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    Re: Dipmeter: was [NAV-L] Wires, back sights and collimation
    From: Alexandre Eremenko
    Date: 2004 Dec 1, 22:10 -0500

    On Wed, 1 Dec 2004, Frank Reed wrote:
    > Myself, I can't think of any technical reason. Can you? I suppose simple
    > conservativism of design would be an explanation...
    The only explanation I can think about is that the inverting scope
    is used for taking large angles. (I am not saying that they
    were thinking of lunars, but probably large horizontal angles
    between the objects on the shore. The arc of this sextant
    permits measuring angles to 140d.
    Now when taking such large angles, very small collimation
    error becomes relevant.
    So they did everything to reduce it.
    a) added the wires
    b) added the collimation screws.
    P.S. By the way, when taking large distances, I always
    have an error which varies between 0.3 and 0.7
    which I cannot explain. It occurs permanently, and I can
    correct for it if I wish. But what is the reason for this error
    I do not understand. Here is my typical recent lunar:
    Dec. 1, GMT 15:32 Lat 40d27.2N, Long 86d55.8W Sun-Moon IC=0.
    Temperature 33F Pressure 30.1 Reducing with Frank's calculator,
    last column is the error.
    15:43:46    127d12.2     0.8
    15:46:15    127d10.8     0.6
    15:47:40    127d09.8     0.3
    15:49:45    127d09.1     0.7
    15:53:28    127d07.2     0.7
    The average error is 0.6, and maximal
    deviation from this average is 0.3
    This 0.6 overshot occurs in all my
    measurements of distances larger than 90.
    But what is this?
    Repeated measurements of the index error
    give zero. Well, never more than 0.2.
    All my attempts to measure collimation error
    also give zero.
    So this is either some kind of
    "personal equation/irradiation"
    or some problems with the arc excentricity.

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