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    Re: Practice Bubble vs. Professional Bubble
    From: Bill B
    Date: 2012 Aug 05, 18:02 -0400

    On 8/5/2012 4:28 PM, David fish wrote:
    > Wondering if folks have an opinion as to whether it's worth spending the
    > $ for the Celestaire professional bubble horizon (vs. practice bubble)
    > for practice ashore.
    I have the Celestaire practice unit.  With some care and averaging you
    can come within 2 or 3 minutes of location. To paraphrase a list
    member's quip from many years ago, "It's like trying to take a sextant
    observation while balancing on a beach ball in a swimming pool."
    Getting a bubble correction number--and observations--are frustrating.
    The bubble, if memory serves, is about 20' top to bottom.  It has no
    limit lines (top or bottom) so the user has to try to center the bubble
    on a line while splitting the sun with the line or kissing the line with
    one limb of the sun with zero magnification. (I have considered kissing
    the line with the upper or lower limb of the bubble and determining
    bubble error that way, but fear the bubble size my change with temperature.)
    It is practically useless after sunset. Note the eyepiece has a slit
    aperture to increase depth of field, which renders the image darker.  I
    have tried to illuminate the bubble through the top portal by LED with
    little success. With a helper and penlight it is useful for obtaining
    rough elevations of a body for star-to-star distances or lunars.
    IMHO it is a useful beginners practice tool for daylight land
    observations without the drawbacks of an liquid artificial horizon.  No
    matter how much care you take, it does not provide adequate feedback for
    refining your technique.
    If you can get past the sticker shock of the pro unit, do consider it.
    Bill B
    > So my thinking is that the dimmer-controlled lighting of the pro bubble
    > might allow me to extend observation time into very late twilight on
    > into early nighttime for star shots.
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