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    Re: Perpendicularity again. was: Adjusting Central Mirror
    From: Michael Dorl
    Date: 2004 Oct 11, 21:03 -0500

    At 06:04 PM 10/11/04 -0500, eremenko{at}MATH.PURDUE.EDU  wrote:
    >Dear Bruce,
    >I am very glad to the opportunity to repeat my question again
    >(The question was addressed to everyone who EVER checked
    >perpendicularity of the index mirror,
    >but surprisingly nobody replied when I asked it
    >for the first time about a month ago).
    >This has nothing to do with SNO-T sextants or adjusting.
    >The question was: how do you properly CHECK perpendicularity
    >of the index mirror to the sextant frame.
    >I was asking a reply of ANYONE WHO HAS ACTUALLY DONE
    >this check on his/her sextant
    >(and nobody replied, which is surprising).
    >There are several versions of this check.
    >The simplest one and most common one says:
    >look into your index mirror so that you see a part of the arc
    >and another part of the same arc reflected in the mirror.
    >If everything is OK, you see the arc as a straight continuation
    >of its reflected image.
    >But whether this is so or not, DEPENDS on the angle of your
    >with respect to the plane of the sextant frame!
    >I just cannot believe that nobody has noticed this.
    >In other words: you can move your eye (or the sextant) so that
    >the point, where the direct and reflected images of the arc meet,
    >moves along the right vertical edge of the index mirror.
    >You cannot get a match of these arcs at every point of this
    >vertical edge. Either they match at the lower point of the edge of the
    >or in the upper point of this edge  or somewhere in the middle.
    >But never everywhere.
    >This does not depend on the particular sextant.
    >Every sextant with sufficiently large mirrors will
    >behave like this (and this can be shown mathematically).
    >So WHERE exactly should you place your eye with respect to the frame
    >for the proper test?
    >No manual says this precisely.
    Mine matches top to bottom.

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