Starcast builds a sky quality score from six variables, then applies a target-specific modifier based on what you're actually trying to shoot. Bortle class is estimated from your distance to major light pollution centers.
Cloud Cover
The dominant variable. Even thin cloud blocks faint objects. Below 10% is ideal; above 50% is a serious penalty.
Moon Phase & Illumination
A full moon raises the sky background enough to wash out Milky Way and DSOs entirely. Penalised up to 18 points.
Bortle Class
Estimated from proximity to city cores. Bortle 1–3 adds no penalty; Bortle 7+ can remove 12+ points from the score.
Humidity & Visibility
Atmospheric moisture scatters light and reduces contrast. Low humidity and high visibility are critical for faint targets.
75–100 — Pristine. Rare. Near-perfect conditions.
55–74 — Excellent. Serious astrophotography possible. Go.
35–54 — Good. Wide field and planetary work well.
0–34 — Poor. Limited or no opportunity.
Bortle estimation is a heuristic based on city proximity, not a measured SQM reading. For verified dark sky data, cross-reference with lightpollutionmap.info or the Globe at Night project.