NavList:

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."
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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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