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    Re: Sextant Filters / Nearsighted CelNav?
    From: Alexandre Eremenko
    Date: 2006 Apr 24, 09:51 -0400

    Dear Greg,
    
    > 1) What (if any) is the specific usage
    > for the different-colored filters (i.e. blue, amber-orange, grey, etc.)?
    > I know that for a really bright sun you'll normally want to have all of
    > them down (4 in my case), but is there ever any case where you'd want
    
    Some sextants have all filters of the same color.
    My one has 2 yellow of different intensity
    and a light grey on each mirror
    + very dark red on the index.
    Even for the brightest Sun, I never use ALL filters together.
    
    But various combinations can be used in various conditions
    for various tasks. Sextant is designed not only for taking Sun's
    altitudes, but for measuring all sorts of angles. For example
    the angles between the objects on the shore. In which case, I would use
    to light yellow filetrs to surpress haze. Sometimes I have to
    use one of the lighter filters on the Moon or even a bright star
    at night. You want to make the two objects to look approx equally bright
    while really they can be of very different brightness.
    
    > 2) How do you near-sighted navigators cope with
    > the problem of finding stars through the sextant?
    
    I am not near-sighted but still this causes some problems sometimes.
    
    a) Take your sextant up side down, point (and focus for your eye) on the
    star, and then search for the horizon. Once you found it, revert the
    sextant to the normal position.
    
    b) Pre-compute (or take the approximate value from the
    star finder) and pre-set the altitude.
    
    c) Detach the scope. Using your normal eyeglasses, you can catch
    the star and the horizon to your field of view, and even bring them
    together. Now attach the scope, and refine the altitude.
    
    
    Alex.
    
    
    

       
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