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    Re: Another Davis Instruments Mark 15 Question
    From: George Huxtable
    Date: 2003 Mar 2, 16:43 +0000

    Trevor Kenchington said-
    >I outlined my experience with my Davis Mark 25 in a posting a week or so
    >ago. It does need care, constant checking of index errors, deliberate
    >efforts to equilibrate temperatures and so forth but, given that, I get
    >altitudes within a minute or so of the calculated value on a regular
    >As for leaving the instrument in the sun: If you intend to take
    >observations with the sun shining on the sextant, you had better leave
    >it in the sun before hand or you will find yourself adjusting the
    >horizon mirror nearly as fast as you turn the micrometer knob! To be a
    >bit more explicit: The frame of the sextant warps as it warms and cools
    >but it does not, in my experience, suffer any permanent warping.
    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
    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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