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    Re: Calibrating a sextant scale
    From: Nicolàs de Hilster
    Date: 2007 Nov 20, 16:15 +0100

    Isonomia wrote:
    > Whilst it is easy to calibrate a sextant's zero reading, is there any
    > way to calibrate any other readings?
    You should start with aligning the telescope with the frame:
    Look through the horizon mirror through the telescope to the horizon (so 
    by reversing the sextant) while you hold the sextant horizontal.  Now 
    the horizon should show as a continuous line running exactly through the 
    middle of the telescope and exactly aligned with the upper edge of the 
    mirror (the silvered part that is).
    Check if the index mirror is perpendicular to the frame. For this you 
    have to look from 'above' the instrument to the divided arc and try to 
    see the arc and its reflected image at the same time. When properly 
    aligned both arcs should be aligned properly as in attached picture.
    Then if the index mirror is perpendicular to the frame. The horizon 
    mirror can be checked by looking at a distant object with the index arm 
    set at exactly 0 degrees (which is checked at the next step and 
    therefore is an iterative action). If you hold the sextant vertical and 
    look at an object with distinct vertical lines you should see the direct 
    and reflected image nicely on top of each other (properly aligned 
    left-right). If not, the horizon mirror needs adjustment. You can of 
    course perform this on the horizon as well by holding the sextant 
    Then calibrate for index error as you already know. If any large errors 
    are found and corrected for you should repeat the last two steps.
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