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    Bubble size and accuracy
    From: Tom Hope
    Date: 2021 Jul 29, 04:14 -0700

    Hi everyone

    New to this - have recently aquired a Mk IX bubble sextant manufactured in 1940; it seems to have been repaired at some point in the not too distant past as everything seems to function as it should (within reason). The only obvious fault I've found with the unit is that the bubble size cannot be reduced beyond a certain level. I can increase the size of the bubble and by positioning it in the 10 o'Clock position, I can then remove the excess and reduce the bubble back down to its' current size - but it'll never go smaller and/or dissappear. The edges (or circumpherence, if you like) of the bubble - when positioned in the centre of the viewfinder - just touches both of the two hairlines as I've tried to show in the attached diagram. I've read a number of different articles and posts from people suggesting bubble sizes which work the best; what are peoples' experiences of this? I've taken both single and multiple shots of the sun, positioning it right in the middle of the bubble - but as I'm still learning this, I'm not sure if my inaccuracy is due to the bubble being too large, calibration of the instrument or perhaps just practice? Can I expect to be wildly out with a bubble this size even if everything else is working OK?

    Would welcome your thoughts!

    Tom

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