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Alex asked->

George,
>By the way,
>can you post or send me
>a picture of your self-made dipmeter?

I don't have a photo, but have made a rough sketch, nearly life-sized, in
.GIF encoding, which I hope you can decode, at
www.huxtable.u-net.com/ebbco.GIF

>And one more question: which model of plastic sextant
>do you use and what sort of precision you obtain with it,
>say for altiutude sights in perfect conditions?

It's an Ebbco micrometer sextant, made by the East Berks. Boat Co., in
Henley, Oxfordshire, England, about 20 miles from here, but more recently
I've seen it marketed under the French Plastimo label.

They claim that the rack is machined, not moulded, in the polycarbonate
body, and maintains a precision of 1 arc-minute. I've had no reason to
doubt that, but have made no determined attempts to check it. Under good
conditions, I expect to be within about 3', which is all I would expect on
my little 26-footer from even the best sextant. It meets my needs

I've had this one for about 8 years now, since I broke its predecessor,
another Ebbco. Usually, if you drop them, they are so light they just
bounce back, which is a great advantage on a small boat where everything
gets dropped. On that occasion, I lost my balance, dropped it, and trod on
it, which was more than it could resist. Its replacement cost me then about
?50, say $90.

It has a 7-inch radius arc. My sketch omits the two horizon shades, off the
left of the picture..

The Ebbco has weaknesses. There's quite a lot of unwanted stray light gets
into the x3 telescope, but the worst aspect is the laminated shades, which
deteriorate over time. The mounting of the horizon mirror is poorly
designed in my view: worse than in the earlier model. It has three screws,
which act against three stainless springs, but one of those screws isn't
opposite its corresponding spring, so there's a small bending moment on the
glass. Worse, one of the screws presses against the silvering, and damages
it locally. I always check index error before and after measurements, and
it appears to be sufficiently stable.

The sketch shows where the back-facing periscope fits. It's supported on a
plastic bracket which is fixed behind the body of the sextant by a couple
of screws, and is easily removable. It doesn't need precise location. I
have to remove one of the three shades in the index path to provide room to
fit it.

I bought the periscope from an optical dealer for about ?5 ($10, say).
Perhaps it was salvaged from some sort of military sight. The body is
aluminium, anodised black, 40mm wide, with three holes 34mm dia. drilled
through to provide a light-path. It has nice front-silvered mirrors, about
40 x 50 mm.

If you have further questions, just ask.

George.

================================================================
contact George Huxtable by email at george@huxtable.u-net.com, by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
================================================================

   

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