NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Night Vision Scopes
From: Gary LaPook
Date: 2005 Jul 10, 19:03 -0700
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. > >