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    Fairchild A10 bubble sextant.
    From: Douglas Denny
    Date: 2009 Jun 13, 09:17 -0700

    Having been on a steep learning curve with the restoration of this sextant I 
    have to confess to having been barking up the wrong tree with the varibility 
    I found in readings of up to 25 minutes of arc.  Though it makes me feel 
    foolish to admit publicly , it is of importance to others to know this is 
    possible,  so report it here.
    The one factor which I assumed was an impeccable reference - the bubble - is 
    the cause of the variability.
    This was proven in calibrating the sextant after dismantling completely, 
    repairing, cleaning, oiling and re-assembly.  The end-float in the worm 
    assembly which blamed, which in fact was only slight, was removed completely 
    with a shim washer and thus all engineering possibilities of the variability 
    were eliminated.
    I am now bemused at not thinking of the obvious all along - the bubble itself 
    is sticking slightly,  which is surprising as it appears to be very lively 
    indeed, and much moreso even than my utterly reliable MkIX sextant which has 
    a telscope magnifying the bubble too.
    This sticking could be seen when calibrating, for when the sextant is tilted 
    forwards and the collimating target examined, one reading is obtained; and if 
    the sextant is then rocked backwards and the target examined again there is a 
    slightly different reading.  Tilt the sextant forwards again and a similar 
    reading (but not necessarily the same)to the first is obtained.
    The bubble size I was using on this sextant was most probably too small. It is 
    a magnified image of the bubble one sees, so quite a small bubble fills the 
    viewing circle aperture making you think it is an adequate size. I was using 
    a bubble diameter about one sixth of the diameter of the aperture, or perhaps 
    a bit less. (i.e. one could use six side by side across the diameter of view.
    The Hughes IX sextant has 'tramlines' in the centre and recommends a bubble 
    slightly less than these which is about a quarter of the aperture.
    Increasing the bubble diameter in the A10 to around a quarter of the view 
    circle has improved matters considerably.  Averaged over four minutes, and 
    with care, sun shots now result in position lines within a couple of nautical 
    miles or so. (where they should be!).
    The down side is of course, as the bubbe is increased, assessing the centre is 
    more difficult and allows for more variability in this.  A compromise is 
    required.  I find a quarter of the view diameter is about optimum, which with 
    the A10 means a physical bubble size of about 4.5 mm.
    When the bubble chamber glass was being cleaned I noted the power of the minus 
    (concave) lens it produces in air was 6.5 dioptres.  With crown glass of 
    index 1.52  this means the radius of curvature of the concave surface of 
    about 52 mm. (a fairly steep curved surface).
    It has been an intersting experience restoring this sextant as I have learned 
    quite a lot about potential error related problems involved, both engineering 
    and physical with the bubble.  I have to say I am annoyed with myself for not 
    even thinking of the obvious straight away, but one expects the bubble 
    reference to be an absolute, and utterly reliable - but not so if it is too 
    small.  That is the main lesson I have learned.
    Douglas Denny.
    Chichester. England.
    Navigation List archive: www.fer3.com/arc
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