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    Re: SNO sextants
    From: Alexandre Eremenko
    Date: 2004 Nov 11, 10:03 -0500

    Dear Joel, Jared and Lee,
    Thank you for your advises about SNO maintenance.
    At this moment I probably have no need in disassembling anything,
    but I will watch the behavior of the horizon mirror adjusting
    screw carefully.
    I was just asking in general, and for the future.
    On Thu, 11 Nov 2004, Yourname Here wrote:
    > I am surprised you get such accurate results. ;-)
    You probably mean the results I posted earlier on this list?
    Why are you surprised? Did you think that SNO are inferior
    to the best sextants? (I thought so myself, but now I am
    not sure; my experience in comparisons is ZERO: I never
    handled any other sextant:-)
    My results are not always so good.
    For example, LONG lunar distances (more than 100 degrees)
    are not good.
    That's why I started to think of very fine adjustment
    of the horizon mirror.
    The "star test" for it always gives normal results
    (the stars come together so that I cannot tell them apart).
    But it seems that there is more precise test:
    I overlap two images of the sun, using two filters of different
    color (red and yellow).Superimposing the two discs gives
    an orange disc. But sometimes one edge of this disc (say left)
    is more yellow, while the other edge (right) is more red.
    This is an evidence of non-perpendicularity.
    My attempts to make a table of the "arc error" by measuring
    distances fail so far (I get contradictive results
    for long distances). I attribute it to
    my unsufficient
    proficiency in getting a precise touch of two stars.
    It also seems to depend on the star.
    The very bright stars (like Vega) are especially difficult.
    I started to try various light filters on these stars
    and the results improved.
    Another thing I found was that filters on my SNO can ditrort
    the measurements.
    I tried all possible combinations of filters on the
    "index correction test" with the Sun.
    And found experimentally that more filters I use larger
    the "index error" is.
    This means that some of my filters are
    For example: I put one red and one yellow filter
    (these give the best results for the Sun) and determine
    my index correction from the sun. I get the values of index
    between 0.0' and -0.2', the average of a long series is -0.15'.
    And the control value of 4SD is within 0.2' of its true value.
    Seems good.
    But then I put ALL my filters (3+4) and obtain the index error
    of -0.4' to -0.5'.

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