Welcome to the NavList Message Boards.

NavList:

A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

Compose Your Message

Message:αβγ
Message:abc
Add Images & Files
    or...
       
    Reply
    Re: Night Vision Scopes
    From: Gary LaPook
    Date: 2005 Jul 10, 19:03 -0700

    I guess then that one cannot see extremely dim objects with the 200 inch
    mount palomar telescope for the same reasons that you mentioned.
    
    Gary LaPook
    
    george huxtable wrote:
    
    > Brooke Clarke wrote-
    >
    >> 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 7x 50 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.
    >
    >
    > What Brooke says about such an optical telescope (and it applies just the
    > same to "night binoculars") is quite correct.
    >
    > But there's an additional point to be made about such optical devices,
    > which is often not appreciated, bur was touched on in earlier
    > discussion of
    > this topic on Nav-l. It's this-
    >
    > No night-glass or telescope or any other such device can do anything to
    > enhance the brightness of a night-scene at the retina, to be any greater
    > that what the naked-eye itself sees.
    >
    > A "night-glass", as Brooke explains, has a big enough objective to
    > collect
    > all the light that will go into the enlarged pupil of a dark-adapted eye,
    > given a certain magnification. In that respect, it's better than a
    > "day-glass", which has a much smaller objective for the same
    > magnification,
    > but is still quite big enough to collect all the light that can go
    > into the
    > much-smaller eye-pupil in daylight (only about 2mm dia. as opposed to
    > 7mm).
    > In daylight, both these oculars will perform exactly the same. Only at
    > night will the night-glass do better. But even then, what you see in a
    > night-glass is no brighter than what you can see without it. In fact,
    > it's
    > somewhat less bright, because of the light-loss inherent in passage
    > through
    > the glass surfaces.
    >
    > To take Brooke's example, a x7 night-glass with a 49 mm. objective can
    > collect all the incident light falling on it and compress it into a
    > narrow
    > pencil 7mm. dia, just big enough to fill the pupil of a dark-adapted eye.
    > If the objective was bigger than 49mm, then that outgoing pencil would be
    > wider than 7mm.,  and light would be wasted in striking the iris rather
    > than in passing through the hole. The ratio between the diameters of the
    > incoming pencil of parallel light (defined by the size of the objective)
    > and the outgoing beam exiting the eyepiece is exactly the same as the
    > magnification of the ocular, 7x in that example. Indeed, that's a
    > valid and
    > simple way to measure the magnification. It's universally true, and
    > doesn't
    > depend in any way on the details of the optical design.
    >
    > If we neglect any light loss in transit through the glass or in crossing
    > its surfaces, then the night-glass collects 49 times as much light-energy
    > to pass into the pupil, compared with the light-energy that would
    > enter the
    > pupil without the night-glass, simply because of the 49x increase of
    > area.  That light now forms an image in the retina. Because of the
    > magnification of x7, every object, focussed on the retina, occupies
    > 49x the
    > retinal area than it did without the glass. So the light-energy per unit
    > area on the retina, which is the definition of brightness, is no greater
    > with the glass than without it.
    >
    > This conclusion seems to contradict common experience. I agree that when
    > you approach a dark harbour, searching for unlit moored craft, a
    > night-glass certainly SEEMS to help. In fact, it helps by making the
    > images
    > bigger, rather than brighter. Surprising, but true. That conclusion
    > surprised me when the question arose, when last discussed on this list.
    >
    > The only way to increase the surface brightness of an image, then, is
    > with
    > a device that can actually feed additional energy, such as the
    > night-vision
    > scopes that Brooke refers to.
    >
    >> Earlier, 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.
    >>
    >
    > From the argument presented above, I suggest that Alex is wrong about
    > the  Galilean "night vision scope" he describes; at least, in comparison
    > with the light-collection of the naked-eye. It would certainly be better,
    > at night, than a "day-glass" with a smaller objective, however.
    >
    > It also seems to me unlikely that his prediction on a "pure theoretical
    > ground" of the inefficacy of an amplifying "night vision device" will
    > hold
    > water. It seems unreasonable for Alex to ask readers to verify such a
    > questonable proposition, before he has presented any arguments in its
    > support.
    >
    > George.
    > ===============================================================
    > 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.
    >
    >
    
    
    

       
    Reply
    Browse Files

    Drop Files

    NavList

    What is NavList?

    Join NavList

    Name:
    (please, no nicknames or handles)
    Email:
    Do you want to receive all group messages by email?
    Yes No

    You can also join by posting. Your first on-topic post automatically makes you a member.

    Posting Code

    Enter the email address associated with your NavList messages. Your posting code will be emailed to you immediately.
    Email:

    Email Settings

    Posting Code:

    Custom Index

    Subject:
    Author:
    Start date: (yyyymm dd)
    End date: (yyyymm dd)

    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site
    Visit this site