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    Re: Calibrating a sextant: with a sextant and a spotting scope
    From: Frank Reed
    Date: 2011 Oct 9, 14:10 -0700

    Bill, you wrote:
    "Routinely, I cannot get repeatablity of index error to better than about 0.3 [minutes]".

    This matches my experience closley. And that is nearly what I wrote in my paper for the "leap second" conference. In my explanation of error limits of celestial navigation, I wrote, "Typical methods for estimating this index correction are accurate to approximately +/-0.25' based on group experiments that I have conducted with experienced students in navigation classes. There are methods for reducing the error in the index correction almost to zero, but they are not widely used."

    There are various methods I have found over the years that can yield IC with a standard deviation smaller than 0.1' but by far the best and simplest is the one I've mentioned in this thread. Just pop out the standard telescope on the sextant and place a spotting scope with a magnification somewhere around 20x (10x to 30x ?) in line with the normal position of the telescope (on a tripod while the sextant is resting on its side on a table). On a recent test, I found this set of IC values, each independent to the extent that the micrometer was turned to some random value before the trial: 0.4, 0.4, 0.4, 0.4, 0.5, 0.4, 0.4. In fact, this methods yields such excellent results that I have decided it's now reasonable to adjust all of my (metal) sextants to zero IE and be done with it.


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