues?
    >
    > Yep - when you leave a MK-3 on the coachroof for an hour at midday,
    > you
    > have to recompensate for index error.
    >
    >
    > >Have any other quantitative measurements been made?
    >
    > #1: experiments with a binary result - it gets me where I've been
    > trying to
    > go.
    > #2: experiments with a somewhat continuous result - from a
    > fairly-stable
    > deck just south of Long Island, I seem to have gotten a quasi-normal
    > distribution of error (wrt the Loran readings) from LOPs, with the
    > mean at
    > 1.2 miles, std dev = .3 mi. (I just checked the log on this & did a
    > quickie
    > on excel, but I'm not about to go back through my log and crunch all
    > the
    > data. My sense is that this particular experiment yielded somewhat
    > typical
    > results.
    > #3: experiments in the real world offshore - I just checked a few old
    > plotting sheets and it seems that I generally get a cocked hat with a
    > max
    > dimension of 2-3 miles offshore. The LOPs used to construct the cocked
    > hat
    > are usually a mixture of advanced LOPs and current observations,
    > though
    > occasionally I can get 2 stars to mix with the moon.
    >
    > >There also is a practical issue of the mass of the sextant when
    > >trying to take a sight from the deck of a pitching and rolling boat.
    >
    > Amen.
    >
    >
    > >In practice, what precision do the MK15 et. al. deliver?
    > >With my Astra IIIB and an artificial horizon consisting of
    > >plate of oil placed upon the south facing deck (of my
    > >condominium), I was able to get a fix that landed within
    > >four nautical miles of that reported by my hand held GPS
    >
    > Wow, John...have you considered changing your oil...or perhaps your
    > GPS
    > batteries? (grin)  Last time I used an Astra III, it beat my MK-3 by a
    > long
    > shot. It's weight made all the difference in trying to get good shots
    > from
    > a 38' sailboat. At the same time, I'd not be willing to part with the
    > extra
    > money to get that improvement. (Ritual and reverence are two separate
    > concepts, please.)
    >
    > Regards,
    >
    > bob
    >
    >
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