Sky quality reference, Bortle scale guide & filter strategy for your site
Light pollution is artificial sky brightness from cities and towns that washes out stars and faint objects. This page tells you exactly what you can see from your sky — and what filters or strategies help. Start by clicking a Bortle class on the reference cards below.
A 1–9 scale describing sky darkness. Bortle 1 is a truly exceptional dark sky (remote wilderness). Bortle 9 is the inner city. Most suburban observers live under Bortle 5–7. Moving to a darker site is the single biggest improvement you can make.
Measures sky brightness in magnitudes per square arcsecond (mag/arcsec²). Higher = darker. A typical suburban sky is ~20 mag/arcsec²; a dark sky site is 21.5+. The scale is logarithmic — each 1-unit difference is ~2.5× in brightness.
The faintest star visible to the naked eye from a given sky. Under Bortle 5, you may see stars to magnitude 5.6. Under Bortle 1, up to magnitude 7.6. Each Bortle class roughly corresponds to about 0.5 mag difference.
Light domes appear as bright patches on the horizon caused by distant cities. Skyglow refers to the general brightening of the entire sky from scattered artificial light. Both reduce contrast for faint objects even in "rural" locations.
A full moon can effectively raise your sky brightness by 2–3 Bortle classes for faint diffuse objects. Even a 30% illuminated moon significantly impacts deep-sky work. Plan imaging sessions around the new moon (within ±5 days).
Narrowband filters (Ha, OIII, SII) pass only specific emission wavelengths, rejecting most skyglow and moonlight. These are highly effective from light-polluted sites for emission nebulae. Broadband imaging (RGB/LRGB) suffers significantly under Bortle 5+ and needs very dark skies.
Use the map's distance rings to see how far you'd need to drive. Look for the direction on the map with the darkest colour (deepest blue or black).
| Location | Bortle | Best Use | Notes |
|---|---|---|---|
| Home / Current | — | Planets, clusters | Your baseline |
| Local Park | ~5 | Clusters, globulars | Away from streetlights |
| Rural Site | ~3–4 | Nebulae, galaxies | 45–90 min from city |
| Dark Site | ~2 | Full Milky Way | Star party location |
Switch datasets above to visually compare years. Most populated areas show Bortle 1–2 class worsening since 2016. Rural and protected areas may be stable.
Click the card that matches your sky's colour on the map above. This sets your Bortle class and updates all the guidance panels. Not sure? Pick Bortle 5 — it's the most common suburban sky.
