Estimating Limiting Magnitude

Limiting magnitude is used to evaluate the quality of observing conditions. It tells the magnitude of the faintest star visible to the unaided eye. The limiting magnitude could also be observed by some instrument. It describes well the sky’s transparency: better transparency means that fainter stars are visible.

Limiting magnitude is used eg. in meteor and deep sky observations. It can be used also to approximate light pollution.

The simplest way to evaluate limiting magnitude is to find suitable stars with known magnitudes from star map and check which of them are visible. A more clever way is to count visible stars inside known star squares and triangles. This method was originally invented by meteor observers.

The Method

  1. Wait for your eyes to dark adapt (at least 30 minutes).
  2. Choose one of the estimating areas from the image map above (or the larger version here) or one of the links below; the resulting page has a larger sky chart and a limiting magnitude table.
  3. Count the total number of stars you see in the area including the corners.
  4. Look up that number in the table and read off your limiting magnitude.

Notes:

  • You may see more (or fewer) stars than are plotted on the charts.
  • Different people may get different results.

The areas used in limiting magnitude estimation:

In observations the area should be choosen so that it is either near the view direction or the zenith, depending on observations and the situation. Meteor observers use the viewing direction. Deep sky observers (in Finland) use the area in 45 degrees altitude.

AreaCorner starsConstellation
1Chi-Zeta-Delta-Xi DraDraco
2Beta-Delta-Zeta PerPerseus
323-Theta-Beta UMaUrsa Major
4Alpha-Epsilon-Beta GemGemini
5Zeta-Gamma-Delta AqlAquila
6Alpha And--Gamma-Alpha PegPegasus-Andromeda
7Alpha-Beta-Delta CepCepheus
8Alpha-Beta-Zeta TauTaurus
9Alpha-Beta-Gamma-Delta LeoLeo
10Alpha-Zeta-Gamma VirVirgo
11Alpha CrB--Gamma-Alpha BooCorona Borealis-Bootes
12Alpha Ser--Beta Lib--Delta OphSerpens-Libra-Ophiuchus
13Beta-Zeta Lyr--Theta-Nu HerLyra-Hercules
14Epsilon-Eta-Gamma CygCygnus
15Beta Dra--Tau-Pi HerDraco-Hercules
16Alpha CVn--Epsilon-Eta UMaCanes Venatici-Ursa Major
17Epsilon-Theta-Delta AurAuriga
18Mu-Gamma-Phi AndAndromeda
19Kappa-Alpha Dra--Beta UMiDraco-Ursa Minor
2042-Beta-Gamma CamCamelopardalis
21Alpha PsA--98-Delta AqrPisces Austrinus-Aquarius
22Beta Lep--Beta Ori--53 EriLepus-Orion-Eridanus
23Delta-Gamma-Epsilon-Beta CrvCorvus
24Beta-Gamma-Sigma-Alpha LibLibra
25Alpha-Epsilon Sco--Chi LupScorpius-Lupus
26Gamma-Alpha TrA--Eta Ara--Alpha CenTriangulum Astrinus-Ara-Centaurus
27Beta Cen-Alpha-Gamma CruCentaurus-Crux
28Beta-Epsilon-Iota CarCarina
29Gamma-Alpha-Beta HydHydrus
30Alpha Tuc--Alpha-Epsilon PavTucana-Pavo

European users may get faster access to the original version of this page which is part of the Finnish Amateur Astronomy Homepage.

Another version of this information (with nice black-on-white star charts) is available at the IMO Major-Shower Observations page.


This page was created by:
Veikko MakelaVeikko.Makela@Helsinki.fi with additions from Rainer Arlt and major modifications by Bill Arnett