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    Re: Introduction
    From: Robert Eno
    Date: 2008 May 3, 11:15 -0400

    Welcome aboard Bruce. I will weigh in on a few things even  though the bar
    is likely closed for the evening.
    
    On choosing a sextant, to be frank, avoid aircraft sextants unless you
    intend to become a collector. I find myself biting my tongue saying that
    because my first sextant was an RAE Mark IX-A and is still in perfect
    working order after 25 years. I am also a collector of aircraft sextants so
    they are very near and dear to my heart. I consider them to be amazing
    pieces of technology and innovation. Nevertheless as you have probably
    gathered from the information passed on to you by our more erudite members,
    aircraft sextants are finicky, cannot be easily repaired and cannot be
    calibrated without special equipment, which in itself is nearly impossible
    to find. To put this in more vernacular terms: aircraft sextants can be a
    supreme pain in the arse.
    
    My advice is to bite the bullet and spend some $$$ on a good marine sextant.
    They are easily repairable, parts can be easily replaced and you can
    calibrate it anytime, anywhere and without the need for special equipment.
    Barring a fall to the steel deck, they will last forever. There are a lot of
    good models out there. I won't go much beyond that as I am an acknowledged
    sextant snob and prefer German models such as C.Plath (no longer in
    existence) and Cassens and Plath (the last German holdout). Nevertheless, I
    keep hearing very good things about the Chinese sextants (Astra) that are
    being sold by Celestaire.
    
    With respect to bubble horizons at sea: forget about it. It will be an
    extremely rare occasion when you can pull off observations with any degree
    of accuracy with a bubble horizon. I know this from experience.
    
    With respect to theodolites, forget about them too. While they are extremely
    accurate, you will never be able to take observations from a ship as
    theodolites require a rock-solid platform from which to operate. They are
    great on land, however, if your intention is to use it to fix your position
    while on a trek, keep in mind that they are heavy, cumbersome and require a
    tripod.
    
    A few back and forth comments were made in this thread with respect to the
    notion of leaving the (marine) sextant outside for 20 minutes or so to allow
    for the metal to adjust to the ambient temperature. In my experience, this
    is good advice. This is especially so when it comes to extreme cold weather.
    Even in moderately cold weather, I have found that the IE will change over a
    period of time if you do not allow the sextant sufficient time to adjust to
    the ambient temperature. The same goes for hot weather when you take your
    sextant out of a relatively cool, dark box, and into the sunlight and
    warmth. Others may disagree with this and may even have formulaic data to
    prove it, however, I can only go on my own personal experience with these
    things.
    
    Just a few observations from the cheap seats.
    
    Robert
    
    
    ----- Original Message -----
    From: "Bruce" 
    To: 
    Sent: Wednesday, April 30, 2008 10:27 PM
    Subject: [NavList 4956] Introduction
    
    
    
    >
    > Anyway, that is my rambling introduction, but I have a few questions for
    > the list.
    >
    > 1)Sextants are expensive precise instruments, but they are still made of
    > brass and aluminum. Why does no one use invar to make them more
    > resistant to thermal expansion?
    >
    > 2) It is supposed to be possible to get exact GMT by observing the moons
    > of Jupiter, but I have yet to find any tables that have more than a
    > minute accuracy.  Has anyone else tried and had better luck?
    >
    > 3) I have looked, and bid on, several a-12 air sextants on e-bay. Has
    > anyone ever had a good result with an e-bay A-12?  Are they fairly
    > rugged? I hate buying a pig in a poke, but celestaire is not cheap (like
    > me).
    >
    > 4)I have heard that it is possible to modify am A-12 to see the horizon
    > by inserting a prism somewhere. Has anyone done it, and can it be done
    > so that the bubble can still be used.
    >
    > Here is a very old Norie's online.
    > http://books.google.com/books?id=OzwEAAAAQAAJ&printsec=frontcover&dq=norie%27s+nautical&lr=&as_brr=1
    >
    > Thank You
    >
    > Bruce Hamilton
    > Vancouver, BC, Canada
    >
    > bruce.hamilton    (antispam you know what goes here) shaw.ca
    >
    >
    >
    > >
    >
    
    
    
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