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Determining the Limiting Magnitude

For a beginning amateur astronomer, there are three categories that the sky conditions can be rated: good, bad, and something in between. These are OK until you attempt to compare different nights, possibly months apart. What is needed is a method that is more objective, and to some degree measurable.

What is Limiting Magnitude?

A measure of a star’s relative dimness is called it’s magnitude. The brightest stars are near magnitude zero (a few are even negative). Stars can be easily seen in suburban skies down to 4th magnitude, and at dark sites, down to greater than 6th magnitude with the naked-eye. The limiting magnitude is the magnitude of the faintest star that can be seen. This would be easy if we could figure out which star was the faintest.

Unfortunately faint stars are not in any order in the sky and comparing two faint stars that are some distance apart is very difficult.
The method presented here has the benefit of working around this problem, yet being easy enough for a beginner. This method is from the International Meteor Organization’s Handbook for Visual Meteor Observers. The tables are from the 1995 edition of that book.

Determining the Limiting Magnitude
cactus.png The procedure for determining the limiting magnitude is very simple.
Look at the charts and find one or more numbered starfields in the area of the sky you are using.
Find the starfield in the sky and count the number of stars you see within it (and yes, count the corner stars too.)
Write down the number of the starfield and the number of stars.
Later you can look-up the limiting magnitude for the starfield, using the tables at the bottom of this page.
It’s that simple.

Things to Remember

There are a few things to remember about using this method.
This method is observer specific.
Other observers may have different limiting magnitudes at the same observing session. This is normal. Items like observer alertness, whether glasses are needed, dark adaptation, age, and experience will all effect the limiting magnitude.
Keep track of your own values. Eventually you will train your eye to see more.

The method laid out in the IMO’s handbook is slightly different, basically, dealing with how these tables are used.

Be sure to read the instructions in the IMO handbook before recording meteor observations.

Corner Stars of Limiting Magnitude Fields






 

Number of stars vs. Limiting Magnitude by Field

2
3
4
5
6
7
8
9
10
11
12
13
14
15
N
LM
N
LM
N
LM
N
LM
N
LM
N
LM
N
LM
N
LM
N
LM
N
LM
N
LM
N
LM
N
LM
N
LM
N
LM
5
4.2
6
5.0
5
4.5
5
4.3
4
4.6
4
4.7
3
4.0
4
4.7
7
4.4
4
4.5
6
4.5
6
5.2
2
4.0
3
4
4
4.9
6
4.9
7
5.1
6
4.6
6
5.0
5
5.1
5
5.2
4
4.5
5
4.8
8
5.0
5
5.8
7
4.7
7
5.3
4
4.3
4
4.8
5
5.1
8
5.0
8
5.4
7
4.8
7
5.1
6
5.2
6
5.4
5
4.6
7
5.1
11
5.6
7
5.9
9
4.9
9
5.4
5
4.5
6
4.9
6
5.5
9
5.2
10
5.6
8
5.2
8
5.3
7
5.4
7
5.7
7
4.9
8
5.3
13
5.7
8
6.0
10
5
11
5.6
6
5.0
7
5
8
5.8
10
5.3
11
5.7
9
5.4
9
5.6
8
6.0
8
5.9
8
5.2
9
5.5
15
6.0
11
6.1
11
5.3
13
5.8
7
5.5
8
5.2
10
6
11
6.0
12
5.8
11
5.7
10
5.7
10
6.2
9
6.2
10
5.4
10
5.9
18
6.1
12
6.4
13
5.7
14
6.4
8
5.7
11
5.5
12
6.1
12
6.1
13
6.0
13
5.8
11
5.9
11
6.4
12
6.3
12
5.5
11
6.0
20
6.3
15
6.5
14
5.8
17
6.5
12
5.9
12
5.7
18
6.3
15
6.3
14
6.1
14
6.0
12
6.1
12
6.5
14
6.4
13
5.9
12
6.1
21
6.4
16
6.7
17
5.9
19
6.7
13
6.0
13
5.9
21
6.4
16
6.4
15
6.2
15
6.1
13
6.2
13
6.6
17
6.5
14
6.0
15
6.2
24
6.6
17
6.8
19
6
20
6.8
15
6.1
14
6
22
6.6
17
6.5
17
6.3
16
6.2
14
6.3
19
6.9
20
6.6
15
6.1
16
6.3
25
6.7
19
7.0
21
6.1
22
6.9
16
6.2
15
6.1
26
6.7
18
6.6
20
6.4
17
6.3
15
6.4
22
7.0
25
6.7
17
6.2
17
6.4
29
6.9
22
7.1
23
6.2
23
7.0
17
6.3
16
6.2
32
6.9
20
6.7
23
6.6
18
6.4
16
6.5
24
7.1
29
6.8
18
6.3
20
6.5
32
7.0
23
7.2
25
6.3
24
7.1
18
6.5
18
6.3
33
7
23
6.8
26
6.7
19
6.5
18
6.6
25
7.2
30
6.9
20
6.4
21
6.6
34
7.1
25
7.3
27
6.4
30
7.2
20
6.8
20
6.4
35
7.1
28
6.9
27
6.8
20
6.6
20
6.7
26
7.3
33
7
22
6.5
23
6.7
38
7.2
26
7.4
30
6.5
23
6.9
24
6.5
38
7.2
34
7.0
29
6.9
23
6.7
22
6.9
27
7.4
35
7.1
23
6.8
26
6.8
40
7.3
31
7.5
32
6.6
27
7.0
28
6.6
48
7.3
41
7.1
31
7.0
25
6.8
23
7.0
40
7.2
26
6.9
28
6.9
44
7.4
36
6.7
32
7.1
32
6.7
54
7.4
46
7.2
35
7.1
27
6.9
25
7.2
43
7.3
33
7.0
29
7.0
45
7.5
39
6.8
36
7.2
34
6.8
61
7.5
55
7.3
42
7.2
29
7.0
26
7.3
46
7.4
41
7.1
31
7.4
45
6.9
42
7.3
36
6.9
60
7.4
44
7.3
33
7.1
30
7.5
49
7.5
48
7.2
32
7.5
52
7
43
7.4
41
7
73
7.5
54
7.4
37
7.2
49
7.3
55
7.1
52
7.5
46
7.1
59
7.5
44
7.3
57
7.4
60
7.2
49
7.2
49
7.4
65
7.5
69
7.3
54
7.3
54
7.5
73
7.4
61
7.4
86
7.5
65
7.5

 

16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
N
LM
N
LM
N
LM
N
LM
N
LM
N
LM
N
LM
N
LM
N
LM
N
LM
N
LM
N
LM
N
LM
N
LM
N
LM
4
4.6
6
4.2
3
4.2
4
5.2
1
4.2
4
4
5
4.3
6
5.2
5
4.5
4
4
7
4.1
5
4.6
4
4
4
4.1
5
4
5
5.1
8
4.6
4
4.3
5
5.4
2
4.4
5
4.5
6
4.4
7
5.8
7
5.2
5
5.2
8
4.9
6
4.7
5
5.4
5
4.7
6
4.2
6
5.7
9
5.1
6
5
6
5.7
4
4.7
6
4.7
7
4.5
8
6
8
5.6
6
5.6
12
5.1
7
5.8
6
5.5
6
4.8
7
4.8
7
5.9
10
5.5
7
5.1
8
6.1
5
5
7
5.6
8
4.8
9
6.4
9
5.7
7
6
14
5.2
9
6.2
7
5.8
7
5
8
4.9
9
6
11
5.9
9
5.2
11
6.4
6
5.4
8
5.7
9
5.5
10
6.5
11
5.8
8
6.1
15
5.3
11
6.4
8
6.4
8
5.6
9
5.1
11
6.2
12
6
10
5.5
12
6.6
7
5.7
9
6
13
5.7
11
6.9
12
6
9
6.2
17
5.5
14
6.7
10
6.5
9
5.7
10
5.2
13
6.3
14
6.1
11
5.7
14
6.7
9
5.8
10
6.4
15
6
13
7.3
13
6.1
10
6.3
18
5.7
15
6.8
14
7
10
6.4
11
5.4
14
6.4
16
6.2
12
5.9
15
6.8
10
6.9
11
6.5
16
6.2
14
7.4
14
6.2
11
6.6
22
5.8
17
7
15
7.2
11
6.5
12
5.6
17
6.6
17
6.3
13
6.1
16
7
13
6
14
6.8
17
6.3
17
7.5
16
6.3
12
6.9
24
5.9
18
7.1
16
7.3
13
6.6
13
5.7
19
6.8
20
6.4
14
6.3
17
7.1
15
6.1
15
7
18
6.4
17
6.6
14
7
31
6.2
19
7.2
17
7.4
17
6.7
14
5.8
22
6.9
26
6.5
17
6.4
18
7.2
16
6.2
18
7.4
19
6.5
19
6.7
15
7.1
32
6.3
22
7.4
18
7.5
20
6.9
15
5.9
24
7
30
6.6
18
6.5
21
7.3
18
6.3
20
7.5
21
6.6
20
6.8
34
6.4
23
7
16
6.2
26
7.1
35
6.7
24
6.6
28
7.4
23
6.4
22
6.7
21
6.9
37
6.5
24
7.1
18
6.3
27
7.2
37
6.8
27
6.7
32
7.5
28
6.6
23
6.8
24
7
38
6.6
26
7.2
19
6.4
28
7.3
41
6.9
31
6.9
33
6.7
24
6.9
26
7.1
40
6.7
29
7.5
20
6.5
33
7.4
44
7
33
7
37
6.8
25
7.2
27
7.3
42
6.8
22
6.6
36
7.5
50
7.1
37
7.2
45
6.9
27
7.3
30
7.4
43
6.9
23
7
55
7.2
41
7.3
50
7
29
7.4
31
7.5
44
7
28
7.1
60
7.3
46
7.4
56
7.1
46
7.1
31
7.3
68
7.4
52
7.5
60
7.2
49
7.2
32
7.5
81
7.5
67
7.3
50
7.4
74
7.4
51
7.5
82
7.5