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I agree with George in his recommendation.
Some years ago I put a piece of thin metal sheet behind the Horizon
mirror of my Ebbco sextant just as he suggests but not for the reasons
he has given.
The design of the Horizon mirror mounting on the EBBCO sextant is such
that the adjustment of the half mirror is made using two hardened steel
grub screws with serrated cup points that bear on the soft plastic of
the mounting. As a consequence with every adjustment made of 'Side
error' and 'Index error', the adjusting screws cut deeper and deeper
into the plastic! This is of course more aggravated when the sextant is
warm.
The index mirror is mounted in the same way and some improvement could
also be made here. However this mirror is not adjusted so often.
A thin piece of tin carefully flattened, cut and bent to fit between the
screws and the plastic mirror frame will take the pressure of the sharp
screw and improve the life of the sextant.
But do take care! Over tightening the grub screws strains the ball and
socket pivot and excessive force will stretch and eventually break the
thin plastic stem between the ball and the mirror frame.
 If this modification is not possible I would set the adjustments
correct, once and for all and live with the remaining errors. I would
observe their value before and after every run of sights and apply the
mean value of the error as a correction.
If possible NEVER ADJUST THE MIRRORS once they are right first time, and
don't attempt to adjust out the last smidgin of error.
I realize this is slightly of topic as I do not know how the adjustments
are designed on the DAVIS Mk 15 Instrument but if I had one of these in
my hands I would be looking very carefully at this part of the design.

As any plastic instrument flexes while it is in the heat of the sun, I
would attempt to keep the sextant in the shade both while making ready
to take the sights and while observing. Do them in the shade of the Sail
or beneath an awning if this is possible. Also any measure which keeps
the sextant cold or at least at a constant temperature will improve the
reliability of the sights.

I am sorry if this message has been a bit delayed but I have only just
rejoined the group after my server changed my address for me. I had some
difficulty in finding how to do it! Entirely my fault.



From Clive Sutherland
jcs@boltblue.com
Abingdon.
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From my experience with an Ebbco, I would concur with Trevor's
statements
about plastic sextants. Trevor says " a minute or two", and I would
normally expect my Ebbco observations to be good within a couple of
minutes
in calm conditions, though I would allow quite a bit of extra leeway on
that in a critical situation, just in case.

Like Trevor, I check for index error shortly before and after a sight,
or
series of sites, because it's so quick-and-easy to do, and increases
one's
confidence level so greatly.

For daytime sights, it's hard to avoid that situation of "taking
observations with the Sun shining on the sextant", but in my Ebbco I do
not
see the dramatic shifts in index error that Trevor relates. Mind you,
the
Sun seldom shines very strongly in UK waters.

Looking at the construction of my plastic sextant, I doubt that much of
the
short-term changes in index error will be due to warping of the frame,
as
Trevor implies. I think a much more sensitive part of a sextant to local
heating will be the brackets holding the mirrors to the frame, with
their
three adjustment screws. If, before taking a sight, sunlight happened to
fall on one half of such a bracket, with the rest still in shadow, that
would have a powerful leverage on the angle of the mirror. I think
there's
room for some improvements in the design of plastic sextants in this
department. To me it appears a simple matter to install a thin
sheet-metal
shield to intercept most or all of the incident light and heat falling
on
every part of the mirror-mountings (except the mirrors themselves, of
course). Do any instruments incorporate this?

Metal sextants are more immune from these effects than plastic ones,
because the coefficient of expansion of metals is less than that of
plastic, but more important, because the much higher thermal
conductivity
allows temperatures of different parts of the sextant to equalise much
better under conditions of unequal heating.

George Huxtable

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