Assess your observing site's light pollution from an SQM reading with Bortle class and observing recommendations.
Enter a Sky Quality Meter reading between 15 and 22 magnitudes per square arcsecond. The index converts it to a Bortle class, then adds practical guidance for both visual observers and astrophotographers.
Two limiting magnitudes are reported: the naked-eye value tied to your Bortle class, and a camera value about 2 magnitudes deeper, reflecting how long exposures accumulate light that the eye cannot.
The recommendation line sets expectations by class: every deep-sky object is fair game through Class 3, bright nebulae and galaxies remain workable through Class 5, Classes 6–7 narrow the menu to bright clusters and double stars, and beyond that stick to the Moon and planets.
It uses the same SQM-to-Bortle conversion but adds imaging-oriented outputs: an estimated camera limiting magnitude about 2 magnitudes deeper than the visual one, plus a per-class recommendation of which target types remain worthwhile from your site.
A sensor accumulates photons for the entire exposure while the eye refreshes in fractions of a second, so long exposures pull faint stars out of the background. The fixed 2-magnitude offset used here is a practical rule of thumb, not a guarantee for any particular setup.
At Class 5 the index still recommends bright nebulae and galaxies — think M42, M31, and the brighter Messier objects. By Classes 6 and 7 it narrows to bright star clusters and double stars, which punch through skyglow far better than diffuse targets.
Readings approaching 22 mag/arcsec² mark the darkest sites on Earth — this index requires at least 21.99 for Bortle 1, where it reports a naked-eye limit of magnitude 7.6 and rates the site excellent for all deep-sky objects. Suburban skies typically meter in the 19–21 range.