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    Re: Another Davis Instruments Mark 15 Question
    From: Trevor Kenchington
    Date: 2003 Mar 1, 00:34 -0400

    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
    basis -- and not all of the error can be attributed to the instrument,
    no matter how much I might want to believe that my own contribution is
    flawless. Maybe I am just lucky with the particular instrument that I
    own but I most certainly don't have errors of several minutes of arc
    when using the filters.
    Granted I have not tried taking serious observations across the full arc
    of the instrument but I have across all altitudes that are commonly used
    in navigation (20 to 60 degrees or thereabouts). Maybe I should try some
    stars at 100+ degrees apart to see how accurate the instrument is at
    extreme angles.
    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. Of
    course, my experience is at around 45 degrees of latitude and air
    temperatures of perhaps -10 to +25 Celsius. Leaving the instrument out
    in the tropical sun might have more lasting effects.
    Still, I'd not argue with +/- 5 minutes for a Davis sextant. My own is
    better but perhaps I am lucky. Extremes of quality control are expensive
    and one of the economies that Davis takes is likely to be that some of
    their sextants fall short of the average precision. George, however,
    quoted a figure of +/- 5 _degrees_, not minutes, which would make a
    sextant utterly useless even for low-precision emergency navigation. I
    would be really surprised if any Davis sextant was anywhere near that
    bad, unless seriously abused by its users.
    Trevor Kenchington
    Marvin Sebourn wrote:
    > I would be surprised if a plastic Davis sextant would have an accuracy
    > of +/- 0.5 minutes throughout the arc. [snip]
    > I would much rather believe +/- 5 minutes over the arc is much more
    > realistic than +/- 0.5', and only then with frequent IC checks. The
    > Celestaire catalog says that the accuracy of the Mark 15/25 is
    > "Unpredictable". [snip]
    Trevor J. Kenchington PhD                         Gadus@iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
                         Science Serving the Fisheries

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