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    Re: Mirror problem
    From: W F Jones
    Date: 2006 May 5, 21:40 -0400

    I bought a USN Mark III sextant (actually two sextants) about fifteen years ago from
    a private source that had acquired them directly from the US government in an
    auction.  The sextants had been disposed of by the US Army.  I was told the Corps of
    Engineers actually used these on a training vessel located on one of the Great
    Lakes.  I immediately knew one sextant had been dropped and the other appeared in
    good condition.  As far as I know, the USN may still buy the same or similar sextants
    from various contracting companies (OEMs) willing to meet the navy's requirements.
    I am sure the demand is low and purchase history sparse.
    My sextant was actually manufactured by Scientific Instruments Inc
    (www.scientif.com).  I think they are now out of the sextant business but still active in
    the survey instrument business.  Oh, I paid way too much for the sextants then but
    they are rather rare and I had to have one.  I have only heard about one or two for
    sale in the last ten years.
    Yes, a special optical sextant tester is used.  It is a large and very impressive device
    manufactured by C. Plath (Germany).  Doubtful that many testers have ever been
    constructed.  There is a sketch in my sextant manual of one.  I have heard that it is
    difficult to position the sextant correctly and some companies only want to test their
    own instruments.  I checked with Scientific Instruments when I acquired the sextants
    about re-certification and they said they had a tester and could re-certify it.  I recall it
    cost almost $200 US for the process.  A replacement mirror set was about $60 then.
    Scientific Instruments estimated a couple of weeks but it took much longer - I
    assume it took them that long to verify that I legally owned the instrument.
    If you look at the arc scale, you will see that it is calibrated for -5 to +125 degrees.
    All marine navigators should know how to do a back-sight.  Personally, I don't like
    high altitude or back-sights because they are simply more difficult to execute.  Still,
    you may encounter a situation where you have no choice but to take a back-sight.
    Sometime, attempt to measure a body with an angle larger than 90 degrees?  The
    body will be behind you!
    Total error across the Mark III arc scale is specified as less than 18 seconds.  That
    means the difference between the most negative and most positive error recorded
    for all measured angles.
    The horizon and index filter specifications may interest some list members.  I will list
    the transmissivity factor (?) below.
    I don't like the idea of using only the filters with a laser.  Seems to me, you should
    use a laser eye protector (like glasses or googles) in combination with the filters.  I
    am very concerned about the telescope in the optical path however.  Do consult an
    expert on this matter first before using one.  I was interested for awhile in
    constructing an artificial star light source.  I just couldn't seem to get it right and never
    found knowledgeable references to support the effort either.
    Horizon Filters
    No. 2     0.012%
    No. 3     0.24%
    No. 4     6.3%
    Index Filter
    No. 1    0.0008%
    No. 2    0.012%
    No. 3    0.24%
    No. 4    6.3%
    I apologize if I strayed off course a little more than necessary.
    I envy your mathematical abilities and most of all, endless patience with the
    diversified crew we make up.  Thanks for sharing your interesting pursuits with the
    Frank J.
    Rochester, NY
    Date sent:              Fri, 5 May 2006 13:44:42 -0400
    Send reply to:          Navigation Mailing List 
    From:                   Alexandre E Eremenko 
    Subject:                Re: [NavList 120] Mirror problem
    Dear Frank J.,
    Your message contains a lot of interesting
    information, besides the mirror story.
    What is "Original Equipment Manufacturer"?
    What "metal sextant" did you send to them?
    Your message makes an impression that they can
    make or alter a sextant on your request.
    Do they have special equipment for testing sextants?
    You also mention back-sights...
    I've never heard of a sextant equiped for taking back
    sights since the beginning of XIX century!
    Could you tell us more about your sextant and OEM?

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