NavList:

I wonder whether any member can help me to find the specification for the USN BuShips Mark II sextant, made in tens of thousands during the Second World War? Some elements of its design seem to me to be perverse and I have recently been seeking explanations.

There are two striking features of difference when the Mark II sextant is compared to most of its contemporaries, particularly German ones and their Japanese followers: the clear aperture of the telescope objective lens is a mere 15 mm, recalling the rather inadequate Galilean telescopes of eighteenth and early nineteeth century sextants, and the mirrors are small to match.
The magnification was a nominal x 3. The aperture of the silvered part of the horizon mirror is only18 x 16 mm or 288 square mm, exactly the same as its Brandis ancestor's, compared to its Carl Plath contemporary,which had a silvered area more than four times as great. There is of course no point in having the light gathering power of large mirrors unless the telescope aperture is large to match.

As navigators use the sun, moon and navigational stars, only one of which has a magnitude greater than 3 (Acamar), light loss through the telescope may seem unimportant. However, accuracy of sights depends quite heavily on getting a clear view of the extended object of the horizon, and in marginal conditions, light grasp is important. While the human eye-brain combination can only just notice a halving in brightness of light, ability to perceive contrast is very much influenced by the relative light levels of the horizon and sky. Why then was the telescope designed as it was (for a lens plan see attachment)?

I suggest that the specification may have called for a telescope giving an erect, wide field image of x3 powers and may not have emphasised brightness of image. A Galilean telescope can easily be designed to give a bright, erect image magnified three times, but the field of view is constrained by the diameter of the objective lens. Unprepared as the USA was for war, it may be that all suitable large diameter objective lenses were reserved for binocular production, though here there were bottlenecks in the production of good quality prisms. In Germany and Japan at the time, Plath and Tamaya were producing sextants having 3 power Galilean telescopes with 40 mm diameter objectives. Even so, the usable field of view as measured by me was only about 5.7°, compared to the 8.6° f.o.v. of the Mark II telescope. Inexperienced navigators sometimes struggle to find and keep in sight heavenly objects, particularly stars, when in any sort of seaway and the wide field of view may well have been regarded as a good trade for some brightness.

The lens plan shown in the attachment, which gives approximate focal lengths and separations, illustrates that there was plenty of scope for light loss, as there were seven lenses, each with two air-glass interfaces at which light could be lost by reflection (about 8 percent per surface) and contrast lost by scattering of light.

In my blog post at http://sextantbook.com/2010/11/30/usn-buships-mark-ii-sextant-some-design-oddities/ I give hints on how to overhaul the telescope and also comment on several other features of the design of the Mark II instrument.

Bill Morris
Pukenui
New Zealand

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