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Plan

  • Tonight
  • Night Outlook
    • Weather
  • Targets
    • Saved targets
  • Plan a night
  • Night plans
    • Logbook

Tools

  • Sky atlas
  • Field of View
  • Exposure Lab
  • Calculators
  • Horizon Studio
  • Sequence Studio
    • Nights
    • Projects

Setup

  • Settings
  • About & Support
  • What’s new

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Sky & timeNew

Sky darkness and limiting magnitude

How faint can I see from my sky?

All 16 calculators
Polar alignmentTripod trailingSky darknessPlanetary imaging

Sky

mag/″²

Telescope

mm

Telescope, camera and sky are shared by every calculator in this visit and kept in the link.

Suburban sky (Bortle 5)Drawn to scale

Faintest naked-eye star

5.8mag

At SQM 20.50 the faintest naked-eye stars are about magnitude 5.8 (Bortle ≈ 5). Under a dark sky a 120 mm telescope reaches about magnitude 13.1.

Bortle class (approx.)
5
Telescope limit, dark sky
13.1 mag
Light grasp vs your eye
294×

With your numbers

NELM = 7.93 − 5·log₁₀(10^(4.316 − SQM/5) + 1) = 5.82 (Unihedron)

TLM = 2.7 + 5·log₁₀ 120 = 13.10

(120 / 7)² = 294×

Bortle is approximate (from the same SQM table AstroBrain uses for your location). The telescope limit assumes a dark sky and an experienced eye.