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    Re: Bubble sextant construction
    From: Hewitt Schlereth
    Date: 2013 Mar 21, 17:25 -0700

    On my Mk IX-A I have found a bubble about twice the size of the sun works 
    well. I keep the bubble in the center of the field and the sun in the center 
    of the bubble. Against GPS locations, intercepts are 2-4 nautical miles.
    
    Hewitt
    
    
    Sent from my iPad
    
    On Mar 21, 2013, at 4:38 PM, "Paul Brewer"  wrote:
    
    > In the case of the RAF Mk IX series of sextants the diaphragm was used to
    > increase the volume of the chamber assembly. There is a reservoir connected
    > to the bubble chamber and eventually this is emptied and air is fed to the
    > chamber. The size of bubble could be controlled by the deflection of
    > the diaphragm. In that type of arrangement there would be essentially no
    > pressure difference from no bubble to full bubble. Some of the American
    > chambers did increase or decrease pressure on a sealed system to change the
    > size of bubble.
    > 
    > As to bubble size, too small a bubble  can make it slow to react. The
    > official guides for the MkIX suggest a bubble size but within a fairly
    > large range. I personally try to match my bubble to my target when I am
    > calibrating but I have little experience with taking sights in anger.
    > 
    > Paul.
    > 
    > On 21 March 2013 14:38, Randall Morrow  wrote:
    > 
    >> ------------------------------
    >> 
    >> Can antone tell me how the older sextants change bubble size? Was it with
    >> application of pressure or by addding-subtracting volume of fluid? Some old
    >> drawing I have seen suggest a bellows for pressure. Also, how much of a
    >> difference in accuracy is gained by matching bubble size to the body
    >> observed? If it is significant, why do the post WWII versions not have
    >> bubble adjustments?
    >> Thanks.
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    > 
    > 
    > 
    > --
    > I am testing Gmail.
    > My Main Email address to which you should reply is
    > 
    > paul---co.uk
    > 
    > I check that one at least daily when I am at home.
    > 
    > 
    > View and reply to this message: http://fer3.com/arc/m2.aspx?i=123024
    > 
    > 
    

       
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