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    Re: Any happy/unhappy Astra IIIB owners here?
    From: Dan Allen
    Date: 2001 Apr 20, 8:02 PM

    The kind of accuracy that these units are spec'd to (12" or 20" of arc)
    is rarely seen in practice on a rolling boat.  Errors of 1-3 miles are
    common and are acceptable for many navigational situations (mid-ocean).
    However, if you want to do almost surveying quality measurements, that's
    where a high power scope and the best quality sextants are needed.
    I continue to test my abilities by using an artificial horizon (the Davis
    in my backyard.  I set the sextant to an even angle, say 30 degrees or
    near to where the sun is about to be.  I think swing the sextant until the
    gets right on the money, without adjusting the sextant.  The minute it is
    I then glance at my GPS or accurate watch and record the time next to the
    which I had already written down.  I take a series of 10 of these
    measurements over
    as many minutes, and then I use Metcalf's nav.c to do the statistical
    analysis of
    my sights.
    I have the artificial horizon for my Astra, but only used it once.  I prefer
    Davis mainly because I can use it with any of my sextants and I don't need
    to keep
    taking on and off the scopes.
    Separately I'll post cnav.c
    -----Original Message-----
    From  Navigation Mailing List
    [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of Yves Arrouye
    Sent: Friday, April 20, 2001 6:41 PM
    Subject: Re: Any happy/unhappy Astra IIIB owners here?
    > I own quite a few metal sextants, and my measurements
    > indicate that Tamaya
    > and Plath are more accurate than the Astra.  I have seen used
    > Tamaya Jupiter
    > sextants sell for about the same as an Astra IIIb and I think
    > I would prefer
    > a Jupiter over an Astra given the choice, but you will be
    > quite happy with a
    > new Astra as well.
    Celestaire's brochure gives 20" of accuracy for the Astra IIIB against 12"
    for the Jupiter. I wish the Astra IIIB had a bronze arc, if that's the
    secret in the reduction of precision.
    I've always wondered: how accurate is a good sextant user? I mean, is 20"
    going to be a limiting factor? That's an error of about 1/3 of a mile
    compared to 1/5 mile for the Tamaya Jupiter, right?
    P.S.: Dan, while reading your thread about your sextant accuracy at
    http://www.i-DEADLINK-com/lists/navigation/9903/0042.html you're mentionning Tom
    Metcalf's nav.c. Where can I find that and the relevant litterature? Do I
    need to get back issues of the journal of the institute for navigation?
    Another question related to what you wrote there: how's the Davis artificial
    horizon compared to the practice bubble horizon sold by Celestaire?

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