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    Re: Sextant precision
    From: Henry Halboth
    Date: 2004 Oct 3, 23:37 -0400

    The several postings with respect to the measurement of interstellar
    distances by sextant, or whatever, have been most interesting and
    instructive. Unless I have misunderstood the intent, the purpose thereof,
    however, from a purely navigational point of view, appears somewhat
    Interstellar distances, to the best of my somewhat antiquated knowledge,
    were considered a second line method of determining centering error,
    which was usually done at intervals of altitude and  increased therewith.
    At one time, an instrument could be sent off for testing by a system of
    collimators, whereby the centering error was quickly and accurately
    determined at given, usually 15-degree, intervals and tabulated as a
    permanent error. As all are undoubtedly aware, index and centering error
    are entirely different animals - an error determined by interstellar
    distance will include both and, theoretically at least, will be valid
    only at the distance measured, unless an independently determined index
    error be negated.
    Hopefully as a matter of interest, the writer offers the following
    discussion on the determination of index error. No effort is made to
    compare the accuracy of the various available methods, however, the
    opinion is advanced that the immediate following, utilizing the sun, is
    the more simple, sensitive and comprehensive approach, as well as being
    the best suited to practical usage.
    Having set the index to zero, mounted the telescope, and turned down
    shades as necessary, or affixed the telescope screen, to counter the
    glare, sight the sun and move the index forward (on the arc) until the
    direct and reflected rim edges of the sun be in contact - now read off
    minutes and seconds "on the arc".
    Next, move the index back (off the arc) until the contrary rims of the
    sun are brought into contact, and read off the minutes and seconds "off
    the arc".
    Half the difference between the foregoing two readings will be the index
    error, independent of any tabular values or other calculations. If the
    sun's diameter be greater on the arc than off the error is subtractive,
    or vice versa. Remember always the old and reliable adage - "when it's
    on, it's off", and "when it's off, it's on".
    Further advantages of this simple method are ....
    1) As the direct and reflected images are passed one over the other, any
    horizontal separation of limbs immediately signals a lack of
    perpendicularity, in either the horizon or index mirrors, to the plane of
    the instrument and demands further investigation of each mirror
    2) By alternating the screens, both at horizon and index mirrors, the
    error (due to a lack in parallelism of the screen surfaces) induced by
    various combinations may be tabulated.
    Otherwise ...
    I have for years used the method of bringing second and third magnitude
    stars into coincidence, also to determine index error. As far as my
    experience is concerned, there has always been an apparent range of
    uncertainty in establishing exact coincidence, necessitating an averaging
    of both on and off the arc measurements in an effort to negate, to the
    extent possible, any error induced by this uncertainty; I have not found
    this uncertainty of contact applicable when using the sun's limbs. Of
    course, any separation of direct and reflected star images in the
    horizontal plane again signals perpendicularity problems. Also in this
    method, a coincident assessment of shade error is not so conveniently
    provided, as when utilizing the sun.
    The sea horizon provides probably the most convenient and least
    complicated method of establishing index error, as well as checking
    perpendicularity but, depending on conditions, may also be the least
    accurate. Regardless, a careful navigator should check his instrument by
    the sea horizon before each sight, or set of sights, as well as perhaps
    thereafter, to insure no significant erroror change thereof, through
    accident or otherwise, since the previous use.

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