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    Re: Problem shooting index error
    From: Tony Oz
    Date: 2019 Oct 11, 05:38 -0700

    Dear Alan,

    The index error is the last to be addressed. You should work in certain order:

    • make sure the index mirror is perpendicular to the sextant frame
    • reduce the so-called side-error
    • with the above adjustments done - correct the index error

    The last step will possibly require to repeat the side-error adjustment.

    The most accurate way to measure the index error by observing the Sun:

    • hold the sextant so its' frame is horizontal
    • put on the proper filters
    • set the index arm at 0°
    • aim the sextant at the Sun, you should see two overlapping images of the Sun
    • turn the drum to decrease the values ("off scale") until the images separate a little, now turning the drum in the increasing direction bring the images to the touching position
    • read the scale and drum. Values to the right on the "0" considered negative, write it down as 359° (or what you actually get), read the drum at the face value and write the minutes part of the measured angle
    • now turn the drum all the way up to the point when the overlapping images just touch on their other sides
    • the readings are now "on scale", write them as 360° + minutes off the drum

    The index error is now calculated thus:

    I.E. = 360° - (onscalevalue + offscalevalue) / 2

    An easy check for the sanity of result - with the same values (the off-scale and the on-scale) do:

    SDSun = (onscalevalue - offscalevalue) / 4

    ...and compare the result with the Sun's semidiameter in NA.

    I'll be happy to help you more if needed.

    The higher the scope magnification - the easier and more accurate are the measurements.

    Warm regards,
    Tony

    60°N 30°E

       
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