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    Re: Sextant precision
    From: Alexandre Eremenko
    Date: 2004 Sep 30, 10:38 -0500

    On Thu, 30 Sep 2004, Robert Gainer wrote:
    
    > One of my sextants was made by Brandis & Sons
    
    I conclude from your description that this is an old one.
    I am very interested in the results of accuracy tests
    you are going to make.
    
    More detail on my SNO-T.
    It permits reading to 0.1'=6" though it has NO vernier.
    I think it is quite unusual construction: the drum has 1'
    divisions, but there is a powerful magnifying glass
    (with built-in fluorescent (non-electric) illumination)
    which permits easy reading to 0.1'.
    
    The certificate says that the instrumental correction
    everywhere on the arc is exactly +10", which indicates that
    this certificate is probably a baloney, I suspect they did not
    really test it:-) The owners manual says that the sextant has
    to be checked in a workshop every year, and a new certificate
    filled.
    
    My own measurements so far indicate that the instrumental error
    (and the index error) is probably less
    than the errors of my most careful measurements, maybe less
    that the "reading-of-the-drum" limit of 0.1'.
    
    I would also like to share some of my experience with
    star-to-star distances, sorry for repeating the things which
    may we well-known to the members of this list. I will appreciate
    any corrections from more experienced navigators.
    
    a) The Russian manual recommends to rotate the drum
    only in one direction, after you catch both objects
    in your field of view. Normally, to decrease
    the angle, when measuring altitudes. So the Sun altitude,
    for example, has to be measured fast, to catch the moment
    of tangency. After the moment has passed, I have to disengage
    the worm from the arc and repeat the whole procedure again.
    There is an additional hurry, to mark the time.
    
    This creates a habit to measure fast.
    But it is different with star-to-star distances. The stars
    do not move, and I don't have to mark the time.
    So star-to-star distances can be done very slowly and carefully,
    until I am really sure that the images of the two stars
    coincide.
    
    b) When I have to hold my sextant in an inconvenient position
    especially with "handle up", my right hand gets tired quickly.
    So I frequently hold the sextant with two hands, when checking
    and double checking
    that the images of the two stars indeed coincide perfectly.
    Then I adjust
    a little, and then hold with two hands again etc., until
    the coincidence is completely satisfactory. You cannot do this with
    the Sun or the Moon which move.
    
    c) It seems inportant to achieve the image coincidence in the center
    of the field of view (many sextant authorities stress this).
    Here the
    cross hairs in the inverting scope help.
    
    d) In my most precise measurements, I sit on a very stable chair,
    with my
    feet
    on another chair, and the right elbow supported my my knee.
    Or lay on the floor for high altitudes.
    
    Alex.
    
    
    

       
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