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The Bortle scale for astrophotography
Updated 20 October 2026 · By William Dutton, developer of Night Logic: Astrophotography
The Bortle scale rates sky darkness from 1 (pristine) to 9 (inner-city). For astrophotography it is the biggest factor in how long you can expose, how much data you need, and which targets are realistic.
The short answer. The Bortle scale ranks sky darkness from Class 1 (pristine dark sky) to Class 9 (inner-city). Every step brighter shortens the useful sub-exposure, raises the total integration you need, and pushes you toward brighter targets or narrowband filters.
What the Bortle scale is
John E. Bortle published the scale in Sky & Telescope in 2001 as a way to describe how dark a site is, using what you can actually see: the Milky Way’s structure, whether M33 is visible by eye, how bright the horizon glows, and the faintest star you can spot. It is a 1–9 scale where lower is darker.
The nine classes
Limiting magnitudes are approximate naked-eye values; different sources vary slightly.
| Class | Name | Naked-eye limit | What it means for imaging |
|---|---|---|---|
| 1 | Excellent dark-sky site | 7.6–8.0 | The best. Faintest targets are in reach; subs can run long. |
| 2 | Typical truly dark site | 7.1–7.5 | Excellent. Milky Way casts faint shadows; almost no airglow concerns. |
| 3 | Rural sky | 6.6–7.0 | Very good. Light domes visible on the horizon but little effect overhead. |
| 4 | Rural/suburban transition | 6.2–6.5 | Good. Most deep-sky targets are fine with a few more hours. |
| 5 | Suburban sky | 5.6–6.1 | Fair. The Milky Way is weak; budget more integration time. |
| 6 | Bright suburban sky | 5.1–5.5 | Challenging. Milky Way only near the zenith; filters start to matter. |
| 7 | Suburban/urban transition | 4.6–5.0 | Hard. Short subs, narrowband for nebulae, lots of data for galaxies. |
| 8 | City sky | 4.1–4.5 | Very hard. Planets, Moon, bright clusters and nebulae; narrowband is the main tool. |
| 9 | Inner-city sky | ~4.0 or brighter | Only the brightest objects show naked-eye. Same strategy as 8, with more patience. |
How Bortle class changes your exposures
- Sub length. A bright sky reaches the sky-limited point quickly, so shorter subs lose almost nothing and reduce saturation. Dark sites let you go longer. See how long to expose.
- Total integration. The sky background adds noise of its own, so the same target needs much more total time under a bright sky. Moving from Bortle 4 to Bortle 7 can easily mean several times the data for similar quality.
- Target choice. Under Bortle 7–9, favor bright, compact targets such as globular clusters, planetary nebulae, and the cores of big emission nebulae. Leave faint galaxies and reflection nebulae for darker sites.
- Filters. Dual-band and narrowband filters are the most effective tool under bright skies for emission nebulae, because they pass only the narrow wavelengths the nebula emits.
How to find your Bortle class
- Use a light-pollution map. Sites like lightpollutionmap.info and Light Pollution Atlas show a colored overlay you can match to a Bortle class for any location.
- Measure it. A Sky Quality Meter (SQM) reports sky brightness in magnitudes per square arcsecond. Higher numbers mean darker skies; roughly 21.5–22 is a pristine site.
- Judge it by eye. Check the Bortle description: can you see the Milky Way, how many stars are in the Little Dipper, is M33 visible?
- Let your planning app estimate it. Night Logic derives a Bortle class from your observing location and applies it to every score, and you can override it if you have measured your own site.
Is it worth driving to a darker site?
Almost always, for faint targets. Dropping two Bortle classes is the single most effective “upgrade” available to most imagers, often worth more than a new camera. Many imagers keep two saved locations: home for bright targets and narrowband, and a dark site for galaxies and faint nebulae.
Frequently asked questions
What Bortle class do I live in?
Most people in a city center are in Bortle 8–9, most suburbs are Bortle 5–7, and rural areas are typically Bortle 3–4. For your exact spot, check a light-pollution map such as lightpollutionmap.info or Light Pollution Atlas, or let an app like Night Logic estimate it from your coordinates.
Can I do astrophotography under Bortle 8 or 9 skies?
Yes, with realistic goals. The Moon, planets, bright star clusters, and bright nebulae like Orion are all workable. Narrowband filters help emission nebulae a great deal, and long total integration can pull out galaxies. Faint, low-surface-brightness targets are the hardest.
Do light pollution filters work?
They help in specific cases. Narrowband and dual-band filters are very effective for emission nebulae because they block most of the skyglow. Broadband “light pollution” filters help less, particularly for galaxies and star clusters, which emit across the spectrum. They are not a substitute for a darker site.
How accurate is the Bortle scale?
It is a rough, subjective scale from 1 to 9 based on what you can see by eye. It is useful as a shorthand but imprecise. Sky Quality Meter (SQM) readings give a measured number in magnitudes per square arcsecond, which is more objective.
Night Logic: Astrophotography is a free iPhone app that ranks tonight’s targets for your telescope, mount, and camera, and builds an exposure plan for each one. Android is coming soon to Google Play.