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    Adjusting Central Mirror
    From: Bruce Stark
    Date: 2004 Oct 11, 18:11 EDT
    I don't know if I remember this right, but it seems Alex mentioned a problem in setting the index mirror of his SNO-T perpendicular to the frame of the instrument.

    Also, as best as I can recall, Joel mentioned some time ago that the reflecting surface of these front-silvered index mirrors is not centered on the pivot, and so can't be checked for perpendicularity in the usual way, by aligning the reflected image of the sextant's arc with a direct image of it. As I take it, that's the reason for the little L shaped vanes in the sextant box.

    Anyway, I had no trouble setting up the index mirror on my SNO-T using the vanes, and just now got to wondering why it didn't work for Alex. I'm thinking he may not have set both vanes exactly the same distance from the center pivot. So I've put the SNO-T on the table and tried it again. To be sure we're talking about the same thing, here is how it's set:

    Index set at 35 degrees. One vane covering 0 degrees on the arc, the other covering 120 degrees. Vanes are set with the long, flat side facing toward the central mirror. The corner edge of each vane is precisely on the inner edge of flat part of the arc.

    Move either of the vanes slightly toward or away from the central mirror and you get the effect Alex described.

    I don't trust my memory of what Alex and Joel had to say and hope they, or other List members, will correct me. There may be more to this than I suppose.

    Bruce
       
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