Welcome to the NavList Message Boards.


A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Add Images & Files
    Bubble sextant test results
    From: Paul Hirose
    Date: 2003 Jan 2, 13:22 -0800

    Over the past few days I've made about 30 sun observations with an
    early 1940s Link A-12 bubble sextant. Comparing my results with the
    USNO online sight reduction page, standard deviation was 1.8 minutes.
    The instrument appears to have about +12 minutes index error.
    Cosmetically it's in beautiful condition, so I don't know how it got
    that far off. But at least the error seems consistent.
    I just took single shots, although the A-12's averager is in working
    condition. It's a simple affair which marks a pencil line on the
    plastic altitude setting drum (which is of large diameter) each time
    you press a thumb trigger. At the end of the observation period you
    set the knob so the marks are evenly distributed either side of the
    pencil lead, then read the altitude.
    Of course you must use the mid time of the observation period to go
    with that average altitude. The bother of doing that is the main
    reason I didn't use the averager. I don't think it would have improved
    my results much anyway.
    The bubble on my A-12 seems a little big, about three Sun diameters. I
    had the feeling a smaller bubble would have made centering easier.
    Along with my A-12 observations, I also took about 20 with a Kollsman
    periscopic bubble sextant I recently bought on eBay. Standard
    deviation for these was 1.6 minutes. One bad shot 5.2 minutes off
    spoiled my numbers. May have been a blunder reading the altitude.
    Index error is -1.6 minutes based on all the observations.
    The Kollsman is rather heavy at 7.4 pounds, and was designed to be
    installed in a mount in the aircraft ceiling. However, with upturned
    palms at about chin level you can support the sextant while
    manuipulating its controls with reasonable ease.
    Field of view through the 2x telescope is generous. In fact it has an
    actual field of view much wider than the non-magnifying A-12. Stars
    look pin sharp in it.
    The clockwork averager on the Kollsman is quite a box of tricks. It
    continuously integrates the altitude for up to two minutes, after
    which a shutter blocks the telescope. Unlike earlier fixed-time
    averagers, it can be stopped manually too. The day my Kollsman arrived
    the sun was going in and out of clouds, and several of my practice
    runs had to stopped early that way. A fixed-time averager would have
    been frustrating in these conditions.
    One delightful feature is a dial on the averager which shows half the
    duration of the averaging run. That makes it easy to determine the mid
    time of the observation period: you simply deduct the "half time" from
    your watch reading at the end of the observation.
    Using an automatic averager is quite a novel experience, so much that
    I haven't tried the Kollsman with single shots yet.
    At first I found it tedious to hold on target for the full two
    minutes. But after some practice I can now do three observations in a
    row without trouble. The sustained concentration maintaining a good
    sight picture is a much a factor as the physical effort of supporting
    the sextant.
    Though I'm well short of his skill, I agree with Dr. Kolbe that visual
    acuity has little to do with operating a bubble sextant. It's more a
    matter of learning the moves to simultaneously steer the bubble and
    the body, and developing the eye to judge that two independently
    wobbling objects are in good average alignment. My results should be
    typical of what a beginner can expect, hand-holding the instrument on
    I'm keen to try the Kollsman on stars but it needs 28 V (AC or DC) to
    run the bubble illuminator. Maybe I'll wire up a couple 12 V lantern
    batteries in series. Anyone know what its bubble looks like at night?
    The A-12 bubble has a poor night lighting arrangement in my opinion.
    You see the it as a dark circle against a lighted background, so in a
    light polluted sky the stars can be hard to see.
    But either sextant works well for Sun shots. Physically the A-12 is
    much lighter and was well designed for use in the hand. The Kollsman
    is easier to read (no vernier), has variable bubble size, and probably
    more accuracy potential. And it definitely has a high "coolness
    coefficient" in use, the averager ticking loudly and ominously like a
    time bomb as you track the body.
    The same guy in Canada who sold me the Kollsman periscopic sextant has
    another one up on eBay now, and if it's in as good a condition as mine
    the price is right. I also saw two British Mark IXs up for auction.
    One thing I've noticed though - eBay bubble sextant sellers are almost
    always utterly clueless about these instruments so you must ask
    questions early, thoroughly explain what you want looked at, and avoid
    any technical words.
    I've bought 20+ items on eBay and haven't been burned yet. A couple
    were below expectations, but they didn't cost much either.
    Bubble sextants are a lot of fun since they let you do real celestial
    at home without the inconvenience and limitations of a separate
    artificial horizon. Just grab watch and sextant, go out in your yard,
    and shoot the sun!

    Browse Files

    Drop Files


    What is NavList?

    Join NavList

    (please, no nicknames or handles)
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.

    Email Settings

    Posting Code:

    Custom Index

    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site