Limiting Magnitude

Aperture (inches)
Aperture (mm)
Limiting Magnitude
(method 1)
Limiting Magnitude
(method 2)
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

Method 1 ML= 3.7 + 2.5 * Log10(D2)  where D = aperture in mm.
From "VISUAL ASTRONOMY FOR THE DEEP SKY" by Roger N. Clark.

Method 2 ML= 9.5 + 5.0 * Log10(D)  where D = aperture in inches.
From "THE OBSERVATIONAL AMATEUR ASTRONOMER" by Patrick Moore.

The differences in columns reflect differences in initial assumptions about
sky conditions, visual acuity and others.
Your results may vary.

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