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Subject: [Nml] Davis Sextants
captainmike7@XXX.XXX
Date: Sat Jan 23 1999 - 15:45:44 EST
At 13:26 1/23/99 EST, Aliscafi@XXX.XXX wrote in part:
>I have taught for many years using the Davis Mark 3 for my students, and I
can
>say with absolute confidence that it would if necessary take you safely
around
>the world with considerable accuracy (say, 2 parts in 1000 n.m). It lacks
>optics (hence, won't gather light from stars or planets very well), a
>micrometer drum (hence, must rely on a vernier scale---as did generations of
>R.N. navigators), and it's plastic (hence, needs to be protected from heat
and
>being sat on). But, pound for pound (or is it Euro for Euro, now?), dollar
>for dollar, it's a great buy---about $40 U.S.
I agree about the Davis sextants. I have used a Mark 25 for years. It
does have a micrometer drum (0.2 minute of arc) and is very accurate.
Taking sights from an exact position and H.E. (e.g. the end of a pier)
often results in intercepts of "0" or "1" nm. Mirrors are adjustable for
reducing perpendicularity and parallelism errors, and it has an LED for
reading the arc and drum when it's dark. The optional horizon prism is
excellent for assuring verticality, eliminating the need for "swinging".
The only real complaint I've had is with the "full-field dielectric beam
converger" horizon glass. It is very easy to kiss the horizon with the
body in full light (especially sun sights) and I think it's the best mirror
under optimum light conditions. But as it gets dark, or if the horizon is
otherwise indistinct, it can be very difficult to see the horizon through
the glass. At such times I would prefer a half-silvered horizon mirror.
Protection from heat is important (as with any delicate instrument, plastic
or not). Tania Aebei began her circumnavigation with a Davis as her only
sextant - and learned the hard way the effects of leaving it sitting in the
sun.
I bought my Mark 25 new in the early '80s for 79 USD and it's the only
sextant I have ever owned. Inflation and, I suppose, lack of demand have
taken their toll - but considering the features and accuracy, it's still a
bargain at today's $189 USD.
P/Lt/C Michael A. LeButt, FC
Balboa (Newport Beach, CA) Squadron
"A ship in harbor is safe,
but that's not what ships are for..."
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