The Bortle scale, explained for astrophotographers

The Bortle Scale is a 1-9 ranking of sky darkness created by amateur astronomer John Bortle in 2001. It's the single most useful number for planning where to image — more useful than zenith brightness in mag/arcsec² for most decisions.

The classes, briefly

• Bortle 1 — Excellent dark sky. Milky Way casts visible shadows. Zodiacal light extends past zenith. Galactic structure visible.
• Bortle 2 — Typical truly dark site. M33 naked-eye averted vision. Milky Way still casts shadows on the brightest portions.
• Bortle 3 — Rural sky. Milky Way structure visible overhead. Faint light domes from towns on horizon.
• Bortle 4 — Rural / suburban transition. Galaxy faint but present overhead. M31 obvious naked-eye.
• Bortle 5 — Suburban. Milky Way still detectable but washed-out. Faint nebulae lost. Decent visual planetary; deep-sky imaging works but needs filters.
• Bortle 6 — Bright suburban. Milky Way visible only directly overhead, if at all. Deep-sky imaging needs narrowband filters.
• Bortle 7 — Suburban / urban. Sky dome glowing. Only Messier brightest visible visually. Solar-system imaging mostly fine; deep-sky needs Ha/OIII/SII.
• Bortle 8 — City. M31 / M42 maybe with binoculars. Imaging restricted to planets and narrowband.
• Bortle 9 — Inner city. Only the moon, planets, and a handful of bright stars visible. Deep-sky imaging effectively impossible.

What Bortle is NOT

It is not a forecast — Bortle is the structural darkness of the site. A Bortle 2 site with 100% cloud cover is still a useless night. Always combine Bortle with current cloud-cover forecast and moonless dark-window length.

Using Bortle in StarryPic

The light-pollution map shows Falchi 2016 sky-brightness which we map to Bortle classes. The dark-site reviews report observed Bortle from community contributions — which can differ from atlas data when sites have changed.