NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Bubble sextant construction
From: Robert Eno
Date: 2013 Mar 21, 20:16 -0400
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?
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.
Robert
