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    Re: Horizons and levels
    From: Ken Gebhart
    Date: 2003 Jul 15, 20:42 -0500

    on 7/15/03 2:03 PM, Jared Sherman at jared.sherman@verizon.net wrote:
    > Is Ken G. from Celestaire with us right now?
    > I ask because they have http://www.celestaire.com/catalog/products/0511.html a
    > "practice bubble level" attachment for $49 which is only stated to give a
    > final observation within 2 minutes of accuracy.
    > That would set "good enough" (or not) and cheap enough (or not) at $49 for
    > a compact device that will allow 2 minutes of accuracy.
    > Better models with more respected names seem to range from $700 to
    > $1800...with no accuracy mentioned.
    Yes, I am here.  We say that the Practice Bubble has an accuracy of 2 min
    under ideal conditions. The biggest problem for bubble horizons is for the
    user to hold the sextant still.  If the sextant is propped up, in a vise for
    example,  and lighting conditions are good, this is an ideal situation.
    Under normal handheld operation the Practice Bubble will give repeatability
    to about 5 min. of arc.
    We have a lot of fun with this at boat shows (where we have access to the
    sun).  We put the actual (GPS) position into a Celesticomp V.  We let
    visitors take a shot of the sun, put the sextant reading into the
    Celesticomp, and get an immediate intercept.  It is usually 2, 5, or even 8
    miles.  The whole thing takes about two minutes of time, and people are
    thrilled that they have taken an actual sight, and been so close.
    FWIW, before we got a collimator, we used to calibrate the Navy Mark V
    aircraft sextant by propping it up, and sighting on polaris.  We would bring
    it down until it just winked on and off at the top of the bubble, and did
    the same at the bottom of the bubble.  We averaged the readings, and got
    repeatability of 1/4 min of arc.  I haven?t tried this with a C. Plath or
    Cassens & Plath bubble on a marine sextant, but I expect the results would
    be similar.  Perhaps Robert Eno can add to this.
    Ken Gebhart

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