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?