NavList:

on 8/21/06 1:55, Geoffrey Kolbe at g.kolbe@btconnect.com wrote:

>
> I wonder if this is the cause of the problem many people have getting
> good altitudes with bubble sextants.
>
> As George and Robert both agree, the optics in a bubble sextant are
> arranged such that the image of the celestial body is superimposed on
> the image of a bubble in a bubble chamber attached to the sextant.
>
> Most aircraft type bubble sextants have spherical bubble chambers
> fashioned such that the sextant is tilt invariant to a certain
> extent. If the sextant is tilted slightly in any direction, the
> bubble follows the image of the celestial body.
>
> In such sextants, the bubble is round. The object is to center the
> celestial object in the middle of the round bubble.
>
> So, the question is, how big should the bubble be? The intuitive
> answer is - just slightly larger than the celestial object of interest.
>
> The bubble size can be altered by adjusting the amount of fluid in
> the bubble chamber. A number of sextants have bubble chambers which
> allow the bubble size to be externally adjusted. Sextants such as my
> Link A-12 come with two bubbles, one for sun (or moon) sightings and
> the other for star sightings.
>
> However, the sight picture of centering the celestial object inside a
> round ring is very similar to a target shooter centering the round
> aiming mark of a target inside the ring of the foresight.
>
> Do experienced target shooters choose the diameter of their foresight
> rings so that there is just a small annulus, "a couple of hairs",
> between the outside of the aiming mark and the inside of the
> foresight ring? Interestingly enough, no. All target shooters go
> through this phase, but in the end the overwhelming experience of
> target shooters is that it is better to have a large foresight ring
> so that there is a wide and generous annulus between the aiming mark
> and the foresight ring.
>
> Given the similarity between the sight picture in a bubble sextant
> and that seen by the target shooter, I think it is reasonable to
> carry over this experience and say that better results will be
> obtained with a fairly large bubble. The bubble in my sextant is
> about a degree in diameter, so the sun is about half the diameter of
> the bubble. I find this to be fine for star sightings as well.
>
> I recommend you try a larger bubble size, you may be surprised at the
> improvement in your altitude sights.
>
> Geoffrey Kolbe
>
>
>> George wrote:
>>
>> When observing the Sun, as I understand
>>> it, the aim is to match the disc of the Sun with the disc image of the
>>> bubble, and try to keep it there.
>>
>> Robert responds:
>>
>> That is pretty much the case, although it is better to have a bubble which
>> is slightly larger than the disc of the sun. Not too much larger but just a
>> couple of "hairs".
>
>
> >
Geoffrey,

That is an interesting analogy about the target shooter.  I have always
believed that "seaman's" eye has been underrated.  All the technical manuals
for the aviation sextants seem to agree that the bubble should be twice the
size of the sun.  As was pointed out, a too big bubble is hard to find the
center of.  A too small bubble loses mobility due to the friction of the
bubble against the top lens of bubble chamber. But why even talk about this
when we are on land? As has been mentioned before, if the sextant is propped
up, and the star is successively positioned against the top and bottom of
the bubble, and the average of the two readings is taken, you have an
extremely accurate observation. Seaman's eye is not needed.  An example is
using an AN 5851-1 (Navy Mk V, or A-14), repeatability of readings is about
.3 minutes. Conversely, when held in the hands, repeatability is 2 to 4
minutes.

Ken


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