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    Re: Sextant and Eye Optics
    From: Bill B
    Date: 2006 Dec 05, 19:05 -0500

    Michael Bradley wrote:
    > Bill, Frank, Alex and all
    > 
    > The posts about using lasers for IE checks and the
    > effect of misalignment along the axis of the telescope
    > have triggered me off ....
    
    Indeed being off axis does matter.
    
    
    Following is one solution: I understand this is a Rube Goldberg and used
    what I have available. Total investment $3. I can imagine a lot of
    solutions, even mounting the laser pointer on a block of wood if one only
    intended to use it with one sextant. A table-top manual or self leveling
    unit, or a professional-grade self-leveling laser unit on tripod could make
    life a lot easier.  I am getting plus/minus 0.5 mm repeatability at 40 ft
    with this setup.
    
    The important point is for accuracy and precision the beam MUST pass down
    the center of the scope.
    
    Tools:  Masking tape, index cards, drawing compass, Xacto knife, 36 index
    cards, cheap dial indicator base and arm, stable TV table, mouse pad,
    bullseye bubble level, tape measure, black magic marker, double-sided tape
    or one-coat rubber cement.
    
    1.  Make a mask for the scope�s eyepiece with a 5mm hole in the center.
    Marker it black.  Place double-sided tape or rubber cement around edge.
    
    2.  With eye about 2 feet from the scope eyepiece, adjust its distance from
    the sextant frame (if possible) so the glass/mirror line is centered in the
    scope eyepiece.
    
    3.  Move the index mirror out and determine where how far above or below the
    physical vertical center the index mirror surface.  This can de done by
    placing a card on the mirror side of the horizon glass, leaving 7 mm of
    mirror exposed and using a zero-mag sight tube.  Mark the card and determine
    any offset
    
    A down and dirty method is to align the axis of the laser pointer with axis
    of the scope, shoot through the hole in the mask, and note where the dot
    hits the horizon glass/mirror juncture.
    
    4.  Place the sextant on the mouse pad (to reduce slipping).
    
    5.  Place the indicator base with laser pointer on 8 index cards.  Do a
    rough height adjustment of the laser pointer so its beam passes through the
    hole in the scope eyepiece mask.
    
    6.  Remove the sextant.  Measure the height of the laser beam from the
    floor.  Project the beam on the far target area and adjust the beams level
    until it strikes the target area the same distance from the floor as
    determined in the previous step.  Mark that spot on the wall/target.
    
    7.  Replace the sextant on the mouse pad.  Roughly line up \the beam with
    the mask hole and predetermined spot on the horizon glass. With the bubble
    level on the sextant frame, level it up for and aft using index cards for
    shims if needed.
    
    8.  Now fine-tune the setup.  The laser pointer beam height can be raised or
    lowering by inserting or removing index cards from under the base.  When the
    relationships are correct, the laser bean passing through the scope and
    horizon glass should not have moved appreciably from the spot marked earlier
    in step #6.
    
    Proceed as per Frank�s instructions.
    
    
    
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