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    Re: Night Vision Scopes
    From: Brooke Clarke
    Date: 2005 Jun 27, 11:10 -0700

    Hi Alex:
    The devices commonly called "Night Vision" are light amplifiers, not IR
    based scopes like were used in W.W. II.  They take the existing light
    and make it brighter.  Although I have not done it, I expect that in the
    middle of an ocean on a moonless night you could in fact see the horizon
    with a night vision scope.
    Astronomers call the optical type of scope you describe a "rich field"
    scope.  That means that the exit pupil diameter is about 7 mm, which is
    the diameter of a night adapted eye.  For example a 7x50 binocular has
    an objective diameter of 50 mm which when divided by the 7 power
    magnification yields about 7 mm exit pupil.  Any scope whose objective
    diameter divided by it's magnification that yields about 7 mm is good
    for viewing with a dark adapted eye.  For daytime use where the eye's
    pupil is only a few mm diameter you can use a scope with a smaller exit
    pupil diameter.
    Have Fun,
    Brooke Clarke, N6GCE
    w/Java http://www.PRC68.com
    w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
    Alexandre Eremenko wrote:
    > I have never used a night vision device,
    > but on a pure theoretical ground I predict
    > that it will NOT help to see the sea horizon:-)
    > (And for the stars and the Moon you don't need
    > any night vision anyway).
    > Can anyone verify this theoretical prediction?
    > :-)
    > Alex.
    > P.S. I mean the common modern night vision devices based on infrared
    > radiation. Another tipe of "night vision scope" was invented
    > in XVIII century, and this was simply a Galileo scope
    > with small magnification and with large
    > object lens diameter. (Approximately of the same type as
    > the standard straight non prismatic scopes of modern sextants).
    > These scopes indeed help with horizon or any other object at
    > night simply because they collect more light.
    > Alex.
    > On Sat, 25 Jun 2005, george huxtable wrote:
    >>Robert Eno wrote-
    >>>Has anyone used a night vision scope for conducting star sights at
    >>>sea?  The idea intrigues me and in theory it sounds like it would work,
    >>>but I do not know anyone who has actually used one at sea.
    >>An interesting question. Presumably you would use it in place of the
    >>telescope, to show up both star and horizon. Otherwise, how well would the
    >>sight-line of such a night scope be defined? And how would you check for
    >>index error?. I only ask, because I am quite unfamiliar with such instruments.
    >>Could a digital camera be used for the same purpose? Amateur astronomers
    >>use quite ordinary arrays for looking at stars.
    >>I think one of the difficulties would be the fast motion of the images with
    >>respect to the screen as the vessel wobbled underfoot. The human eye is
    >>good at assessing the relative positioning of star and horizon, even while
    >>they are whistling across his field of view. An electronic screen could
    >>find that job to be more difficult. Perhaps some image stabilisation would
    >>be needed also.
    >>But a sextant with lots of electronics attached: wouldn't that be getting
    >>the worst of both worlds? The most satisfying aspects of a sextant are its
    >>precision and its basic SIMPLICITY.
    >>Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222,
    >>or from within UK 01865 820222.
    >>Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
    >>5HX, UK.

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