Optolong L-eXtreme Filter
A dual narrowband filter passing 7nm bands at Hydrogen-alpha and Oxygen-III, and blocking nearly everything else, including most city skyglow. That is what makes emission nebulae workable from a Bortle 8 balcony. The trade is exposure time, because rejecting that much light means much longer sub-exposures.
The filter that makes a city balcony viable
Emission nebulae do not glow across the whole spectrum. They emit at specific wavelengths, principally hydrogen-alpha at 656nm and doubly-ionised oxygen at 501nm. Skyglow, by contrast, is broadband. That asymmetry is the whole opportunity.
The L-eXtreme passes two 7nm windows centred on those two lines and rejects almost everything else. Most of what a Bortle 8 sky throws at the sensor is simply not allowed through, while the signal you actually want passes largely intact.
What it costs you
Blocking that much light means the sensor sees far less of it. Sub-exposures have to get longer, and total integration time has to grow, sometimes substantially. A filter like this does not make imaging from a bright sky easy. It makes it possible, in exchange for time.
It is also useless on the wrong target. Galaxies, star clusters and reflection nebulae shine by broadband starlight, and a dual narrowband filter throws most of that away. Putting an L-eXtreme in front of a galaxy is a wasted night.
Stars go strange, and that is expected
A common surprise for first-time users is what happens to stars. With only two narrow windows of colour reaching the sensor, star colour is largely gone and bright stars can look bloated or oddly magenta. This is not a defect. It is the predictable consequence of discarding most of the spectrum, and it is normally handled in processing rather than at capture.
7nm against 3nm
Tighter is not automatically better. A 3nm filter rejects more skyglow and also cuts more signal, so it demands longer integration and is less forgiving of a fast optical system, where the steep light cone shifts the effective passband. The 7nm L-eXtreme is the more tolerant choice.
Who it suits
Anyone imaging emission nebulae from a light-polluted location with a colour camera. From genuinely dark skies the case is weaker, and broadband work will give better results on more target types.