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    Re: 7/8 Scale Sextants More
    From: Joel Jacobs
    Date: 2004 Aug 2, 07:53 -0400

    Let me add something that I thought of after mailing.
    
    In boat and ship design when you scale something up, everything changes in a
    geometric progression rather than an arithmetic one.
    
    For example, one foot by one foot is one square foot, but if you increase
    the size to two feet squared, the result is four square feet or four times
    greater.
    
    George, you're the mathematician. Doesn't the reverse hold true when you
    shrink something?
    
    Joel
    
    
    ----- Original Message -----
    From: "Joel Jacobs" 
    To: "Navigation Mailing List" 
    Sent: Monday, August 02, 2004 7:39 AM
    Subject: 7/8 SCALE SEXTANTS
    
    
    > Hello George,
    >
    > Some additional comments on reduced scale sextants (7/8):
    >
    > The small size mirrors collect less light to pass through the optical
    system
    > and consequently make twilight observations of stars and planets more
    > difficult. There is another equally important disadvantage. When taking
    high
    > altitude LAN sights the small surface area of the mirrors allows the
    object
    > to jump off the mirror more easily than with large size mirrors.
    >
    > In respect to the telescopes on the reduced scale sextants, they have a
    > small objective and ocular lens. The Freiberger scope is about 2.5 x 25mm
    > compared to a normal 4 x 40 scope and passes less light through the
    system.
    > Again this makes observations of stars and planets more difficult.
    >
    > If there are any shooters amongst us, they may comment on the preferred
    > weight of a rifle. Some people believe that light weight rifles are harder
    > to hold on target, and less accurate even though carrying them is easier.
    > The same holds true of a sextant. A very light weight one is thought to be
    > more difficult to use.
    >
    > I'm not sure of this, but I think Tamaya was the first to introduce a 7/8
    > scale sextant around 1975. It was the MS 933 Venus which, when sold today
    > used, sells for about the same as a NEW Freiberger of the same size.
    > Freiberger's list price is $600.00. Here's a link to my listing of the
    > original Venus sextant. GO: http://stores.ebay.com/LAND-and-SEA-COLLECTION
    > It is the fourth one down. I don't know when Tamaya  discontinued
    production
    > of this model, but they appear on ebay fairly regularly and sell quickly.
    >
    > Here is a link to a European Freiberger Yachtsman Sextant site since
    > Claussen Instrument's is down.
    > http://www.busse-yachtshop.de/dae_Yachtsextant.html Note that 600 Euros is
    > about $720 over there.
    >
    > If you do mostly sun sight navigation, the small size and light weight
    might
    > appeal to some, but the 7/8 scale sextant has never had much of a
    following
    > by celestial navigators.
    >
    > Joel Jacobs
    >
    >
    > ----- Original Message -----
    > From: "George Huxtable" 
    > To: 
    > Sent: Monday, August 02, 2004 5:40 AM
    > Subject: Re: Celestaire vs Freiberger Yacht Sextant
    >
    >
    > > Joel Jacobs  wrote:
    > >
    > >
    > > >From my experience, if your going to do serious navigation relying on
    > > > twilight sights, the 7/8 scale sextants are very lacking. Their optics
    > are
    > > > not very good, and there small size mirrors are not anywhere as
    > effective
    > > > when taking star sights or high altitude sun sights.
    > > ===============
    > >
    > > Something about that puzzles me. Why should it be so, I ask?
    > >
    > > First, I should make it clear that I have never even handled such a 7/8
    > > size sextant; and that observations on my own small boat have never
    passed
    > > beyond a plastic job, though I have used many "posh" sextants, belonging
    > to
    > > others. So I make no claims to being an expert on sextants.
    > >
    > > Yet, it seems to me that if you were to shrink a sextant to 7/8 of its
    > > original size, and shrink its mirrors correspondingly (in both
    directions)
    > > while preserving the same angular field-of-view of its telescope, then
    > > (because the distance from the eye to those mirrors is also reduced to
    > 7/8)
    > > the patch of sky that the mirrors subtend would be exactly the same as
    > > before.
    > >
    > > So, in those circumstances, why should the smaller mirrors present any
    > > disadvantage?
    > >
    > > George.
    > >
    > > ================================================================
    > > contact George Huxtable by email at george---.u-net.com, by phone
    at
    > > 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    > > Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    > > ================================================================
    >
    
    
    

       
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