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    Re: Navy MK 5 Octant Using Natural Horizon
    From: Greg Rudzinski
    Date: 2012 May 1, 15:36 -0700


    The index error doesn't have to be odd, even, or a whole number. For bubble aircraft octants there is no need to try for fractions of a minute and since the micrometer drum is scaled in even minutes the index error is best determined to the nearest whole minute. On my octant the true index error is probably somewhere between 5' and 6' moa but 6' moa looks to be the closer whole moa. This 6' moa can be adjusted out but is a difficult bit of business unlike a marine sextant.

    Greg Rudzinski

    MK 5 Octant Using Natural Horizon
    From: Alexandre Eremenko
    Date: 1 May 2012 17:37

    > Spread or scatter has been used here on the list to describe
    > the difference between maximum and minimum values. Not sure what the
    > statistics term is (variance ?).

    I don't think there is a standard term, but "spread" is Ok.
    Variance is different: it is usually defined as
    the square root of the average of the squares of individual deviations.

    > On preset observations the micrometer is set exactly on an
    > even minute of arc then the time is mark when the Sun's lower limb
    > touches the horizon (fingers are off the micrometer).

    This is the usual method recommended with ordinary sextant (when observing
    the Sun). At least Russian manuals recommend it for the Sun.

    One thing which surprises me is that your Index Correction happens
    to be so nearly close to 6' (even minutes!).
    Suppose it were 5' in reality.
    Then you would determine it either as 4' or as 6',
    and your whole series would have a systemanic bias of 1'.

    The difficulty of determining the Index Error with high precision was
    many times discussed on this list.

    Also I suppose that your scope has power 1.5x or something like this.
    Anyway, there are many reasons I could imagine which would lead to
    errors much larger than you have.


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