NavList:

Ken wrote:
"Bill thought the
astigmatizer was primarily for bubble sextants, and I, that it was
for marine sextants. As it turns out both of us were partly correct
( I think)."

( I think it might have been the other way around) The astigmatiser did get a one line mention in "The Nautical Sextant." It has a longer history than just in bubble sextants, which is the only place I have seen it. A J Hughes in his "Book of the Sextant" (1st edition, 1915) calls it a "lenticular" and says "This is a cylinder lens fitted in between the index and horizon mirror to intercept the star and draw the image out into a line or band of light parallel to the horizon." This could of course be fitted so that the line would be at right angles to the sextant's frame and thus help in avoiding tilt.

Of the bubble sextants that also have a horizon prism, e.g. Mk IV, A7, AN5851-1 and AN5854-1, the official manuals all say that the astigmatiser can be used with the bubble and with the natural horizon if desired. While it is true that in the AN5854-1 the glasses of the bubble unit are spaced so that the bubble touches both, to give a clear image through the centre, in other sextants in which the light path also passes _through_ the bubble chamber, a clear view cannot be obtained and the user is instructed to align the object alongside the bubble. In those that use the Booth type of bubble horizon, in which, by contrast, a reflection of the bubble at infinity is diverted into the light path, generally a choice can be made as to whether to put the object in the centre or alongside the bubble.

And thank you Ken for the subtle promotion of my book...

Bill Morris
Pukenui
New Zealand
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