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    Re: Practice Bubble Horizon
    From: Bill B
    Date: 2014 Mar 02, 14:30 -0500

    On 2/28/2014 6:26 PM, Alan S wrote:
    > Re the Practice BH, while I suppose they work, in daylight anyhow,it
    > takes a steadier hand than I have. Also,as I mentioned, I never could
    > understand the BC Factor, perhaps I'm dense.
    First a correction. Brackets on the reticle would not help much with the
    bubble as the reticle lines do not show up on the bubble. The limit
    lines would have to be on the spirit level, or perhaps the mirror that
    reflects the level.
    Regarding the bubble error/correction or SIC (Sextant Index Correction
    as Celestaire calls it):
    As an analogy, imagine you purchased a 4 ft plastic carpenter's level at
    you local big-box store. You bring it home, use a magnifying glass to
    center the spirit level and some ingenuity to secure the level int that
    position. Then you check level with a section of water-filled clear
    tubing.  You find it is a 1/16" off of perpendicular (no island gravity
    mound). You can compensate for that in use, or figure, "What the heck,
    it's a carpenter's level so 4.5 minutes of an arc doesn't really mater
    when you have seen 8' walls 1" out of plumb. Clearly that would be too
    much error for a sextant.
    To obtain bubble error I first doubled checked IE using tangent images
    of the sun's limbs. Then I did a LAN observation with an AH to use as a
    sanity check/baseline. I also establish a true south a range. For
    several days following that I tripod mounted the sextant using
    photographic grip hardware and pointed the scope true south. Then I did
    my best to center the bubble, lock it down, and do LAN observations
    (time and Ho precalculated). (I found it easier and more repeatable for
    me to bisect the sun with the reticle line rather than bringing a limb
    tangent to the line, but that's a personal preference.)
    Once you reduce the bubble LAN observations (no dip correction) and
    compare the Ho to Hc, you obtain an error that includes any index error.
    Back out the IE and you have your bubble error. That's my story and I'm
    sticking to it ;-)
    Note: I did several observations and averaged them as precisely
    centering the bubble from day to day was not a given for this operator,
    even with a tripod. In some situations lighting may cause the bubble to
    have a highlight on one long edge and a shadow on the other producing
    fluting. This optical illusion further complicates matters.
    I will follow up with questions I have regarding determining this units
    bubble error with a natural horizon.
    Hope that helps

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