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    Re: Bubble sextants on eBay
    From: Paul Hirose
    Date: 2007 Jan 01, 15:51 -0800

    Alexandre E Eremenko wrote:
    >
    > I was not considering a periscopic sextant yet, because
    > I thought they are too heavy (aren't they supposed to
    > be hanging on some hook in the airplane during an
    > observation?) What is the weight of your periscopic sextant?
    
    Periscopic sextants were designed to install in a mount in the aircraft
    ceiling. Weight is about 6 pounds, I think. That's not bad. I find the
    mental effort of a 2-minute observation worse than the physical effort!
    It requires nonstop concentration and coordination to maintain the
    correct sight picture.
    
    > That's a big problem of course. As I never held one in my
    > hands, I doubt that I can give to a seller good
    > instruction.
    
    The instructions for the AN5851 (also called the A-14) are printed on
    the sextant.
    
    > So AN 5951 does not require any battery for observation,
    > correct?
    > Is the bubble usually illuminated by the day light when
    > available? Or there are models where you have to use
    > electricity even in daylight?
    
    Yes, the AN5851 needs no power for bubble illumination, day or night. I
    have read that the radium paint in the bubble cell can flake off, which
    is why I asked that eBay seller to check it. The half life of radium is
    extremely long, so that's not an issue.
    
    I heard that it's not really bright like modern tritium night lighting,
    so if you don't give your eye a little time to adjust to the dark, you
    might think the radium paint isn't working. However, I've never handled
    one of these sextants.
    
    On some periscopic sextants, the original clockwork averager (mine has
    that type) was replaced with an electronic averager. I don't know what
    power it uses -- maybe 28 volts.
    
    Other than that, I don't know of any bubble sextant which needs electric
    power for hand-held day observations. At night a periscopic sextant
    needs 28 V for bubble illumination. I guess you could connect four 6 V
    lantern batteries in series, but I've never tried that.
    
    The A-12 sextant is much lighter than a periscopic. It was designed to
    be held in the hand. Years ago I had a couple batteries which were
    modified to fit to illuminator, so I have done some star shots with my
    A-12. The bubble illumination system seemed poorly designed. It simply
    lit up the translucent top of the bubble chamber, making the bubble
    visible as a dark ring against an illuminated background. (You view the
    bubble through the transparent bottom of the chamber.) The trouble
    was, this illumination also washed out the stars! Combined with a
    light-polluted sky, it gave me trouble on all but the brightest stars. I
    think side lighting (bubble appears as a bright ring against a dark
    background) would be better. However, the A-12 is the only sextant
    I've ever used at night, so I can't compare it to other designs in this
    respect.
    
    By the way, those modified C cells that did fit my A-12 appeared to be
    ordinary carbon-zinc cells with the protective jacket removed. That is,
    the zinc "can" was exposed. But nobody seems to sell plain old
    carbon-zinc batteries anymore; at least, none of the places I go. It's
    all alkalines. I've tried to cut those down without success.
    
    For daytime shots the A-12 is nice. It's comfortable to hold. There are
    only two different shades (compared to eight on the periscopic), but I
    don't mind. One real disadvantage is the bubble size is fixed. If the
    cell loses a little fluid as the years pass, you can't adjust for that.
    
    Note that the A-12 is a naked eye instrument. Depending on your
    eyesight, you may prefer the magnifying optics with focus adjustment on
    the AN5851 and periscopic sextants. On the other hand, sharp vision is
    not vital with a bubble sextant since the celestial body and bubble are
    constantly wobbling around. Eyesight is far less important than
    coordination.
    
    Robert Eno wrote:
    >
    > The late John Luykx, who was a regular contributor to the Navigator's
    > Newsletter and one of the Navigation Foundation's directors, ran a
    > company called "Infocenter" and specialized in bubble sextant sales. I
    > do not know if this firm has been carried on by John's partner but I can
    > try to find out. I do know that he had several different types of
    > aircraft sextants for sale, all of which were overhauled and refurbished.
    
    Their site is still up, anyway:
    http://www.paxp.com/mac/navtrak/index.html
    
    A couple months ago I requested a catalog by email. It didn't bounce,
    but nor was there any reply.
    
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