Making a sextant monocular mounting

28 01 2009

In the Yaho sextants group recently, Mike Bowman of Darwin Australia asked about the availability of a prismatic monocular for his Freiberger Trommelsextant and Federico Rossi, writing from Italy, wondered about the possibility of cutting a pair of binoculars in half and mounting a half on a sextant. Had anyone experience of this?

Until this afternoon, I would have answered in the negative for myself, but wanting a break from writing my new manual on restoring the A10 and A 10A bubble sextants, I retired to the workshop for a few hours to see if I could prove a concept I had in my mind. It would be very difficult to retrofit a mounting like the one shown in the next picture, in which a huge 7 x 50 monocular is mounted on a 1957 Tamaya sextant as original equipment

Tamaya 7 x 50 prismatic monocular

Tamaya 7 x 50 prismatic monocular

The mounting of the very old 10 x 30 monocular shown in the next picture gave me the germ of an idea that started me rooting around in one of my many treasure chests for a suitable piece of aluminium alloy plate.

Ancient prismatic monocular

Ancient prismatic monocular

In this ‘scope, the monocular backplate is continued as a fork that has a vee machined on one side. It was probably originally mounted on a top-of-the-range Heath and Co sextant, made around the beginning of the twentieth century, and there is no room for the fore-part of the monocular in a modern sextant, so I decided to use the objective lens mounting to hold a vee and flat type of mounting, to match the Tamaya shown above, as well as nearly all modern sextants.

I had only scraps of 4 mm plate in a suitable material, so screwed two pieces together to save having to convert a 12 mm hunk of material mostly to metal chips. I won’t bore you with all the steps, but the mounting started out as in the following picture.

Starting point

Starting point

After about three hours of machining, sawing and filing, it ended up like this:

Finished article

Finished article

The 4 mm plate is counterbored to a depth of 3 mm to accommodate the outside diameter of the objective lens mount while the threaded part of the lens mount passes through the smaller diameter hole and screws into its rightful place in the monocular, with an increase in optical path length of only 1 mm.  The next picture shows it in place.

Mounting in place

Mounting in place

It works well, but the weak point of the system, apart from the ease with which the threads of the objective lens mount can get crossed, is that when you halve most binoculars, you also have to halve the focussing arrangment. You can find binoculars with individual focussing, but these tend to be vintage WW II US Navy ones, worth almost as much as a second hand sextant of the same vintage. I haven’t for the moment any suggestions, but you can always let me have your ideas in the comments section; and you could encourage me by buying my book, The Naked Nautical Sextant and its Intimate Anatomy.

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One response

20 11 2013
K E Froeschner

I found a 7×50 binocular with individual focus eyepieces made by ‘Astra’ on e-bay for about $25. Mounted it on my Astra III sextant, in about the way you did, but I split the ring so I did not have to disassemble the objectives from the body. The split is closed around the (now) monocular, with 2-56 SHCS and is very inconspicuous. Beautiful result. And easy to do.

Just found another, made by Wuest, which I hope to get for my Tamaya T Series Spica and Jupiter.

Gorgeous web-site.

K E Froeschner

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