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Limiting Magnitude Table

Limiting Magnitude

DISCLAIMER: The larger the aperture, the more light a telescope will gather and the fainter the object you can see.  Here is a range of magnitudes you might expect to see. Given excellent sky conditions and good optics, a well trained eye can sometimes add up to 0.8 magnitude to these values.

Limiting Magnitude depends on much more than the instruments aperture.
Factors include:

  • seeing, the transparency of the atmosphere, the quality of the telescopes optics, their cleanliness, magnification, and observers experience and use of averted vision.

  • If ANY of the conditions are not met, expect to see less!


Two formulas are provided to give a representative range of possible magnitudes.  One formula is for aperture in inches the other is for aperture in millimeters. They are NOT equivalent formulas - if they were, no range would result.

~ The differences in the columns reflect differences in initial assumptions about sky conditions, visual acuity and others. ~

Aperture (inches)

Aperture (mm)

Limiting Magnitude
ML= 3.7 + 2.5 * Log10(D2)
D = aperture in mm

Limiting Magnitude
ML= 9.5 + 5.0 * Log10D
D = aperture in inches
1/4 6 7.5 6.3
.31 8 8.2 6.9
2 50.8 12.2 11.0
3 76.2 13.1 11.8
4 101.6 13.7 12.5
5 127.0 14.2 12.9
6 152.4 14.6 13.3
8 203.2 15.2 14.0
10 254.0 15.7 14.5
11 279.4 15.9 14.7
12 304.8 16.1 14.8
14 355.6 16.4 15.2
16 406.4 16.7 15.5
17 431.8 16.8 15.6
18 457.2 17.0 15.7
20 508.0 17.2 16.0
22 558.8 17.4 16.2
24 609.6 17.6 16.4
30 762.0 18.1 16.8
32 812.8 18.2 17.0
200 5080 22.2 21.0

Back to Magnification Calculator
by Matt Oltersdorf