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    Re: Error of Perpendicularity
    From: Bill B
    Date: 2006 Apr 18, 17:39 -0500

    > Furthermore, I know of a few techniques for adjusting the index mirror for
    > perpendicularity, including the eyeball method and placing objects (such as a
    > pair of dominos) of exactly the same height at opposite ends of the sextant
    > limb but it seems to me that these methods are rather coarse and not nearly as
    > precise as those for reckoning index and side errors.
    > Any comments on this most fundamental topic are welcome.
    When I first obtained my Astra with front-silvered index mirror, I went
    through the process.  (I had not yet obtained the matched brass cylinders.)
    All the dominos I could find were too big (double size).  I took a
    micrometer to dice and other objects about the right size but they were not
    a close enough match.  A little starter kit of Lego blocks (a few bucks at a
    toy store) were the solution.
    I mixed and matched them on a flat surface (a 1/2"-thick piece of float
    glass).  You don't have to measure them, one can feel a difference of a
    couple thousands of an inch.  I got really anal with the best match (with a
    few thousands difference) and slapped a piece of fine sandpaper face up on
    the float glass and lapped the "high" Lego down for an exact match (within a
    few ten thousandths).
    That being done I placed the sextant on a flat surface (3/8" thick float
    glass table top), placed the Legos on the arc, and measured from the table
    top to the top of the blocks.  A shim of two index cards under one leg made
    both block tops equal height from the table top and level fore/aft and
    port/starboard.  (A little circular bubble level on the frame could work
    Then I attached an index card to a small scrap block of wood (a base to hold
    the card upright) and placed a pencil mark at the same height as the block
    tops.  I placed this on the table top  between my eye and the sextant.  That
    gave me a reference mark to make certain my eye was aligned with the top of
    the bricks.
    NOTE:  I needed a lot of light for this, to get enough depth of field in the
    old eyeball to have both the index card and Lego in focus.
    Seems to have worked.

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