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    Re: Navy MK 5 Octant Using Natural Horizon
    From: Greg Rudzinski
    Date: 2012 May 1, 15:57 -0700

    Alex,

    To answer this very good question you asked:

    "But you did not answer my question: how did you choose the preset values?
    And why they were such as we see them."

    The preset is not picked. The Sun's lower limb is dropped to within a third of a solar diameter from the horizon. The micrometer drum is then inspected and moved to the nearest mark. Now back to business with no fingers on the micrometer drum. When the lower limb touches the horizon in perfect tangency time is marked. I look at my watch subtracting one second of time for the time it takes for the eye to go from scope to watch.

    Greg Rudzinski

    P.S. There are U.S. Navy MK 5's without the averager. They are lighter and compact so would be more practical on a small craft. The whole works would fit nice in a large camera bag.


    [NavList] Re: Navy MK 5 Octant Using Natural Horizon
    From: Alexandre Eremenko
    Date: 1 May 2012 18:17
    Greg,

    > The results are not extraordinary in that any data set from
    > any sextant can be made to look good by adjusting the index error.

    Does your octant have adjustment for the index error?
    I understand that with ordinary sextants the index error can be
    adjusted. The problem is to determine it with 0.2 accuracy:-)

    Besides, we know that even good ordinary sextants can have substantial
    arc error, etc. Which is again hard to find.

    Now, the 0.2 average error in your 30 observations shows that the
    index error as you determined it is very close to the truth.
    Did you adjust it? I supose not, (otherwise why would you adjust
    it to 6')

    > is why I look at the maximum and minimum values of a set
    > of data to judge a sextant and or navigator performance.

    The maximum spread of 2.7 shows that the errors are all less then
    1.2 or 1.4 in magnitude.

    But you did not answer my question: how did you choose the preset values?
    And why they were such as we see them.

    Alex.

    > Keep an eye open on ebay for U.S. Navy MK 5's which can be
    > had for under $100 but use caution on the buy.

    I don't know how can one "use caution" with a sextant on e-bay,
    especially with an air sextant:-)
    You can only take the risk.
    99.9% of e-bay sellers would say in advance that they know nothing about
    sextants, which is true, about half of them photograph a normal sextant
    upside down. Once I asked a seller what is the radius of the arc, and his
    reply was that he is not sure what the "radius" is :-)
    So I do not expect them to answer such questions as whether the bubble is
    present or not. I simply thought that $100 or $140 that I payed is a sum
    I can afford to risk:-)

    I was lucky: the only thing which did not work was the light,
    and I found how to fix it and fixed in few hours.
    But repairing a bubble is definitely beyond my possibilities:-(

    Alex.

    P.S. By the way, you promote this sextant for use at sea.
    Is it possible to detach the averaging mechanism? It is a general
    agreement that the averager is of no use at sea, while it takes most
    weight and bulk. Ken told me that he just sawed it off from his
    MkIX A, thus converting them to MkIX.


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