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    Re: Geometry of SNO-T
    From: Bill Murdoch
    Date: 2004 Oct 15, 10:49 EDT
    In a message dated 10/14/04 12:26:18 PM Eastern Daylight Time, eremenko{at}MATH.PURDUE.EDU writes:


    I have no clue why they designed it like this.
    It seems they did it intentionally, and they knew the adjustment
    will be trickier, that's why they supplied the visors as standard
    accessories.

    The closest to SNO-T Western sextant is Freiberger.
    Question to Freiberger owners: does it also have this feature?


    I have a Freiberger Trommelsextant and a Davis Mk 25.  I took a piece of coat hanger wire and bent a 3 in diameter circle in it forming a base which rests on a table.  I bent the wire up from the circle then horizontally cutting it off as it passed over the center of the circle.  I ground the cut end to a point and painted it black with a magic marker.  I placed first one then the other of the sextants on the table with the point just over the sextant index mirror.  With my eye a foot or two above the table, I repeatedly swung the index arm from -5d to +120d moving the sextant until the point was over the spot on the index mirror holder which did not shift in relationship with the point.  That should be the center of the index arm axis.  I marked that spot on the mirror holder.  I could find the spot to within 0.5 mm.

    For the Davis sextant the point is 2.5 mm behind the front surface of the mirror.  The mirror is silvered on its rear and is 2.0 mm thick

    For the Freiberger sextant the spot is 4.0 mm behind the front surface of the mirror.  The mirror is silvered on its front.

    For the Davis sextant the effect that Alex mentions is quite small (or nonexistent) and difficult to see.  I set the index at 55d so that I am comparing the 0d and 120d areas on the arc.  The index mirror frame, the edge of the mirror between the front and silver surface, and the mirror clip all get in the way and make it hard to compare the two parts of the arc as the viewing angle is changed from one corner of the index mirror to the other.

    For the Freiberger sextant the effect that Alex mentions is easily noticeable.  I set the index at 30d so that I am comparing the 0d and 120d areas on the arc.  Because the mirror is silvered on its front and because the mirror frame is behind the front of the mirror and almost invisible it is easy to compare the two parts of the arc when the viewing angle is changed from one corner of the index mirror to the other.

    Bill Murdoch
       
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