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    Re: Reliable Index Correction to a Tenth Minute of Arc
    From: Frank Reed
    Date: 2010 Feb 17, 18:05 -0800

    Antoine, you wrote:
    "While we are at sextant index corrections, I will be most interested at reading your method."

    Ok. I was going to cover this along with a few other tricks during the Navigation Weekend at Mystic Seaport in June, and after that on NavList, but I don't want to keep you all waiting, especially for something as simple as this.

    Place your sextant on its side on a table either outside or looking out a window where you can see some well-defined thin vertical line in the distance, like a radio tower or a skyscraper miles away. Remove the sextant's telescope. Next get a spotting scope with a magnification of 25x or better. Place it on the table in line with the normal position of the telescope. The rest is obvious. You align the two images seen through the telescope just as you would with an ordinary horizon test for index error. The high magnification lets you see differences smaller than a tenth of a minute of arc easily. It's extremely sensitive. You will probably discover quickly with most sextants that direction counts. That is, you can see that if you're turning the micrometer in the positive direction on each trial you get a different index error compared to turning the micrometer in the opposite direction. Using this method, I find a standard deviation of about 0.05 minutes of arc in individual trials. In other words, to the precision of the micrometer, the results rarely differ from one trial to the next. It's repeatable, and when I've had other people try it, they also get excellent, repeatable results. I had Greg Rudzinksi try this out in California, by the way. A good metal sextant's index error will not change much over time so if we're very careful it should be possible to measure this on land and then count on it at sea. But of course, if the instrument's horizon mirror is adjusted, or if the sextant is banged about at all, then you can't count on it anymore.

    You also wrote:
    "Obviously you need a well calibrated sextant with the 2 images extremely close to one another at minimum distance. I also have observed that when the images do not exactly cover one another, it does not really matter as regards the final result and it is even easier to best appreciate when both are "horizontal"."

    That works (and I agree with your last comment, too), but it's been my experience personally, and also comparing notes with others, that this method is not terribly reliable. I've had results using this method that "feel" good when the observation is done, but when I compare with a better test later, they may differ by several tenths of a minute of arc. And if the result you found by this method differed by, let's say, 0.4 minutes of arc from the "true" I.E. for your sextant, how would you know? Most of celestial navigation can tolerate errors of that order of magnitude.

    And you concluded:
    "But I also have to confess that I have had the extreme luck of an super-sharp eyesight, very useful earlier as a fighter pilot, still very much apreciated nowadays for sextant shooting, and even funny when in a group of friends for far distance acquisition of (feminine) target(s) for example :-))"

    Ha! Does this super vision of yours come with some sort of "heads-up" display when distant targets are acquired? ;-)


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