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    Re: Problem with a sextant
    From: Bill B
    Date: 2006 Apr 25, 01:26 -0500

    > What can this be???
    > I tried Bill's Astra (with my scope) and the star distances were OK.
    > Bill tried my SNO with his scope and was -1' off.
    For the record, this was scope A (30 X 6 ) that we were testing.  And yes,
    the Astra was close with scope A, but by both our (repeated) measurements
    the Sno-T was over by close to 1' (with scope A) in the 30d range. Yet close
    to spot on with the Sno-T and its inverting scope.  Frustrating indeed.
    > The only explanation I can imagine is that the arc is deformed near 0.
    > (Worn after so many index checks, as Bill joked:-)
    Not joking.  After a large change in index error overnight while trying to
    use the sextant as a range finder by measuring parallax at 100 yards I
    discovered a speck barely visible to the human eye on an arc tooth. (Thank
    you Ken G.)  I know I get a brass-alloy buildup on the arc near 0d from
    repeated IE checks.
    Theoretically I like the idea of adjusting the mirror so 2.0' on or off the
    arc is nearly 0.0' index error to check for that problem.  Practically there
    is still worm gear "residue" at 2.0' from 0.0' But a good start on reducing
    2nd- or 3rd-order variables.
    In the global overview, we (you and I at least) are striving for a 0.1' to
    0.2' intercept with a static land-based environment.  I am not clear the
    nautical almanac coupled with sextants represent a system that can achieve
    that accuracy repeatedly.
    On the flip side of the coin, we are testing the *precision* of a
    combination of sextants and scopes to measure the same distance under the
    same (semi) static conditions--almanac and refraction be damned. Under those
    conditions we should have some level of confidence that the instruments
    (factoring in operator skill levels) perform up to their certificate level.
    That being plus/minus 10" to 20" plus operator error sigma.  Here I share
    your frustration that changing scopes should not result in an averaged 1.0'
    change on average.
    In summary, I think we need to leave cel nav behind and pick up
    land-surveying tools and tables. 
    Bonus question.  "Within two miles" seems to be the standard for cel nav.
    Most observations appear to be running fixes from the sun.  What is the
    level of expectation from nearly simultaneous star/planet observations?

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