NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: JC Sutherland
Date: 2003 Mar 10, 00:41 -0000
I
am sorry this is a bit late. I composed it a week ago in reply to George but I
have only just got back online. I am sending it without modification.
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.
=========================
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
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
==============================================================
contact
George Huxtable by email at george@huxtable.u-net.com
or by phone at 01865 820222 (from outside
