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    Re: Is the focus on the mirror or on the star
    From: Frank Reed
    Date: 2017 May 26, 15:56 -0700

    Randall Morrow, you wrote:
    "It seems that the direct and reflected images cannot both be perfectly focussed at the same time, which makes no sense to me."

    That's an interesting clue. Could it be that your sextant's index mirror is slightly convex or concave. That's not unheard-of, and it would yield a slight difference in focus. A couple of experiments to try:

    • Try measuring an angle between two bright stars. Do you find that that the direct image looks a little out-of-focus compared to the reflected image? That observation takes the artificial horizon optics out of the equation.
    • Try a star-based index error check. When you place a bright star's reflected image directly on top of its direct image, do they stay exactly togther as you move them around in the field of view? If not, that may be evidence of some non-flatness of the sextant's own mirrors.
    • Try looking at a star in the artificial horizon with a pair of binoculars. Then look at the same star direct in the sky through those binoculars. Do you see any difference in focus? If not, then the focus issue you're seeing is very likely originating in your sextant's own mirrors.

    Frank Reed

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