How to Read a Bortle Scale
The Bortle scale rates how much artificial light pollution washes out a night sky, from 1 (the darkest skies on Earth) to 9 (inner-city). It was designed for naked-eye and visual observing, but it's just as useful as a shorthand for what's realistically imageable through a camera. The same sky glow that hides the Milky Way from your eyes also raises the background noise floor a sensor has to image through.
The nine classes, roughly grouped
| Bortle | What it looks like |
|---|---|
| 1 | Excellent dark-sky site |
| 2 | Typical truly dark site |
| 3 | Rural sky |
| 4 | Rural/suburban transition |
| 5 | Suburban sky |
| 6 | Bright suburban sky |
| 7 | Suburban/urban transition |
| 8 | City sky |
| 9 | Inner-city sky |
StarYard groups these into three practical tiers when it flags a concern on a target:
- Bortle 1–4 (Low concern) : Dark enough that light pollution rarely limits what's worth imaging; other factors (Moon phase, seeing) usually matter more.
- Bortle 5–6 (Moderate concern) : A typical suburban sky. Bright, compact targets (galactic cores, planetary nebulae, star clusters) still image well; large, faint emission nebulae get noticeably harder.
- Bortle 7–9 (High concern) : Urban skies. Faint extended targets are largely washed out; narrowband filters targeting specific emission lines can still cut through better than broadband imaging can. When you add a filter to your equipment, this will be taken into account.
How StarYard gets your number
When you add a location, StarYard estimates its Bortle class automatically from satellite-measured night-sky radiance data. If you've measured your own sky (or you know your site better than a satellite does), you can override the estimate for that location.
Why it affects your results
Your location's Bortle class feeds directly into the object suitability scoring on the Results page. It's one of the reasons a target might be flagged as a harder image that night, alongside altitude, Moon separation, and forecast sky conditions (see Seeing vs. Transparency for the other side of that: even a dark site can have a rough night for imaging).