Tools

Limiting Magnitude Calculator

What This Calculator Tells You

Limiting magnitude is the faintest star you can see through your telescope under your current sky conditions. It depends on your telescope's aperture (light-gathering power) and the darkness of your sky (naked-eye limiting magnitude).

Calculator

The diameter of your telescope's primary mirror or lens.

Faintest star you can see without a telescope (typically 5-7).

13.3
Limiting Magnitude

You can see stars 7.3 magnitudes fainter than with your naked eye.

816x
Light gathering power

vs. 7mm dark-adapted eye

+7.3
Magnitude gain

over naked eye

Naked Eye Limiting Magnitude Guide

Sky ConditionsNELMBortle Scale
City center3.0-4.08-9
Suburban4.5-5.55-6
Rural5.5-6.53-4
Dark site6.5-7.01-2

Rules of Thumb

  • Doubling aperture: Improves stellar limiting magnitude by up to ~1.5 magnitudes under similar sky conditions
  • Practical limits: For visual observing, practical limits are often around magnitude 15–16
  • Extended objects are harder: Galaxies and nebulae spread light over area, making them harder than their integrated magnitude suggests
  • Averted vision adds ~1 magnitude: Looking slightly to the side uses more sensitive rod cells

Common Mistakes & Misconceptions

  • "I can see 15th magnitude easily" - This is theoretical limit; actual results depend heavily on sky conditions, experience, and the object type.
  • Ignoring sky conditions - Light pollution reduces your limiting magnitude regardless of aperture.
  • Same limit for all objects - Point sources (stars) are easier than extended sources (galaxies) of the same magnitude.

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