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  The only bubble sextant that I have that has an opaque bubble is the
A-7 made in 1942 which is an improved version of the Pioneer sextant
from the early '30s, the same kind of sextant used by Fred Noonan. It
has a provision to use the visible horizon and also has an astigmatizer.
Since you can't see through the bubble you must align the sun alongside
the bubble. I sometimes use the astigmatizer which then stretches the
sun out so that it extends out both sides of the bubble but I haven't
found any difference in the accuracy with or without the astigmatizer.
Go to:
https://sites.google.com/site/fredienoonan/topics/pionneer-octant and
click on Air Navigation Manual, H.O. 216 (1941) Page 180 to see
illustrations of the collimation of the body with and without the
astigmatizer.

My other bubble sextants, an A-10A (that I bought from Ken in 1977 and
that is still going strong, I've never had to refill the bubble
chamber), an MA-2 and a half dozen Kollsman periscopic sextants. I
consider the periscopic Kollsmans as the perfection of (or at least the
highest development of) the bubble sextant since they were continued in
use in great numbers by the U.S. Air Force into the 21st century , more
than 50 years after other bubble sextants were abandoned. The Kollsman
instruments (the periscopes and the MA-2) have clear bubbles and no
astigmatizers so it appears that lining the body up in the center of the
bubble has stood the test of time and turned out to be the better design
in actual use.

gl

On 10/8/2010 9:43 AM, Douglas Denny wrote:
>
> Ken Gebhart wrote:
> "As for bubble sextants, as far as I know, astigmatizers are only
> found on bubble sextants that also have prisms that allow them to be
> used as marine sextants as an option. But coincidentally, these same
> bubble sextants also had a bubble that was considered opaque. This
> meant that the bubble only contacted the upper part of the chamber,
> and not the bottom of it. This meant that even though you could see
> through the bubble, the center part of it was not necessarily
> accurate, and it was recommended that the body be put alongside of
> the bubble instead of in its center."
> -----------
>
> I know of no astigmatiser used on any British bubble sextants, and
> have some going back to the 1920's. I suspect they were only used on
> American bubble sextants.
>
> I am not sure what you mean by an 'opaque' bubble; and you seem to
> suggest the bubble can contact the bottom of the chamber. I know of no
> bubble chamber where the bubble can contact the lower surface, and by
> definition the bubble is floating in liquid up against the upper
> surface which is concave by about 8 dioptres.
>
> It is true that it is probably slightly more accurate to adjust the
> star or Sun when taking a sight up against the side of the bubble as a
> tangent to it, as the only other way is to estimate the centre
> position, which is not quite as easy to do. Both ways require an
> estimate to be made and an astigmatiser might help to bisect the
> bubble more accurately, but as it is not generally an option in bubble
> sextants one must assume it has been tried and found wanting.
>
> Douglas Denny.
> Chichester. England.
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