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    Sextant and Eye Optics
    From: Michael Bradley
    Date: 2006 Dec 5, 10:31 +0000

    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 ....
    
    If your sextant has a telescope using lenses with
    'spherical' surfaces, as I assume most do, its design
    will be based on the classical 'dioptre' based optical
    theory. This theoretical treatment produces some
    simple design algebra, but I believe that it only
    works for what is called the paraxial ray which runs
    down the centre of the optical components. It's not a
    great surprise if different results follow from set
    ups that have different ray paths, away from that
    central axis. But then if you have a more expensive
    telescope.....
    
    And to refer back to some of the hail of previous
    posts on index error.... Anyone doing a little
    scholarly work in a visual optics textbook will also
    come across descriptions of the process by which
    change of focus can indeed, in some eye circumstances,
    change the size of the image on the retina. Maximum
    power in the image is not always the point of accurate
    focus. Accurate focus is needed for accurate rendition
    of the image size on the retina. General advice is
    always to focus in from the 'short sighted' side.
    
    Then you take the sextant out in the dark when the
    eye's focusing mechanism doesn't always have enough
    power in the image to operate properly.  Those of us
    who use spectacles can test this by going out in the
    middle of a dark night ( moon perhaps too full at the
    moment ), wait for your eyes to adjust, then look at
    the stars ( no instrument ) with and without the
    specs. You will find that the specs don't make any
    difference at all....
    
    In summary, the sextant may be a beauty, but you have
    to consider its telescope and after that the eye.
    
    I'm no optician, but following the summer's furore I
    briefly borrowed Tunnicliffe ' An Introduction to
    Visual Optics', published by Brit. Assoc. Dispensing
    Opticians, for the limited research I'm reporting
    here.
    
    Michael Bradley
    
    
    
    
    
    
    
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