It depends on the target, your sky quality and your filters. As a rule of thumb, bright galaxies like M31 need 2-4 hours of broadband integration and emission nebulae need 4-8 hours (rule of thumb, 2026-09). Narrowband needs roughly double, and Bortle 6 skies need roughly 2x the time of Bortle 3-4 skies (rule of thumb, 2026-09).
Total integration time matters most, because noise drops as total hours grow. Choose a sub length your mount can repeat reliably for all those hours, then keep collecting subs until you reach the total (rule of thumb, 2026-09).
Plan the hours first, then pick a sub length your equipment can hold for those hours.
| Target type | Broadband minimum | Narrowband minimum |
|---|---|---|
| Bright galaxies like M31 | 2-4 h | 6-10 h |
| Emission nebulae | 4-8 h | 8-20 h |
| Small planetary nebulae | - | 6-12 h |
| Dark nebulae | 4-8 h | - |
Rule of thumb values, 2026-09. A dash means the combination is uncommon for that target type.
Light pollution raises the noise floor, so the same target needs more integration under bright skies. Rule of thumb: Bortle 6 skies need roughly 2x the integration of Bortle 3-4 skies for the same result (rule of thumb, 2026-09).
In practice this means the same object is ready after one night under a dark sky and needs several nights under a city sky.
Two hours is enough at the low end of the broadband ranges under dark skies, for example bright galaxies like M31 at the start of its 2-4 h range (rule of thumb, 2026-09). For narrowband targets, plan from 6-10 h upward (rule of thumb, 2026-09).
