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    Re: Bubble sextants on e-bay
    From: Bill Morris
    Date: 2009 Jun 7, 04:21 -0700

    I think Jean-Philippe and Douglas are talking about different instruments, J-P 
    of the A12 and Douglas of the A10 series.
    
    J-P is correct about the A12. The bubble unit itself can also be adjusted, but 
    if you try, you may regret it, as you may find your instrument's reading 
    swinging wildly from one side to the other. Fine adjustment is not possible 
    by adjusting the bubble unit.
    
    The A10 series adjustment has to start by adjusting the Veeder readout to zero 
    when the direct image of a distant horizon viewed past the left edge of the 
    fixed prism lines up with the indirect image. A grub screw attaches the 
    one-toothed clutch to the shaft of the counter and the latter is provided 
    with a screw-driver slot to make the adjustment easier. If there is side 
    error present, it may be removed by adjusting the fixed prism Its mounting is 
    provided with three Allen grub screws, which are often locked solid by 
    sixty-odd years of corrosion. To get at these screws and to view the direct 
    horizon, you have first to remove the shade that snaps on and off one of the 
    index prism fixing screws.
    
    Next, the bubble must be collimated by adjusting the mirror lens at the bottom 
    of the bubble unit assembly up or down. This needs special wrenches, drawings 
    of which are given in the official manual, but improvisations are possible. 
    At the same time, with the instrument sighted on a distant horizon or a 
    horizontal collimator, the bubble is centred.
    
    Finally, with a distant horizon and applying dip appropriately, or an object 
    of known elevation, or a horizontal collimator, residual index error is 
    removed using the single screw of the fixed prism mounting. This will 
    introduce some side error, which is removed by adjusting one of the pair of 
    other fixed prism screws.
    
    
    Removal of the glasses from the A10 series bubble unit is relatively simple, 
    provided the bottom one is removed first. After removing the bottom locking 
    ring, give the unit a sharp rap on the bench top, and the bottom, plane glass 
    will usually drop out, as there is a generous edge clearance. There is very 
    little clearance for the top, concave, glass and it needs to be gently 
    pressed out from below using a piece of dowel or the end of a pencil. There 
    are several methods of refilling and resealing them, depending on type and 
    how permanent you want the result to be. In any case, the fluid you use 
    should have a refractive index close to that of glass (about 1.5), presumably 
    why xylene and hexane were chosen by American and British makers 
    respectively.
    
    It is some time since I worked on an A12 bubble chamber, but from memory I 
    think they used lead washers. Since they are easy to refill,probably the best 
    way of re-sealing them is to use shellac. If you suspect that a bubble 
    chamber has been sealed with shellac, then you have to heat up the whole 
    chamber on a hot plate or electric iron at between 160 and 180 celsius to 
    melt the shellac, after which it becomes easier to undo the locking rings and 
    extract the glasses!
    
    Seized A10 index prism shafts can be very difficult to deal with. The shaft 
    is, I think, of brass, while the bearing seems to be of white metal, which is 
    an alloy mainly of tin with a little copper and lead added. As the A10 is 
    notorious for seizing, I surmise that the combination is a poor one. Warmth, 
    time and a good penetrating compound should always be used first, as the 
    white metal is a little plastic and it is possible to distort it by heavy 
    hammering and pressing. You are lucky indeed if the shaft just slides out of 
    the bearing when pressed. My experience is that the seized bearing gives way 
    with a loud bang and is then found to be a little proud of where it should 
    be, so that the worm and sector no longer run together properly. The bearing 
    then has to be coaxed back into shape. A functioning instrument can be 
    obtained following this, but you need to be fairly well equipped.
    
    It is essential that the anti-backlash spring is correctly repositioned and 
    the assembled bearing adjusted so as to allow it to work while leaving no end 
    play. There is a trick to this. Hint: use a loop of button thread to wind up 
    the spring before repositioning the index prism shaft.
    
    
    Perhaps I may be allowed a plug for my A10/A10A Restoration Manual, which 
    gives a more detailed account of fixing the instrument in an e-book of 100 
    pages and 104 labelled figures, along the same lines as my Mark IX series 
    Reconditioning Manual. Details may be found in my blog at 
    www.sextantbook.com.
    
    Bill Morris
    Pukenui
    New Zealand
    
    
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