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Light pollution and the Bortle scale
Light pollution brightens the sky background and adds noise, so faint objects need much more stacking time from a city than from a dark site. Bright nebulae and clusters still work well from town, especially with a dual-band filter; faint galaxies benefit most from darker skies.
What light pollution does to a photo
Street lights and buildings light up the air above a town. That glow lands on every pixel, on top of the light from the nebula. A stacking app can subtract the average glow, but not its noise: that randomness stays and hides faint detail. The only cures are more exposure time, a filter for the right kind of object, or a darker sky.
The Bortle scale
The Bortle scale rates night skies from 1 (truly dark, the Milky Way casts shadows) to 9 (inner city, only a few stars visible). Most people in Western Europe live under Bortle 6 to 8. Moving from Bortle 8 to Bortle 4 cuts the sky glow by more than ten times.
- Bortle 1–3: dark rural sky, Milky Way full of structure.
- Bortle 4–5: countryside and outer villages, Milky Way clearly visible.
- Bortle 6–7: suburbs, Milky Way faint or only overhead.
- Bortle 8–9: city centres, only bright stars and planets.
The same 30 minutes under four skies
When a filter helps
Emission nebulae glow in a few narrow colours (hydrogen-alpha and oxygen III). A dual-band filter passes those and blocks most of the sky glow, so it helps a lot from town. Galaxies, star clusters and reflection nebulae shine in all colours like the sky glow itself, so a filter cannot separate them. For those, darker skies and more time are the answer.
Getting the most from a city sky
- Pick targets high in the sky: near the horizon you look through more polluted air.
- Start with bright nebulae and clusters, keep faint galaxies for darker trips.
- Use a dual-band filter on emission nebulae if your telescope has one.
- Plan longer sessions: twice the noise needs four times the frames.
- Avoid the Moon: a bright Moon acts like extra light pollution.
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