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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


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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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