Determining the Limiting Magnitude the Limiting Magnitude...
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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 at- tempt 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 magni- tude 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 cor- ner stars too.) Write down the number of the starfield and the number of stars. Later you can look-up the limit- ing 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- 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 Messier Logbook Limiting Magnitude Page 173
Transcript of Determining the Limiting Magnitude the Limiting Magnitude...
Determining theLimiting
MagnitudeFor 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 at-tempt to compare different nights, possibly months apart.What is needed is a method that is more objective, andto some degree measureable.
What is Limiting Magnitude?
A measure of a star’s relative dimness is called it’smagnitude. The brightest stars are near magnitude zero(a few are even negative.) Stars can be easily seen insuburban 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 fainteststar that can be seen. This would be easy if we could figureout which star was the faintest. Unfortunately faint starsare not in any order in the sky and comparing two faintstars that are some distance apart is very difficult. Themethod presented here has the benefit of working aroundthis problem, yet being easy enough for a beginner.
The method for determining the limiting magnitudeis from the International Meteor Organization’s Handbook
for Visual Meteor Observers. The tables are from the 1995edition of that book.
Determining the Limiting Magnitude
The procedure for determining the limiting magni-tude to very simple. Look at the charts and find one ormore numbered starfields in the area of they sky you areusing. Then find the starfield in the sky and count thenumber of stars you see within it (and yes, count the cor-ner stars too.) Write down the number of the starfieldand the number of stars. Later you can look-up the limit-ing magnitude for the starfield using the tables on pages180–1. It’s that simple.
Things to Remember
There are a few things to remember about using thismethod. This method is observer specific. Other ob-
servers may have different limiting magnitudes at the sameobserving session. This is normal. Items like observeralertness, whether glasses are needed, dark adaptation,age, and experience will all effect the limiting magnitude.Keep track of your own values. Eventually you will trainyour eye to see more.
The method laid out in the IMO’s handbook isslightly different, basically, dealing with how these tablesare 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 DRA2 β PER – δ PER – ζ PER3 23 UMA – θ UMA – β UMA4 α GEM – ε GEM – β GEM5 ζ AQL – γ AQL – δ AQL6 α AND – γ PEG – α PEG7 α CEP – β CEP – δ CEP8 α TAU – β TAU – ζ TAU9 α LEO – β LEO – γ LEO – δ LEO10 α VIR – ζ VIR – γ VIR11 α CRB – γ BOO – α BOO12 α SER – β LIB – δ OPH13 β LYR – ζ LYR – θ HER – ν HER14 ε CYG – η CYG – γ CYG15 β DRA – τ HER – π HER16 α CVN – ε UMA – η UMA17 ε AUR – θ AUR – δ AUR18 µ AND – γ AND – φ AND19 κ DRA – α DRA – β UMI20 42 CAM – β CAM – γ CAM21 α PSA – 98 AQR – δ AQR22 β LEP – β ORI – 53 ERI23 δ CRV – γ CRV – ε CRV – β CRV24 β LIB – γ LIB – σ LIB –α LIB25 α SCO – ε SCO – χ LUP26 γ TRA – α TRA – η ARA –α CEN27 β CEN – α CRU – γ CRU28 β CAR – ε CAR – ι CAR29 γ HYD – α HYD – β HYD30 α TUC – α PAV – ε PAV
Messier Logbook Limiting Magnitude Page 173
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Messier Logbook Limiting Magnitude Page 181
Use these tables to look-up the limiting magnitude. After counting the number of stars you can see in the star field, lookup the count (N) for that field to find the limiting magnitude (LM).
1 2 3 4 5 6 7 8 9 10N LM N LM N LM N LM N LM N LM N LM N LM N LM N LM5 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 4.56 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 5.88 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 5.99 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 6.010 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 6.111 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 6.412 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 6.515 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 6.716 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 6.817 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 7.018 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 7.120 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 7.223 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 7.328 6.9 27 6.8 20 6.6 20 6.7 26 7.3 33 7.0 22 6.5 23 6.7 38 7.2 26 7.434 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 7.541 7.1 31 7.0 25 6.8 23 7.0 40 7.2 26 6.9 28 6.9 44 7.446 7.2 35 7.1 27 6.9 25 7.2 43 7.3 33 7.0 29 7.0 45 7.555 7.3 42 7.2 29 7.0 26 7.3 46 7.4 41 7.1 31 7.460 7.4 44 7.3 33 7.1 30 7.5 49 7.5 48 7.2 32 7.573 7.5 54 7.4 37 7.2 49 7.3
59 7.5 44 7.3 57 7.449 7.4 65 7.554 7.5
11 12 13 14 15 16 17 18 19 20N LM N LM N LM N LM N LM N LM N LM N LM N LM N LM6 4.5 6 5.2 2 4.0 3 4.0 4 4.9 4 4.6 6 4.2 3 4.2 4 5.2 1 4.27 4.7 7 5.3 4 4.3 4 4.8 5 5.1 5 5.1 8 4.6 4 4.3 5 5.4 2 4.49 4.9 9 5.4 5 4.5 6 4.9 6 5.5 6 5.7 9 5.1 6 5.0 6 5.7 4 4.710 5.0 11 5.6 6 5.0 7 5.0 8 5.8 7 5.9 10 5.5 7 5.1 8 6.1 5 5.011 5.3 13 5.8 7 5.5 8 5.2 10 6.0 9 6.0 11 5.9 9 5.2 11 6.4 6 5.413 5.7 14 6.4 8 5.7 11 5.5 12 6.1 11 6.2 12 6.0 10 5.5 12 6.6 7 5.714 5.8 17 6.5 12 5.9 12 5.7 18 6.3 13 6.3 14 6.1 11 5.7 14 6.7 9 5.817 5.9 19 6.7 13 6.0 13 5.9 21 6.4 14 6.4 16 6.2 12 5.9 15 6.8 10 6.919 6.0 20 6.8 15 6.1 14 6.0 22 6.6 17 6.6 17 6.3 13 6.1 16 7.0 13 6.021 6.1 22 6.9 16 6.2 15 6.1 26 6.7 19 6.8 20 6.4 14 6.3 17 7.1 15 6.123 6.2 23 7.0 17 6.3 16 6.2 32 6.9 22 6.9 26 6.5 17 6.4 18 7.2 16 6.225 6.3 24 7.1 18 6.5 18 6.3 33 7.0 24 7.0 30 6.6 18 6.5 21 7.3 18 6.327 6.4 30 7.2 20 6.8 20 6.4 35 7.1 26 7.1 35 6.7 24 6.6 28 7.4 23 6.430 6.5 23 6.9 24 6.5 38 7.2 27 7.2 37 6.8 27 6.7 32 7.5 28 6.632 6.6 27 7.0 28 6.6 48 7.3 28 7.3 41 6.9 31 6.9 33 6.736 6.7 32 7.1 32 6.7 54 7.4 33 7.4 44 7.0 33 7.0 37 6.839 6.8 36 7.2 34 6.8 61 7.5 36 7.5 50 7.1 37 7.2 45 6.945 6.9 42 7.3 36 6.9 55 7.2 41 7.3 50 7.052 7.0 43 7.4 41 7.0 60 7.3 46 7.4 56 7.155 7.1 52 7.5 46 7.1 68 7.4 52 7.5 60 7.260 7.2 49 7.2 81 7.5 67 7.369 7.3 54 7.3 74 7.473 7.4 61 7.4 82 7.586 7.5 65 7.5
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21 22 23 24 25 26 27 28 29 30N LM N LM N LM N LM N LM N LM N LM N LM N LM N LM4 4.0 5 4.3 6 5.2 5 4.5 4 4.0 7 4.1 5 4.6 4 4.0 4 4.1 5 4.05 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 4.26 4.7 7 4.5 8 6.0 8 5.6 6 5.6 12 5.1 7 5.8 6 5.5 6 4.8 7 4.87 5.6 8 4.8 9 6.4 9 5.7 7 6.0 14 5.2 9 6.2 7 5.8 7 5.0 8 4.98 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 5.19 6.0 13 5.7 11 6.9 12 6.0 9 6.2 17 5.5 14 6.7 10 6.5 9 5.7 10 5.210 6.4 15 6.0 13 7.3 13 6.1 10 6.3 18 5.7 15 6.8 14 7.0 10 6.4 11 5.411 6.5 16 6.2 14 7.4 14 6.2 11 6.6 22 5.8 17 7.0 15 7.2 11 6.5 12 5.614 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 5.715 7.0 18 6.4 17 6.6 14 7.0 31 6.2 19 7.2 17 7.4 17 6.7 14 5.818 7.4 19 6.5 19 6.7 15 7.1 32 6.3 22 7.4 18 7.5 20 6.9 15 5.920 7.5 21 6.6 20 6.8 34 6.4 23 7.0 16 6.2
22 6.7 21 6.9 37 6.5 24 7.1 18 6.323 6.8 24 7.0 38 6.6 26 7.2 19 6.424 6.9 26 7.1 40 6.7 29 7.5 20 6.525 7.2 27 7.3 42 6.8 22 6.627 7.3 30 7.4 43 6.9 23 7.029 7.4 31 7.5 44 7.0 28 7.1
46 7.1 31 7.349 7.2 32 7.550 7.451 7.5
Messier Logbook Limiting Magnitude Page 183
