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    Re: Navy Mark V Sextant, Accuracy
    From: Paul Hirose
    Date: 2004 Dec 24, 13:31 -0800

    Peter Henry wrote:
    > I understand that the way to use this instrument is to center the bubble on
    > the cross in the field of view and then to bring the object down to a point
    > parallel to the horizontal line of the cross on either side of the bubble.
    > Occasionally with stars I can bring it down centered in the bubble due to
    > there small size.  I also understand that the bubble should be twice as big
    > as the object.  Is this the right way to use this sextant?
    It's not absolutely clear to me where you're placing the body and what
    bubble size you use, so I'll repeat what I recently posted in the
    Yahoo Sextants group:
    My 1955 edition of Air Force Manual 51-40, "Air Navigation", says
    "With the A-14 [aka AN5851-1 or Navy Mk V], the body is viewed
    alongside of the bubble, the bubble in the A-14 is opaque."
    Regarding collimation and bubble size, it says,
    "Collimation is effected when the body is placed squarely in the
    center of the bubble or horizontally centered alongside the bubble if
    it is the opaque type. The error will be small if the bubble is
    anywhere on the vertical line of the bubble chamber field, as long as
    it does not touch the top or bottom of the bubble chamber field. For
    greatest accuracy, the bubble should be in the center of this field.
    The illustration at the right shows examples of proper and improper
    [The illustration labeled BEST POSITION shows the bubble centered in
    the chamber and three possible positions of the body: centered in the
    bubble, or touching the bubble's 3 or 9 o'clock edge.]
    [The illustration labeled NEGLIGIBLE ERROR shows the bubble near the
    chamber's 12 or 6 o'clock edge, and the body within or alongside the
    bubble as described before.]
    [The illustration labeled LARGE ERROR shows various other scenarios.
    The only one I think is non-obvious has the bubble near the chamber's
    3 or 9 o'clock edge and the body within or alongside the bubble.
    Despite the good appearance, this is poor collimation.]
    To sum up, keeping the bubble and body near the vertical centerline
    and coincident or alongside each other gives the correct sight
    "Bubble size affects the accuracy of a sextant observation. The ideal
    situation is to have a small bubble for ease in determining the
    center; however, it is also desirable to have a bubble that is active,
    since too small a bubble is sluggish. Experience has shown that best
    results can be obtained with a bubble approximately one and a half
    times the diameter of the sun or moon."
    (You have to join the group in order to read the messages.)
    On land, I'd expect the Mark V to perform at least as well as my A-12
    (a naked eye instrument, no help to my mediocre eyesight). Of 30 sun
    lines I shot with that sextant in an accuracy test, about 2/3 were
    within 1.8 arc minutes of the correct value.

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