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    Re: Bubble sextants on e-bay
    From: Douglas Denny
    Date: 2009 Jun 8, 15:25 -0700

    I am grateful for your assistance with this sextant.
    
    I have now opened the main casing and dissasembled the index sector gear and 
    re-adjusted the anti-backlash spring to it's maximum rotation consistent with 
    it not binding as fully wound;  and lengthened the application point of the 
    spring further along the index sector to increase the lever arm of the force 
    applied.
    I am now satisfied the maximum returning force is being appleid by the spring.
    
    I have now checked the sextant against a collimated light source for backlash 
    and have proved to myself the index prism sector arm and worm gear is _not_ 
    the cause of the problem.
    By gently rocking the index prism whilst looking at the (pinhole) light 
    source, one can easily see the slight shift in position of the light (its 
    only a few minutes of arc anyway) and the very positive returning motion to 
    exactly its same position each and every time.  The backlash in the index 
    sector/worm gear is indeed eliminated.
    
    There is still variability however when the light source is approached from 
    the bottom or from the top. It is quite consistent in value (most of the 
    time) when a series of checks is made from one way then the other.  It is 
    still in the order of 20 minutes of arc.
     
    This means there can only be one other source of movement causing the apparent 
    shift of reading and that is movement of the whole worm shaft itself slightly 
    up or down with the force applied by  the worm gear when approaching from one 
    way or the other.
    
    In other words there must be "end-float" of the whole shaft in its mountings 
    with the worm gear pushing it one way then the other.
    
    I shall have to investigate further to see if this can be eliminated by some 
    engineering 'fix'.  If not, then the sextant has a serious fundamental flaw 
    built-in to the design from the very start and indicates very poor design 
    engineering - which is a pity if so as it seems to be a nicely built 
    instrument otherwise.
    
    Douglas Denny.
    
    Chichester.  England.
    
    
    
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