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 measureable.
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.
The method for determining the limiting magnitude
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
The procedure for determining the limiting magnitude to very simple. Look at the charts and find one or
more numbered starfields in the area of they sky you are
using. Then 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 on pages
180–1. It’s that simple.
Things to Remember
There are a few things to remember about using this
method. This method is observer specific. Other ob-
Messier Logbook
servers 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
Field
Corner Stars
1
χ DRA – ζ DRA – δ DRA – ξ DRA
2
β PER – δ PER – ζ PER
3
23 UMA – θ UMA – β UMA
4
α GEM – ² GEM – β GEM
5
ζ AQL – γ AQL – δ AQL
6
α AND – γ PEG – α PEG
7
α CEP – β CEP – δ CEP
8
α TAU – β TAU – ζ TAU
9
α LEO – β LEO – γ LEO – δ LEO
10
α VIR – ζ VIR – γ VIR
11
α CRB – γ BOO – α BOO
12
α SER – β
LIB – δ OPH
13
β LYR – ζ LYR – θ HER – ν HER
14
² CYG – η CYG – γ CYG
15
β DRA – τ HER – π HER
16
α CVN – ² UMA – η UMA
17
² AUR – θ AUR – δ AUR
18
µ AND – γ AND – φ AND
19
κ DRA – α DRA – β UMI
20
42 CAM – β CAM – γ CAM
21
α PSA – 98 AQR – δ AQR
22
β LEP – β ORI – 53 ERI
23
δ CRV – γ CRV – ² CRV – β CRV
24
β
LIB – γ
LIB – σ
LIB – α LIB
25
α SCO – ² SCO – χ LUP
26
γ TRA – α TRA – η ARA – α CEN
27
β CEN – α CRU – γ CRU
28
β CAR – ² CAR – ι CAR
29
γ HYD – α HYD – β HYD
30
α TUC – α PAV – ² PAV
Limiting Magnitude
Page 173
Page 174
Limiting Magnitude
Messier Logbook
CANIS MAJOR
MONOCEROS
CANCER
LEO MINOR
LEPUS
GEMINI
4
ORION
LYNX
URSA MAJOR
8
AURIGA
17
2
ERIDANUS
TAURUS
PERSEUS
20
CAMELOPARDALIS
ARIES
7
DRACO
CETUS
PISCES
ANDROMEDA
CEPHEUS
18
TRIANGULUM
CASSIOPEIA
URSA MINOR
6
LACERTA
PEGASUS
AQUARIUS
CYGNUS
14
Messier Logbook
Limiting Magnitude
Page 175
ANTLIA
PYXIS
PUPPIS
MONOCEROS
GEMINI
VELA
CANIS MAJOR
ORION
CARINA
28
VOLANS
FORNAX
CHAMAELEON
MENSA
HYDRUS
29
APUS
OCTANS
RETICULUM
HOROLOGIUM
ERIDANUS
DORADO
CAELUM
PICTOR
COLUMBA
LEPUS
22
TAURUS
TUCANA
PHOENIX
PAVO
30
SCULPTOR
CETUS
TELESCOPIUM
INDUS
GRUS
PISCES
AQUARIUS
SAGITTARIUS
MICROSCOPIUM
PISCIS AUSTRINUS
21
Page 176
Limiting Magnitude
Messier Logbook
CORVUS
VIRGO
16
CRATER
LEO
DRACO
CANES VENATICI
COMA BERENICES
BOOTES
CORONA
BOREALIS
HERCULES
SEXTANS
9
LEO MINOR
URSA MAJOR
19
3
HYDRA
URSA MINOR
20
CANCER
GEMINI
PERSEUS
8
LEPUS
ARIES
TRIANGULUM
TAURUS
ORION
2
CANIS MAJOR
MONOCEROS
CANIS MINOR
4
AURIGA
17
CASSIOPEIA
CAMELOPARDALIS
LYNX
CEPHEUS
ANDROMEDA
Messier Logbook
Limiting Magnitude
Page 177
SCORPIUS
LUPUS
CORVUS
VIRGO
NORMA
26
27
CRUX
APUS
MUSCA
ANTLIA
HYDRA
SEXTANS
TRIANGULUM
ASUTRALE
CIRCINUS
CENTAURUS
23
CRATER
LEO
28
CARINA
MENSA
VOLANS
OCTANS
CHAMAELEON
VELA
PUPPIS
CANCER
TUCANA
29
CAELUM
HYDRUS
RETICULUM HOROLOGIUM
DORADO
PICTOR
LEPUS
COLUMBA
CANIS MAJOR
MONOCEROS
CANIS MINOR
PHOENIX
22
FORNAX
ERIDANUS
ORION
Page 178
Limiting Magnitude
Messier Logbook
SCUTUM
AQUILA
5
SERPENS
SAGITTA
VULPECULA
14
CYGNUS
LACERTA
HERCULES
OPHIUCHUS
13
LYRA
7
12
LIBRA
SERPENS
CORONA
BOREALIS
DRACO
1
15
CEPHEUS
11
BOOTES
19
16
VIRGO
URSA MAJOR
3
10
COMA BERENICES
CANES VENATICI
URSA MINOR
CAMELOPARDALIS
CORVUS
LEO MINOR
LYNX
CRATER
LEO
9
CANCER
Messier Logbook
Limiting Magnitude
Page 179
SCUTUM
SAGITTARIUS
PISCIS
AUSTRINUS
GRUS
MICROSCOPIUM
CAPRICORNUS
AQUILA
HERCULES
SERPENS
TUCANA
INDUS
30
TELESCOPIUM
CORONA
AUSTRALIS
PAVO
SCORPIUS
OPHIUCHUS
ARA
25
24
HYDRUS
OCTANS
APUS
26
TRIANGULUM
AUSTRALE
MUSCA
27
MENSA
CHAMAELEON
CIRCINUS
LUPUS
NORMA
LIBRA
12
SERPENS
VOLANS
28
CENTAURUS
10
CARINA
23
CORVUS
VIRGO
VELA
ANTLIA
CRATER
PUPPIS
PYXIS
HYDRA
LEO
Page 180
Limiting Magnitude
Messier Logbook
CETUS
ARIES
PISCES
TRIANGULUM
PERSEUS
2
18
6
PEGASUS
LACERTA
AQUARIUS
ANDROMEDA
CASSIOPEIA
CAMELOPARDALIS
14
EQUULEUS
DELPHINUS
VULPECULA
CYGNUS
7
CEPHEUS
URSA MINOR
1
AQUILA
5
SAGITTA
13
DRACO
LYRA
19
SERPENS
OPHIUCHUS
SERPENS
CANES VENATICI
CORONA
BOREALIS
BOOTES
HERCULES
SCUTUM
15
URSA MAJOR
Messier Logbook
Limiting Magnitude
Page 181
21
PHOENIX
HYDRUS
29
EQUULEUS
DEPLPHINUS
PAVO
ARA
CIRCINUS
26
TRIANGULUM
AUSTRALE
MUSCA
APUS
5
CORONA
AUSTRALIS
SAGITTARIUS
AQUILA
TELESCOPIUM
CHAMAELEON
OCTANS
30
INDUS
MICROSCOPIUM
CAPRICORNUS
MENSA
TUCANA
GRUS
PISCIS
AUSTRINUS
AQUARIUS
DORADO
RETICULUM
HOROLOGIUM
SCULPTOR
ERIDANUS
CAELUM
FORNAX
CETUS
PICSES
PEGASUS
25
OPHIUCHUS
CENTAURUS
LUPUS
SCORPIUS
SERPENS
NORMA
SCUTUM
Use these tables to look-up the limiting magnitude. After counting the number of stars you can see in the star field, look
up the count (N) for that field to find the limiting magnitude (LM).
1
2
N
5
6
8
9
10
11
12
15
16
17
18
20
23
28
34
41
46
55
60
73
LM
4.2
4.9
5.0
5.2
5.3
6.0
6.1
6.3
6.4
6.5
6.6
6.7
6.8
6.9
7.0
7.1
7.2
7.3
7.4
7.5
N
6
7
9
10
11
13
14
17
19
21
23
25
27
30
32
36
39
45
52
55
60
69
73
86
11
LM
4.5
4.7
4.9
5.0
5.3
5.7
5.8
5.9
6.0
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
7.0
7.1
7.2
7.3
7.4
7.5
Page 182
3
N
6
7
8
10
11
12
13
14
15
17
20
23
26
27
29
31
35
42
44
54
59
LM
5.0
5.1
5.4
5.6
5.7
5.8
6.0
6.1
6.2
6.3
6.4
6.6
6.7
6.8
6.9
7.0
7.1
7.2
7.3
7.4
7.5
N
6
7
9
11
13
14
17
19
20
22
23
24
30
12
LM
5.2
5.3
5.4
5.6
5.8
6.4
6.5
6.7
6.8
6.9
7.0
7.1
7.2
4
N
5
6
7
8
9
11
13
14
15
16
17
18
19
20
23
25
27
29
33
37
44
49
54
LM
4.5
4.6
4.8
5.2
5.4
5.7
5.8
6.0
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
7.0
7.1
7.2
7.3
7.4
7.5
N
2
4
5
6
7
8
12
13
15
16
17
18
20
23
27
32
36
42
43
52
13
LM
4.0
4.3
4.5
5.0
5.5
5.7
5.9
6.0
6.1
6.2
6.3
6.5
6.8
6.9
7.0
7.1
7.2
7.3
7.4
7.5
5
N
5
6
7
8
9
10
11
12
13
14
15
16
18
20
22
23
25
26
30
LM
4.3
5.0
5.1
5.3
5.6
5.7
5.9
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.9
7.0
7.2
7.3
7.5
N
3
4
6
7
8
11
12
13
14
15
16
18
20
24
28
32
34
36
41
46
49
54
61
65
14
LM
4.0
4.8
4.9
5.0
5.2
5.5
5.7
5.9
6.0
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
7.0
7.1
7.2
7.3
7.4
7.5
6
N
4
5
6
7
8
10
11
12
13
19
22
24
25
26
27
LM
4.6
5.1
5.2
5.4
6.0
6.2
6.4
6.5
6.6
6.9
7.0
7.1
7.2
7.3
7.4
N
4
5
6
8
10
12
18
21
22
26
32
33
35
38
48
54
61
15
LM
4.9
5.1
5.5
5.8
6.0
6.1
6.3
6.4
6.6
6.7
6.9
7.0
7.1
7.2
7.3
7.4
7.5
7
N
4
5
6
7
8
9
12
14
17
20
25
29
30
33
35
40
43
46
49
LM
4.7
5.2
5.4
5.7
5.9
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
7.0
7.1
7.2
7.3
7.4
7.5
N
4
5
6
7
9
11
13
14
17
19
22
24
26
27
28
33
36
16
LM
4.6
5.1
5.7
5.9
6.0
6.2
6.3
6.4
6.6
6.8
6.9
7.0
7.1
7.2
7.3
7.4
7.5
Limiting Magnitude
8
N
3
4
5
7
8
10
12
13
14
15
17
18
20
22
23
26
33
41
48
49
57
65
LM
4.0
4.5
4.6
4.9
5.2
5.4
5.5
5.9
6.0
6.1
6.2
6.3
6.4
6.5
6.8
6.9
7.0
7.1
7.2
7.3
7.4
7.5
N
6
8
9
10
11
12
14
16
17
20
26
30
35
37
41
44
50
55
60
68
81
17
LM
4.2
4.6
5.1
5.5
5.9
6.0
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
7.0
7.1
7.2
7.3
7.4
7.5
9
N
4
5
7
8
9
10
11
12
15
16
17
20
21
23
26
28
29
31
32
LM
4.7
4.8
5.1
5.3
5.5
5.9
6.0
6.1
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
7.0
7.4
7.5
N
3
4
6
7
9
10
11
12
13
14
17
18
24
27
31
33
37
41
46
52
18
LM
4.2
4.3
5.0
5.1
5.2
5.5
5.7
5.9
6.1
6.3
6.4
6.5
6.6
6.7
6.9
7.0
7.2
7.3
7.4
7.5
N
7
8
11
13
15
18
20
21
24
25
29
32
34
38
40
44
45
LM
4.4
5.0
5.6
5.7
6.0
6.1
6.3
6.4
6.6
6.7
6.9
7.0
7.1
7.2
7.3
7.4
7.5
N
4
5
6
8
11
12
14
15
16
17
18
21
28
32
19
LM
5.2
5.4
5.7
6.1
6.4
6.6
6.7
6.8
7.0
7.1
7.2
7.3
7.4
7.5
N
4
5
7
8
11
12
15
16
17
19
22
23
25
26
31
10
LM
4.5
5.8
5.9
6.0
6.1
6.4
6.5
6.7
6.8
7.0
7.1
7.2
7.3
7.4
7.5
N
1
2
4
5
6
7
9
10
13
15
16
18
23
28
33
37
45
50
56
60
67
74
82
20
LM
4.2
4.4
4.7
5.0
5.4
5.7
5.8
6.9
6.0
6.1
6.2
6.3
6.4
6.6
6.7
6.8
6.9
7.0
7.1
7.2
7.3
7.4
7.5
Messier Logbook
N
4
5
6
7
8
9
10
11
14
15
18
20
21
LM
4.0
4.5
4.7
5.6
5.7
6.0
6.4
6.5
6.8
7.0
7.4
7.5
N
5
6
7
8
9
13
15
16
17
18
19
21
22
23
24
25
27
29
22
LM
4.3
4.4
4.5
4.8
5.5
5.7
6.0
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
7.2
7.3
7.4
Messier Logbook
N
6
7
8
9
10
11
13
14
17
23
LM
5.2
5.8
6.0
6.4
6.5
6.9
7.3
7.4
7.5
N
5
7
8
9
11
12
13
14
16
17
19
20
21
24
26
27
30
31
24
LM
4.5
5.2
5.6
5.7
5.8
6.0
6.1
6.2
6.3
6.6
6.7
6.8
6.9
7.0
7.1
7.3
7.4
7.5
N
4
5
6
7
8
9
10
11
12
14
15
25
LM
4.0
5.2
5.6
6.0
6.1
6.2
6.3
6.6
6.9
7.0
7.1
N
7
8
12
14
15
17
18
22
24
31
32
34
37
38
40
42
43
44
46
49
50
51
26
LM
4.1
4.9
5.1
5.2
5.3
5.5
5.7
5.8
5.9
6.2
6.3
6.4
6.5
6.6
6.7
6.8
6.9
7.0
7.1
7.2
7.4
7.5
Limiting Magnitude
N
5
6
7
9
11
14
15
17
18
19
22
27
LM
4.6
4.7
5.8
6.2
6.4
6.7
6.8
7.0
7.1
7.2
7.4
N
4
5
6
7
8
10
14
15
16
17
18
28
LM
4.0
5.4
5.5
5.8
6.4
6.5
7.0
7.2
7.3
7.4
7.5
N
4
5
6
7
8
9
10
11
13
17
20
23
24
26
29
29
LM
4.1
4.7
4.8
5.0
5.6
5.7
6.4
6.5
6.6
6.7
6.9
7.0
7.1
7.2
7.5
N
5
6
7
8
9
10
11
12
13
14
15
16
18
19
20
22
23
28
31
32
30
LM
4.0
4.2
4.8
4.9
5.1
5.2
5.4
5.6
5.7
5.8
5.9
6.2
6.3
6.4
6.5
6.6
7.0
7.1
7.3
7.5
Page 183