Next message: Geoffrey Kolbe: "Re: Night Vision Scopes"
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 ,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.
>
>
