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    Re: sextant index error measurement
    From: George Huxtable
    Date: 2006 Nov 5, 10:00 -0000

    Oh, dear. I AM getting error-prone...
    I wrote, in NavList 1588, about estimating the offset between the two
    views from a sextant, when it's set to zero on the arc.
    | Or you can do it by geometry. For that, you will need the effective
    | distance d between the parallel reflecting surfaces of the two
    | mirrors. You can measure this between the upper part of the horizon
    | glass and the lower part of the index glass, where there will
    | be some overlap between those parallels. If they are front-silvered,
    | that's easy. Otherwise, if you want to be precise, you will have to
    | add two-thirds of the combined thickness of the two glasses; the
    | two-thirds factor allowing for the refractive index of glass.
    | And you need the angle A by which the horizon mirror is tilted from
    | the plane that's at right angles to the collimation line of the
    | telescope. You should be able to estimate this using some sort of
    | protractor (or course plotter). Then the spacing between the
    | sightlines is then d x cos 2A.
    But it isn't, I'm afraid. It's d x sin 2A, if A is defined in the way
    I had suggested. Sorry about that. I thought there was something
    wrong, when comparing the offsets by the two different methods I had
    I have two very different sextants, a plastic Ebbco micrometer-type,
    and a Vernier Heath-clone, and in both the offset angle A is pretty
    near 20 degrees, by a rough visual check using a Douglas protractor;
    perhaps a degree or so less. A better method may be to lay the sextant
    flat down, photograph it from vertically above, and measure off the
    photo with a protractor, but I haven't done that (yet).
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK..
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