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    Re: Geometry of SNO-T
    From: Bill B
    Date: 2004 Oct 13, 22:19 -0500

    > Here is some simple math behind the perpendicularity test
    > and Frank Reed's experiment on his kitchen floor.
    Perhaps simple for someone who bemoans the simple pleasures lost when
    switching from the log tables to calculators, "One important part of the
    pleasure, computing with the logarithms, was gone forever."  Heck, no one is
    stopping you from deleting the computer program(s) you wrote and doing it
    that way. 
    I had posted a question a while back, and have received no response.  Since
    Alex seems to have a good grasp of this now, I will ask him and others to
    address it.
    "If the front-silvered mirror is no longer on the axis of the rotation, will
    this affect the sextant's performance?"
    I admit my ignorance.
    What I think I know is that if I scribed a line on the index mirror at it's
    axis of rotation, and the plane of that mirror was on the axis, that line
    would appear at the same elevation in the horizon mirror as I moved the
    I also think that if the plane of the index mirror moved off the axis, that
    line would move vertically as I moved the arm.
    From the reference point of the angles of the index and horizon mirror, no
    problem, that relationship will remain the same.
    From the reference point of reflection, so what if the area of the mirror
    reflecting or the body changes,  Angle in, angle out.
    This is as far as my skill sets get me.  I understand the movements of a
    view camera, and the differences between what the axis of rotation for
    swings and tilts in relation to the center of the lens can do to the image
    from an end-user standpoint, but someone with (at least) better schooling in
    that area (and probably a lot brighter) did that work for me.
    Now most likely I am comparing apples to oranges.  Common-sense test--
    manufactures would not do something that would compromise the geometry of
    their instrument, but I gnaws at me.
    So I ask again.  When you move the plane of the mirror off the rotational
    axis (as in a front-silvered mirror in a rear-silvered mirror mount) will
    this affect the geometry of the instrument?  If so, uniformly so it can be
    adjusted out, or nonuniformly?
    Thank you

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