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Alex;

If you get multiple or overlarge bubbles as you describe, Then, when
setting up before relaxing the screw on the  bubble chamber, and while
you are looking through the eyepiece, turn your head plus sextant until
the bubble is sitting over the opening at the 10 oclock position.Slowly
release the screw to suck the vapour back up, until the bubble is the
right size.
  If you have already relaxed the screw then you will have to screw it
up again until just  a hint of another bubble appears. then move the
main bubble up to 10 oclock and unscrew as above.

The size of the bubble will change with time according to how much
tension you have left on the screw. for example if the screw is too
tight while you are observing then the bubble will slowly increase in
size, and vice versa if it is has been released.
It is a matter of trial and error. For best results, get the bubble just
about the same size as the sun and with just enough tension on the
bubble screw to keep it the same size for long enough to get your sights.

By the way These sextants are quite old now and I find that the bubble
can stick just a little to the glass in the chamber, they were designed
to be used in propellor aircraft and on a jet there may not be enough
remaining vibration  to keep the bubble mobile. It is a good idea to add
just a little of your own, the clockwork helps!

  Clive

On 15/03/2012 15:14, Greg Rudzinski wrote:
>
> Alex,
>
> I found in the archives one of your posts on bubble octant trials with
> a MK IXa (NavList 2170). If this bubble octant is radium free then I
> wouldn't worry about using it on a commercial flight here in the U.S..
> It would probably be easier to carry in a padded camera bag than the
> original box.
>
> Your trial results are pretty good. On land anything better than 4 NM
> from GPS is good and in the air (with averager) anything better than 8
> NM is good. See the linked img 1112 for my latest clutch of LOP's
> taken from a stable platform (no averager). All are better than 5 NM
> from GPS with a fairly uniform distribution around the GPS position of
> observation.
>
> Greg Rudzinski
>
> P.S. Aircraft bubble octants won't work on small boats even if using
> an averager. If equipped with a natural horizon then it will work as a
> conventional marine sextant (bubble not used as horizon).
>
>
> [NavList 2170] Bubble sextant trials
> From: Alexandre Eremenko
> Date: 14 Feb 2007 09:19
> Today I made my first observations with my new MkIXa
> bubble sextant (purchased on E-bay for $41).
>
> Position 40d27'.2N, 86d55'.8W Sun altitude, refraction -1.5
> 1. Without the clockwork averager:
> GMT: Reading: Error:
> 16:30:40 32d30' -1.0
> 16:32:05 32d38' 0.0
> 16:33:00 32d42'.5 -0.1
> 16:34:05 32d49' 1.0
> 16:34:50 32d52' 0.3
> 16:35:32 32d54.5 -0.7
> 16:36:30 32d58.5 -1.3
> Average error = -0'.26, sigma=0'.8
>
> 2. Now with the clockwork averager:
> Time at the end of observation: 16:39:10, reading: 22d08'.5.
> Assuming the average GMT=16:38:10 I obtain the error of 0'.7.
>
> Remarks. The manual says to time the beginning and the end
> of an observation. However I do not see how to time the
> beginning with sufficient precision: one needs time to record it,
> then to align the images before you trig the clockwork.
> Instead, I measured the time that clockwork works: it is 1m57sec,
> so in my observations I time the end and then subtract one minute
> to get the average GMT of the observation.
>
> Further remarks. The averager seems useless for observations from
> a stable platform. Besides, it is very heavy, and my hands get tired
> in the two minutes needed for an observation. So Ken's idea to cut
> the averager off seems reasonable for backyard observations.
> I wonder if the averager will help on a small boat.
>
> I noticed that something strange happens with the bubble:
> as I observe, small bubbles come from 10 oclock direction and
> join my main bubble. So it grows:-)
> Do I have to rotate the bubble wheel anticlockwise (relaxing the
> tension
> of the membrane) after the size of the bubble is adjusted?
>
> The observation I posted above was my 3-d trial. In the first two I
> was
> lerning how to keep the bubble and the Sun in a good position, how
> to read the scale and what is the right size of the bubble.
> It seems that smaller bubble gives better results.
>
> Will try some stars if weather permits.
>
> Alex.
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