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    Re: Another Davis Instruments Mark 15 Question
    From: JC Sutherland
    Date: 2003 Mar 4, 20:10 -0000

    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
    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
    From my experience with an Ebbco, I would concur with Trevor's
    about plastic sextants. Trevor says " a minute or two", and I would
    normally expect my Ebbco observations to be good within a couple of
    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,
    series of sites, because it's so quick-and-easy to do, and increases
    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
    see the dramatic shifts in index error that Trevor relates. Mind you,
    Sun seldom shines very strongly in UK waters.
    Looking at the construction of my plastic sextant, I doubt that much of
    short-term changes in index error will be due to warping of the frame,
    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
    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
    room for some improvements in the design of plastic sextants in this
    department. To me it appears a simple matter to install a thin
    shield to intercept most or all of the incident light and heat falling
    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
    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 UK, +44 1865 820222)
    or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon, UK.

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