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    Re: Problem with a sextant
    From: George Huxtable
    Date: 2006 Apr 25, 11:20 +0100

    Bill wrote, about the problems Alex is finding with his sextant-
    | > 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.
    If there's wear near zero degrees, then presumably it's more likely to be on 
    the (softer) aluminium-alloy rack rather than on the
    (harder) worm. Any error due to wear on the worm would be expected to repeat 
    at one-degree intervals, so could affect readings close
    to an integral number of degrees (and zero minutes) anywhere on the arc. If 
    wear (or metal transfer between the surfaces) really is
    the cause of the problem, then it would likely be worse near the zero-point, 
    because that setting is used so often.
    I expect that Alex is quite accustomed to tweaking his mirror adjusting screws 
    and would be prepared to do so again. If so, may I
    suggest the following, to test for whether there really is a wear-problem on index-zeroing?
    Why not really misadjust the index mirror, in a big way, so that it remains 
    perpendicular, but (in the other plane) the zero-setting
    is shifted well away from 0 degrees? I don't know how much latitude the 
    adjusting screws would allow, but my guess is that a couple
    of degrees (that is, 1 degree tilt of the mirror from its normal position) 
    might well be possible. Ideally you would want to shift
    far enough so that the worm had moved to a completely different set of rack 
    teeth from the ones it normally occupies at the zero
    setting, and that depends on how many teeth are in contact with the worm at a 
    time. Ideally, one might wish to shift the zero by
    perhaps two and a half degrees, to use an unworn part of the worm as well as 
    an unworn part of the rack, when making an index-error
    check. It wouldn't matter much whether the deliberate offset was on the arc or off it.
    Now, if Alex repeats his checks, allowing for that new, gross, index 
    correction, then if his discrepancies disappear, he can be
    pretty sure that his problems result from local wear of his track/worm combination. And if not, not.
    It's a potential problem, that the older, Vernier sextants didn't suffer from. 
    Another disadvantage of progress, perhaps?

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