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    Re: Bubble sextant construction
    From: Robert Eno
    Date: 2013 Mar 21, 20:16 -0400
    I will stick my neck out and take a stab at this:
    ----- Original Message -----
    From: Randall Morrow
    To: enoid@northwestel.net
    Sent: Thursday, March 21, 2013 5:38 PM
    Subject: [NavList] Bubble sextant construction

    Randall asked: Can antone tell me how the older sextants change bubble size?
    Robert responds:
    Some of them, like the C.Plath bubble attachment for a marine sextant, used a bellows, while others, like the RAE MK IXA used a diaphragm.
    Randall asked: Was it with application of pressure or by addding-subtracting volume of fluid?
    Robert responds:
    I can tell you that this is definately the case for the C.Plath (having fixed a lot of these babies over the years) and I do believe it is the same for the RAE MK IX: pressure was applied or relaxed to push fluid in and out of the bubble chamber.
    Randall asked:
    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?
    Robert responds:
    Yes, as I stated above, I know that C.Plath used a metal (brass) bellows.
    In my experience -- and I have had a good 30 years tinkering with bubble attachments and bubble sextants -- having an adjustable bubble makes a big difference in terms of being able to take an accurate observation. For sun and moon observations, I try to adjust the bubble to the same diameter as these bodies. This does not hold true for stars, which are quite small as compared to the sun and moom, however, I still try to get the bubble as small as possible when observing stars and planets. Keep in mind that this does not work well if the fluid in the bubble chamber is viscous. This is because the bubble becomes sluggish as it is reduced in size. For this reason, I refilled my bubble attachment with hexane which provides for a lively bubble, regardless of the size. The RAE MK IX A, with which I am most familiar, used hexane in the bubble chamber.
    Randall asked: If it is significant, why do the post WWII versions not have bubble adjustments?
    Robert responds:
    Just a slight correction: Post WWII C.Plath bubble attachments had a provision for adjusting the size of the bubble right up until, I believe, the late 1960s after which they replaced it with a bubble attachment with a fixed-size bubble. This also holds true for a number of aircraft sextants such as the Kollsman Periscopic sextant which had a provision for adjusting the size of the bubble.
    As for why this was done, I suggest that it was simply a matter of cost. Adjustable bubble = more moving parts = more cost.  With the advent of electronic navigation systems -- even prior to GPS --  and the subsequent decrease in reliance on astro, the market for these delicate and expensive instruments started to drop. That's progress.
    With all of the collective wisdom and experience on this sight, I am certain that others will be able to chime in and fill in any gaps I have left. Or conversely, tell me that I am talking rubbish.

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