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    Re: Calibrating a sextant scale
    From: Alexandre Eremenko
    Date: 2007 Nov 26, 00:31 -0500

    
    Frank,
    
    > That doesn't mean a sextant is
    > "poor". It just means the certificate may be irrelevant.
    
    You mentioned that the 0.8' error that you eliminated
    returned after a highway trip. My statement that this was
    a poor sextant was related to this, not to the difference
    between the current arc error and the certificate.
    And to make it even more precise, I mean not "poorly made"
    but "in poor condition".
    
    > Some people aren't worried about that very fine level of
    > measurement. Mike was talking about a
    > plastic Ebbco sextant. For an
    > instrument like that, star-to-star
    > angles should provide all the necessary
    > accuracy.
    
    No doubt. In all this discussion of calibrarion
    I was not talking about 1' arc errors or plastic sextants.
    
    > could be done in a relatively short period
    > of time (a night or two?) maybe
    
    Hardly. There are usually not enough good distances
    to measure in a "night or two" to determine the arc
    error. Aiming at "ultimate accuracy" I use at least 1/2
    of the full cycle of Sun and Jupiter. This takes 2 weeks
    under extremelly lucky weather.
    And measure the same distance MANY times, on several days.
    (Again I am NOT talking of plastic sextants and 1' errors.
    I am talking of making a table of errors which can be used
    reliably when doing lunars.)
    
    > once every five or ten years
    > by using special apparatus
    
    Russian manual says:
    every two years. Instead of a certificate they issue a little
    book with empty forms for the arc corrections. The forms
    have to be filled up every two years in "Navigation Chambers"
    Besides the arc correction they measure backlash.
    
    > Otherwise, I wouldn't drive with them at all, right?
    
    I afraid that at least 95% of this list
    members have to drive
    from home to reach their boats:-)
    (I am in even worse condition, I have to take an airplane,
    drive to the airports etc.) So from my personal perspective
    a sextant is useless if it can develop a 0.8' ARC ERROR
    after a transportation in a car. (I don't mean the index
    or side error which is relatively
    easy to measure and adjust).
    
    > *eventually* change the adjustment
    > of a sextant and require
    > re-certification.
    
    Frank, let us make a distinction  between re-adjustmewnt
    and certification. Arc error is not adjustable.
    It can change if you either deform the frame,
    or if something serious happens with the arm pivot or with
    the worm screw assembly. I think only the third named thing
    can be adjusted without special equipment, and even this
    hardly.
    (Any such adjustment is explicitly prohibited by the
    Russian manual).
    
    > One of the things that I really like about the SNO-T is
    > its very sturdy design.
    > I do expect that your sextant could survive more
    > banging around than average sextants.
    
    When I bought it, it was sold "as new". And it looked new,
    never used and in factory packing, rubber parts covered
    with talc. The factory certificate
    contained only the original factory arc correction.
    It's true, this original certificate is total nonsense
    (which shows that nobody really measured anything and that
    the person who filled the certificate did not understand
    what s/he was doing). I suppose that my sextant was kept
    in some warehouse from 1990 when it was made to 2003 when
    it was sold.
    
    And of course I use utmost care in transportation:-)
    
    Alex.
    
    
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