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    Re: "Vernier acuity" of horizon IC tests
    From: George Huxtable
    Date: 2009 Jul 8, 09:45 +0100

    I had written, about Bill Morris' horizon observations-
    "Those statistics strongly suggest (though are not yet conclusive) that
    there's a real difference between the index zeros, depending on the
    telescope that's in use. If that's really the case, can anyone suggest a
    possible physical cause? It eludes me."
    To which Douglas Denny has responded
    "The answer most likely is:
    Slight prism effect through the telescope system if not viewed directly down
    the parallactic centre axis.  If looking slightly offset from centre-line
    then prism effect will shift the apparent axis. It does not require much of
    an offset from the optical centre to give a large effect of prism in
    powerful optics"
    I haven't come across that use of the word "parallactic" before, and wonder
    whether Douglas was referring to axial and paraxial rays passing through a
    It's certainly plausible that different telescopes might have slightly
    differently aligned optic axes, when assembled into the same fitting on the
    sextant. And that could alter slightly the angle of light passing through,
    just as a prism would, as Douglas says. But that would only call for the
    sextant/eye combination to be twisted a bit in direction, to look at the
    same spot on the landscape. I've considered that as a possibility, but as I
    see it, it would do nothing to shift the reflected image RELATIVE to the
    direct image, which is what would need to happen to change the index error.
    Indeed, even if a prism was deliberately inserted into the optical system,
    where a telescope normally goes, then by my assessment the sextant would
    work exactly the same, as long as the eye position was shifted
    appropriately, to look into it. That's based on reasoning rather than
    practical trial, however.
    So I'm still unconvinced as to what could possibly be the cause of that
    shift of index-zero, between using various telescopes and no-telescope, if
    the effect is real. And it seems likely, though not certain, that it IS
    real, to judge by reported observations.. I just can't see how what you do
    with the light after it's passed the two mirrors, can cause a relative shift
    between the two images.
    Here's a conceivable mechanism, though I don't propose it as a likely or
    reasonable one. Say the weight of the telescope was sufficient to distort
    the sextant frame, enough to alter the parallelism of the mirrors by a few
    tens of arc-seconds (half the index-error shift we appear to see), then it
    could cause the effect that Bill seems to observe. Trouble is, if that were
    the case, it would give rise to other obnoxious effects.
    Until it's understood, anyone attempting high-accuracy sextant work would be
    wise to follow Bill's advice, and use the same telescope for obtaining index
    error as is used for the observation itself.
    contact George Huxtable, at  george@hux.me.uk
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    Navigation List archive: www.fer3.com/arc
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