Understanding Transparency, Seeing & the Bortle Scale
1. Transparency
| Score | Rating, Description & Visual Test |
| 0 | Zero Worst case. Clouded out. Possible precipitation. No stars visible. |
| 1 | Very Poor. Cloud covering most of sky. Momentary patches of clear sky. |
| 2 | Poor. Patchy cloud or heavy haze. Alpha and Beta Crux visible. |
| 3 | Somewhat clear. Cirrus or Moderate haze. α, β, γ Crux visible. |
| 4 | Partly clear. Slight haze. α, β, γ, δ Crux visible. |
| 5 | Clear. No clouds. α, β, γ, δ, ε Crux visible. |
| 6 | Very Clear. No clouds, little sky glow, very little light pollution. Milky way visible. All stars of Crux visible plus Coal Sack. |
| 7 | Extremely Clear. Best Case. No clouds, minimal sky glow, Dark skies. ω Centauri and 47 Tucanae visible to naked eye. |
2. Seeing
| Score | Description |
| 1 | Perfect, no movement of the image even at high magnification (100X plus). Best Case. |
| 2 | Slight quiver with calm moments lasting several seconds. |
| 3 | Moderate quiver with larger tremors, blurring of image. |
| 4 | Poor constant undulation of image. |
| 5 | Very Bad not able to make even a rough sketch of image. Worst case. |
3. Bortle Scale
| Score | Description & Observational Magnitude Limit |
|---|---|
| 1 | Excellent dark-sky site. Best case. Constellations may be barely recognizable amid the large number of stars. Limiting magnitude with 12.5″ reflector is 17.5. |
| 2 | Typical truly dark site Milky Way is highly structured. Many Messier objects and globular clusters are naked-eye objects. Limiting magnitude with 12.5″ reflector is 16.5. |
| 3 | Rural sky Light pollution and clouds are evident at the horizon. The Milky Way still appears complex. Limiting magnitude with 12.5″ reflector is 16. |
| 4 | Rural/suburban transition Pollution domes visible in several directions. The Milky Way is still impressive, but lacks detail. Limiting magnitude with 12.5″ reflector is 15.5. |
| 5 | Suburban Sky Light pollution is visible in most directions. Clouds are noticeably brighter than the sky. The Milky Way is very weak or invisible near the horizon Limiting magnitude with 12.5″ reflector is 15. |
| 6 | Bright suburban Sky Light pollution makes the sky within 35° of the horizon glow gray. White clouds anywhere in the sky appear fairly bright. Background surroundings are easily visible. The Milky Way is only visible near the zenith. Limiting magnitude with 12.5″ reflector is 14.5. |
| 7 | Suburban/urban transitionSky Light pollution makes the entire sky light grey. Clouds are bright. Strong light sources are evident in all directions. The Milky Way is nearly or totally invisible. Limiting magnitude with 12.5″ reflector is 14. |
| 8 | City Sky The sky is light gray or orange. Worst Case. Stars forming familiar constellation patterns may be weak or invisible. Limiting magnitude with 12.5″ reflector is 13. |
| 9 | Inner-city Sky Worst Case. White. |
