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So is it possbile to build a system from lenses and mirrors that can make faint gas nebulas brighter or can be used as nightvision?

If you increase the size of the aperture of a telescope it will collect more light, but the exit pupil will be also bigger, so not all light will enter the eye.

In order to direct all light into the eye you'll need to shrink the exit pupil and you'll need a stronger eyepiece. But this will increase the magnification too and the collected light will spread on a larger image so the surface brightness will remain the same.

Is it possible to work this limitation around?

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Ok, I've found this:

http://www.cv.nrao.edu/course/astr534/Brightness.html

I proves that it's not possible to build such optical system.

The conservation of brightness also applies to any lossless optical system, a system of lenses and mirrors for example, that can change the direction of a ray. No passive optical system can increase the specific intensity or total intensity of radiation. If you look at the Moon through a large telescope, the Moon will appear bigger (in angular size) but not brighter. Many people are disappointed when they see a large, nearby galaxy (e.g., Andromeda) through a telescope because it looks so dim; they expected to see a brilliantly glowing disk of stars, as in the photograph below. The difference is not in the telescope; it is in the detector—the photograph appears brighter only because the photograph has summed the light over a long exposure time.

Though the Andromedia example is not necessarily correct... Because it consists of many stars which have huge surface brightness. I think if we could have large enough aperture can could resolve Sirius as a disk it would be more eye damaging sight than the Sun...

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It is not possible. If you could build such an optical system, you would have a perpetual motion machine of the second kind.

Suppose you would have a passive (no batteries allowed) optical system capable of creating images brighter than the source at which you aim it. You aim this device at the sun and thereby create an image brighter and hotter than the sun. That goes against the second law of thermodynamics which tells us that entropy increases and therefore heat 'evens out'.

The concept of entropy translates into a key optical concept: 'etendue'. When light propagates through an optical system, the etendue can not decrease. It basically is the second law of thermodynamics applied to optics.

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