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    Re: Instumental error?
    From: Bill B
    Date: 2005 Apr 21, 01:34 -0500

    >> Bill wrote:
    >> Could we located other stars in the the
    >> same separation range as the problem areas
    >> that might offer a different
    >> (easier) sextant position?
    > Alex responded:
    > I am always chosing the stars for this business in
    > convenient position.
    > Locating two good pairs at the SAME distance is
    > of course very hard.
    Or at least a labor of love ;-)
    > I also don't think that the scope is relevant.
    > What sort of deficiency in a scope can lead to
    > a systematic bias? I suppose, only collimation.
    > Collimation I checked, and in general, it can give only very small
    > error.
    As a practitioner of photographer, not a an optical engineer, I can only
    suggest possibilities; and cannot predict the degree they might affect
    observations to the extent you have observed.
    In a brief on-list exchange with a fellow photog, we lamented the quality of
    scopes in general as opposed to quality fixed-focal-length camera lenses.
    A couple suspects might be:
    Spherical distortion in the scope's optics.
    Chromatic aberration. Look at a quality prime (fixed focal length) camera
    lens.  Along the focus/depth-of field scale there should be a red dot.  That
    is the focus shift for infrared photography.  The point being that even the
    color of the star (Taurus the Bull's eye) can shift the focus/image.  If the
    focus is off, circles of confusion would make the body appear larger.
    Flair from the optics could also make the body appear larger.
    The always elusive erradiation.
    Mechanical possibilities:
    Could it be the bearing on the index arm?  You would think this would creep
    up on you for separations close to those you are observing unless it/they
    have a small but pronounced flat spot.
    Index mirror off index arm rotational axis?  I would guess that would be a
    nice smooth curve.
    Gears? You measure the same pair of stars for a given separation. You have
    done this pair many times, so eliminate the the worm gear worm for now (you
    have done many other observations, and a glitch in the worm gear would show
    up in other observations at the same minute reading).  Trusting your
    ability, which I do, that leaves the acme thread of the arc gear.  Even a
    slight burr could throw you off.  Think about it.  Our handheld instruments
    are measuring one tenth of 1/60th of 360. A fly's wing in the arc gear could
    be a big deal.  So find something to measure that is a notch or two off on
    the arc gear where you get the abnormal results.  If the results are
    similar, then you have a problem unrelated to the arc gear.  If not, an
    off-spec gear tooth.  So no need to throw out the baby with the bath water.
    Like my HO229 tables where you noticed a few blank pages, it is
    inconvenient, but if you are willing to wait four minutes, no big deal.
    Point being, as shrinks like to say, "If you continue to repeat the same
    behavior and expect different results, you're crazy."  Put another way for
    Star Trek fans, Captain Kirk was the only academy student to win the
    Koribache (Koribache spelling?) Test.  Once he realized it was a no-win
    exercise, he sneaked in and reprogrammed the computer--and won.  Referencing
    Richard Feynman "Cargo Cult Science" again, eliminate variables to be sure
    you are measuring what you intended to measure.  In this case it *may* be
    nothing more than a couple of errant arc gear teeth.

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