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    Re: Reliable Index Correction to a Tenth Minute of Arc
    From: Frank Reed
    Date: 2010 Feb 17, 18:16 -0800

    Bill Morris, you wrote:
    "I think I am correct in saying that as the sextant reading approaches zero, so does the error from mis-collimation reduce. At a reading of 10 degrees, for example, Simms gives the error as only about 5 seconds for a tilt of a whole degree (an amount that should be noticeable to most people)."

    Yes. For index corrections, telescope collimation doesn't matter. We can check this easily by doing an index error test with the telescope intentionally mis-aligned using some sort of shim. This also means that there's no problem using a telescope which isn't attached to the sextant at all. In the method I described in my last post, the sextant is placed on its side on a table with the standard scope removed, and a spotting scope with magnification of 25x or better is rather roughly aligned with it. Collimation fortunately doesn't matter.

    And you wrote:
    "I suggest that those who believe they can correctly estimate their index error to a tenth of a minute on the basis of a single estimate deceive themselves. I would invite them to estimate it thirty times in a row and if they can get the same result within a tenth of a minute each time, I will be among the first to congratulate their instrument."

    Using the standard textbook tests, I agree with you. And of course some sextants are more stable than others. I have only tried the tabletop "spotting scope" index error test that I've described with four sextants. With all of the metal sextants, the standard deviation in individual tests was 0.1 minutes of arc or smaller and 0.05 minutes of arc for the newer instruments (so long as the final adjustment of the micrometer was always done in the same direction). For a Davis plastic sextant, the standard deviation was 1 minute or arc or larger.

    -FER


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