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Gentlemen,

For many years I have used a Zeiss Artificial Horizon in the bush for
celestial navigation and position fixing. It has a standard three- legged,
adjustable mount and came supplied with a dark, machine-ground glass
reflector, for sun observations. Needing to do star observations I had made
a front-silvered mirror from a piece of float glass. This was obviously not
machined but as the glass was floated when molten, it should have been
reasonably accurate due  to the force of gravity on a liquid. To do star
sights I simply removed the dark glass plate from the frame and inserted the
front silvered mirror. This arrangement has given great service over the
years and I am normally not out by more than 2 nautical miles from known
positions on land (arithmetic calculation errors being the exception).

However, I have often wondered just how flat and accurate is the front
surfaced, glass mirror. I recently took it to an optical engineering
workshop here in Sydney who assessed it as being accurate to 7 wavelength's
of light. They advised that while this was not perfectly level anything
under 10 wavelengths of light is so accurate that it would not add any
meaningful error to that already produced by someone holding a hand held
sextant. It was their belief that even after index error and instrument
error was accounted for the, minute deficiencies in the sextant would still
produce greater error than the 7 wavelengths of light in the horizon. Also
it is clear that the levelling process for the horizon itself i.e. defects
in the bubble levels that I use and operator error eg in reading the bubbles
would produce error.

Does anyone on the list have a comment. What is the error in a ground piece
of glass? The engineering workshop told me they could get it down to
practically zero wavelengths. What is a 7 wavelength's error? Can this be
translated into minutes of arc? Can the error in the horizon mirror be
eliminated through adjustments in the sight reduction process?

Your advice would be appreciated.

Kieran Kelly

   

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