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    Re: Sextant precision, SNO-T
    From: Alexandre Eremenko
    Date: 2004 Oct 4, 17:08 -0500

    Dear Fred,
    
    On Mon, 4 Oct 2004, Fred Hebard wrote:
    
    > In your first set of
    > measurements, for example, with an off' measurement of 32.4'
    > and an on'
    > of 32.6', the index error is equal to (32.6-32.4)/2 = +0.1'.  The
    > average index error across all six measurements is +0.34' of arc.
    
    Yes. But I think my tables show that my star observations
    are more reliable, and this +0.34' is probably more related
    to my "human error" rather than index correction of my sextant.
    
    By the way, your table shows pretty large variation of the index
    error from day to day. This seems surprising to me.
    Is this what it should be? Is this related to the air temperature?
    
    The last conjecture could explain why with Sun observations
    I always get somethings small but positive,
    though small, like the 0.35'
    in the table I sent you while with stars I consistently get 0.
    The temperature of my sextant is very different in these two
    cases!
    
    > It appears that you are judging the two images to be in
    > contact when they actually are separated by 0.4' of arc.
    
    Yes. I need to practice more. It is somewhat hard to detect
    the moment of precise tangency. Have to experiment with the
    best colored filters combination.
    
    > Because of the fairly large variation
    > in the measurements from one to
    > another, I would not put much stock in the average index error of
    > +0.3'.
    
    Yes, this is why I trust my star observations more. The
    star index tests have no noticeable variation.
    
    > From your sun sight and your star-to-star measurements,
    > it appears that
    > your sextant is quite accurate, although the star-to-star measurements
    > don't extend beyond 60 degrees.
    
    I will certainly try when I find two appropriate stars.
    
    > I started out with
    > inferior equipment to what you have,
    
    ?????? What do you mean? Cassens and Plath?
    Or Husun? In the table you sent me you seem to mention
    Cassens-Plath, if I understood your abbreviation "C+P" correctly,
    which everyone says is the top sextant of those still produced.
    
    When I was deciding what to buy as my first sextant, the choice
    was essentially between a 1945 "Gothic" Husun (full size
    brass frame,
    two scopes) and a "new" 1990 SNO-T which was cheaper.
    I think I decided for SNO-T for some sentimental reasons,
    as a former Soviet citizen, etc:-)
    
    
    Alex.
    
    
    

       
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