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    Removing Sextant Errors
    From: Mike Hannibal
    Date: 2005 Nov 18, 09:12 +1100
    I have had an interesting experience over the last 24 hours of fitting a new index mirror. I thought it might be of interest to the list.

    The manufacturer of my sextant provided a replacement index mirror because the original mirror was producing substantial stray reflected light from the margins of the mirror. The manufacturer felt that the mirror hadn't been finished correctly.

    When the new mirror arrived I removed the old one and replaced it with the new. I have large hands and between my partner and I it was like wrestling (gently) with a slippery pig to get it secured with the retaining springs. I'd hate to have to do that in a small boat at sea...

    Having done that I checked and corrected the perpendicularity error and then tested for side error. There was substantial side error so I adjusted the horizon glass and then checked index error: it was 1 degree 35 minutes off the arc!!

    Prior to replacing the index mirror t! he index error was 1.3 minutes off the arc. After replacing and before adjusting it was 15 minutes off the arc.

    Back to square one. I checked perpendicularity again - it was fine. I then asked my partner to check it and she said it was substantially out. I have no binocular vision - blind in one eye. This is not usually a problem but it was clear that I couldn't correctly see the perpendicularity error (a lesson for the future). So we set it to my partner's view of the world and then returned to side error.

    My logical brain said that if you have two diagonally offset adjustment screws then screwing one in is the same as screwing the other out. Well it isn't! By the time I got to zero side error and an index error of 2 degrees off arc I knew it was time for a rethink.

    I began to ignore the simple view - if it's offset one way, tilt the mirror the appropriate direction - by either screw - to bring it back. That doesn't work. Instead I begn ! to operate counterintuitively and instead sought to understand which screw I needed to turn which way to minimise index error. I used a distant flagpole with a distinctive truck.

    Very quickly I found myself turning both screws in the same direction and getting rid of side error and index error. I know about breaking mirrors but the spring loaded design makes this unlikely.

    In next to no time I had only the tiniest side error and index error of 0.3 minutes on the arc.

    What I did come to understand also was the complex interaction between perpendicularity, side error and index error. A lesson never to be forgotten.

    I'd be interested in other members' expereinces.



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