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    Re: Navy Mark V Sextant, Accuracy
    From: Ken Gebhart
    Date: 2004 Dec 24, 15:07 -0600

    on 12/23/04 10:34 PM, Peter Henry at peterghenry@EARTHLINK.NET wrote:
    > I would like to receive some advice on two questions regarding the standard
    > accuracy with this sextant.  I have been using the instrument occasionally
    > for approximately 1 year and learned to use a sextant from books.  I have
    > done calculations by hand but also with the computer program mac nav.
    > I find that I get wildly varying accuracy from the most accurate of 1.5nm to
    > 30nm approximately.  I will also take multiple sites and have a varying
    > range of 15 nm between the sites.
    > 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?
    > Secondly can a magnifying scope be added to this sextant to increase ones
    > ability to precisely place the object on the horizon?
    > What is the standard level of accuracy for this sextant?
    > I also find that I must refill the bubble level every few months, that
    > initially after filling the bubble I can change it's size up and down, but a
    > few months out I can no longer reduce the bubble size.  Obviously the bubble
    > or one of the storage reservoir is leaking but I see no fluid though the
    > sextant has a distinct smell which I assume is the fluid being turned into a
    > gas.  How do I find and fix this leak?
    > Peter Henry
    You are centering the bubble correctly, but the bubble should be about twice
    the size of the sun.  You should not put the body in the center of the
    bubble because there could be distortion as the light passes through the
    bubble.  The reason for this is that unlike all other aircraft sextants, the
    Mark V bubble touches only the top surface of the chamber instead of both
    top and bottom.
    It sounds like you are taking some sights with the bubble and some with the
    natural horizon prism.  They could have much different correction values if
    the sextant has not been calibrated properly.  This could be the cause of
    your 30nm error.
    If you are filling the bubble chamber (with xylene) through the pin at the
    rear, you could put some paint or glue around where the pin enters to act as
    a seal, and see how it goes.  For a more lengthy discussion about other
    possibilities, and accuracy, you may phone me at +1-316-686-9785.
    Ken Gebhart

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