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    Re: Suitable Sextants - Mirrors
    From: Frank Reed CT
    Date: 2005 Oct 12, 20:29 EDT

    Joel Jacobs wrote:
    "Your analysis though  interesting, fails to take into account that a
    sextant's mirrors are not used in  a static state, and hence size does make a
    difference. Consider that the  platform is moving directionally, and rolling and
    pitching all at the same  time."
    
    But since the mirrors, telescope, and other components of the  sextant are
    all experiencing the same motion, this really isn't relevant to  mirror size.
    They're either big enough to fill the field of view, or they're not  --no matter
    how much pitching and rolling there is.
    
    And:
    " There also  is motion introduced by the user when he rocks his arm to align
    for  perpendicularity."
    
    Why would that depend on mirror size? Many people  mistakenly rock the
    sextant about the axis to the horizon in which case the Sun  or star's image sweeps
    across the field of view from one side of the horizon  glass/mirror to the
    other. When "rocking" for perpendicularity, the sextant is  supposed to be
    rotated about an axis that points to the Sun or star. And when  this is done
    correctly, the Sun or star remains centered in the field of view.  For objects that
    are low in the sky this is nearly the same motion as the  "incorrect"
    procedure. But for objects at high altitudes, it's very different.  If the Sun or star
    is in the zenith, "rocking" the sextant amounts to spinning  about vertically.
    And in every case, the Sun or star stays centered in the field  of view so
    mirror size does not appear to be relevant.
    
    Just so there's no  misunderstanding, I agree with you that a sextant should
    have (or at least  potentially accept) a telescope with a relatively large
    aperture and the mirrors  should be compatible in size with that --large enough
    to fill the field of view.  I don't agree that it is because the mirrors
    "collect more light" but this  theoretical issue is really a minor concern compared
    to your experience in  actual use.
    
    -FER
    42.0N 87.7W, or 41.4N  72.1W.
    www.HistoricalAtlas.com/lunars
    
    
    

       
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