Updated 15 Feb 1998

Amateur Astronomer's Notebook

Astronomical Data: Magnitude Comparision Tables

The tables below contain data which will tell you how much brighter (or fainter) star images will appear in one size telescope compared to another. For example, if you now own a 90mm refractor and are trading up to an 8" reflector, the table shows that stars will appear about 1.75 maginitudes brighter in the 8" as compared to the smaller scope. In another example, if you now own a 20" reflector and are considering a 6" Dobsonian for a "second" scope, stars in the 6" will appear about 2.75 magnitudes dimmer in your new scope.

Important: Data in the tables below is based on calculations using the area of the telescope primary mirror or objective; losses such as absorption or reflection losses are not included. In addition, all telescopes larger than 6 inches diameter are assumed to be reflectors; a secondary obstruction of 20% of the primary diameter has been included in the data for scopes 6" and larger. Rounding of data has been performed (in case you spot small discrepancies). I used MATLAB (software) to generate the numbers used in these tables.

How to use the tables: Select the table applicable to the telescope size you now own. Go to the line in the table which corresponds to the size of the telescope you are moving up (or down) to and read the "times brighter" and "magnitudes brighter" value. This is how much brighter (or fainter) stars will appear in your new scope as compared to your old one. If you are not sure how much brighter the "magnitude" number represents, a Star Magnitude Comparision Table is included at the end of this article so you can make a "real world" comparision.

Tables are included for the following popular size amateur telescopes:

If you are not sure of what the different magnitudes represent, use the following table to help you identify some popular stars to make the comparison:

Table for a 2.4" (60mm) telescope

Present Scope
New Scope
Stars will appear this many times brighter
Stars will appear this many magnitudes brighter
2.4" (60mm)
90mm
2.1267
0.8
2.4" (60mm)
4"
2.7778
1.1
2.4" (60mm)
4.5"
3.5156
1.35
2.4" (60mm)
5"
4.3403
1.6
2.4" (60mm)
6"
6.0000
1.95
2.4" (60mm)
7"
8.1667
2.25
2.4" (60mm)
8"
10.6667
2.55
2.4" (60mm)
9.25"
14.2604
2.9
2.4" (60mm)
10"
16.6667
3.05
2.4" (60mm)
11"
20.1667
3.25
2.4" (60mm)
12"
24.0000
3.45
2.4" (60mm)
14"
32.6667
3.75
2.4" (60mm)
16"
42.6667
4.05
2.4" (60mm)
17.5"
51.0417
4.25
2.4" (60mm)
18"
54.0000
4.35
2.4" (60mm)
20"
66.6667
4.55
2.4" (60mm)
24"
96.0000
4.95


Table for a 3.5" (90mm) telescope

Present Scope
New Scope
Stars will appear this many times brighter
Stars will appear this many magnitudes brighter
3.5" (90mm)
60mm
0.4702
0.8 fainter
3.5" (90mm)
4"
1.3061
0.3
3.5" (90mm)
4.5"
1.6531
0.55
3.5" (90mm)
5"
2.0408
0.8
3.5" (90mm)
6"
2.8212
1.15
3.5" (90mm)
7"
3.8400
1.45
3.5" (90mm)
8"
5.0155
1.75
3.5" (90mm)
9.25"
6.7053
2.05
3.5" (90mm)
10"
7.8367
2.25
3.5" (90mm)
11"
9.4824
2.45
3.5" (90mm)
12"
11.2849
2.65
3.5" (90mm)
14"
15.3600
2.95
3.5" (90mm)
16"
20.0620
3.25
3.5" (90mm)
17.5"
24.0000
3.45
3.5" (90mm)
18"
25.3910
3.5
3.5" (90mm)
20"
31.3469
3.75
3.5" (90mm)
24"
45.1396
4.15


Table for a 4" (101mm) telescope

Present Scope
New Scope
Stars will appear this many times brighter
Stars will appear this many magnitudes brighter
4" (101mm)
60mm
0.3600
1.1 fainter
4" (101mm)
90mm
0.7656
0.3 fainter
4" (101mm)
4.5"
1.2656
0.25
4" (101mm)
5"
1.5625
0.5
4" (101mm)
6"
2.1600
0.85
4" (101mm)
7"
2.9400
1.15
4" (101mm)
8"
3.8400
1.45
4" (101mm)
9.25"
5.1338
1.75
4" (101mm)
10"
6.0000
1.95
4" (101mm)
11"
7.2600
2.15
4" (101mm)
12"
8.6400
2.35
4" (101mm)
14"
11.7600
2.65
4" (101mm)
16"
15.3600
2.95
4" (101mm)
17.5"
18.3750
3.15
4" (101mm)
18"
19.4400
3.2
4" (101mm)
20"
24.0000
3.45
4" (101mm)
24"
34.5600
3.85


Table for a 4.5" (114mm) telescope

Present Scope
New Scope
Stars will appear this many times brighter
Stars will appear this many magnitudes brighter
4.5" (114mm)
60mm
0.2844
1.35 fainter
4.5" (114mm)
90mm
0.6049
0.55 fainter
4.5" (114mm)
4"
0.7901
0.25 fainter
4.5" (114mm)
5"
1.2346
0.25
4.5" (114mm)
6"
1.7067
0.55
4.5" (114mm)
7"
2.3230
0.95
4.5" (114mm)
8"
3.0341
1.2
4.5" (114mm)
9.25"
4.0563
1.55
4.5" (114mm)
10"
4.7407
1.7
4.5" (114mm)
11"
5.7363
1.9
4.5" (114mm)
12"
6.8267
2.05
4.5" (114mm)
14"
9.2919
2.45
4.5" (114mm)
16"
12.1363
2.75
4.5" (114mm)
17.5"
14.5185
2.9
4.5" (114mm)
18"
15.3600
2.95
4.5" (114mm)
20"
18.9630
3.2
4.5" (114mm)
24"
27.3067
3.55


Table for a 5" (127mm) telescope

Present Scope
New Scope
Stars will appear this many times brighter
Stars will appear this many magnitudes brighter
5" (127mm)
60mm
0.2304
1.6 fainter
5" (127mm)
90mm
0.4900
0.75 fainter
5" (127mm)
4"
0.6400
0.5 fainter
5" (127mm)
4.5"
0.8100
0.25 fainter
5" (127mm)
6"
1.3824
0.35
5" (127mm)
7"
1.8816
0.65
5" (127mm)
8"
2.4576
0.95
5" (127mm)
9.25"
3.2856
1.25
5" (127mm)
10"
3.8400
1.45
5" (127mm)
11"
4.6464
1.65
5" (127mm)
12"
5.5296
1.85
5" (127mm)
14"
7.5264
2.2
5" (127mm)
16"
9.8304
2.45
5" (127mm)
17.5"
11.7600
2.65
5" (127mm)
18"
12.4416
2.75
5" (127mm)
20"
15.3600
2.95
5" (127mm)
24"
22.1184
3.35


Table for a 6" (152mm) telescope

Present Scope
New Scope
Stars will appear this many times brighter
Stars will appear this many magnitudes brighter
6" (152mm)
60mm
0.1600
2.0 fainter
6" (152mm)
90mm
0.3403
1.15 fainter
6" (152mm)
4"
0.4444
0.85 fainter
6" (152mm)
4.5"
0.5625
0.65 fainter
6" (152mm)
5"
0.6944
0.4 fainter
6" (152mm)
7"
1.3067
0.3
6" (152mm)
8"
1.7067
.55
6" (152mm)
9.25"
2.2817
.9
6" (152mm)
10"
2.6667
1.05
6" (152mm)
11"
3.2267
1.25
6" (152mm)
12"
3.8400
1.45
6" (152mm)
14"
5.2267
1.8
6" (152mm)
16"
6.8267
2.05
6" (152mm)
17.5"
8.1667
2.25
6" (152mm)
18"
8.6400
2.35
6" (152mm)
20"
10.6667
2.55
6" (152mm)
24"
15.3600
2.95


Table for a 7" (178mm) telescope

Present Scope
New Scope
Stars will appear this many times brighter
Stars will appear this many magnitudes brighter
7" (178mm)
60mm
0.1176
2.35 fainter
7" (178mm)
90mm
0.2500
1.5 fainter
7" (178mm)
4"
0.3265
1.25 fainter
7" (178mm)
4.5"
0.4133
0.95 fainter
7" (178mm)
5"
0.5102
0.75 fainter
7" (178mm)
6"
0.7053
0.4 fainter
7" (178mm)
8"
1.2539
.25
7" (178mm)
9.25"
1.6763
.55
7" (178mm)
10"
1.9592
.75
7" (178mm)
11"
2.3706
.95
7" (178mm)
12"
2.8212
1.15
7" (178mm)
14"
3.8400
1.45
7" (178mm)
16"
5.0155
1.75
7" (178mm)
17.5"
6.0000
1.95
7" (178mm)
18"
6.3478
2.0
7" (178mm)
20"
7.8367
2.25
7" (178mm)
24"
11.2849
2.65


Table for an 8" (203mm) telescope

Present Scope
New Scope
Stars will appear this many times brighter
Stars will appear this many magnitudes brighter
8" (203mm)
60mm
0.0900
2.6 fainter
8" (203mm)
90mm
0.1914
1.8 fainter
8" (203mm)
4"
0.2500
1.5 fainter
8" (203mm)
4.5"
0.3164
1.25 fainter
8" (203mm)
5"
0.3906
1.0 fainter
8" (203mm)
6"
0.5400
0.65 fainter
8" (203mm)
7"
0.7350
0.35 fainter
8" (203mm)
9.25"
1.2834
0.25
8" (203mm)
10"
1.5000
0.45
8" (203mm)
11"
1.8150
0.65
8" (203mm)
12"
2.1600
0.85
8" (203mm)
14"
2.9400
1.15
8" (203mm)
16"
3.8400
1.45
8" (203mm)
17.5"
4.5938
1.65
8" (203mm)
18"
4.8600
1.7
8" (203mm)
20"
6.0000
1.95
8" (203mm)
24"
8.6400
2.35


Table for a 9.25" (235mm) telescope

Present Scope
New Scope
Stars will appear this many times brighter
Stars will appear this many magnitudes brighter
9.25 (235mm)
60mm
0.0673
2.95 fainter
9.25 (235mm)
90mm
0.1432
2.1 fainter
9.25 (235mm)
4"
0.1870
1.85 fainter
9.25 (235mm)
4.5"
0.2367
1.55 fainter
9.25 (235mm)
5"
0.2922
1.35 fainter
9.25 (235mm)
6"
0.4039
1.0 fainter
9.25 (235mm)
7"
0.5498
0.65 fainter
9.25 (235mm)
8"
0.7181
0.35 fainter
9.25 (235mm)
10"
1.1220
0.15
9.25 (235mm)
11"
1.3576
.35
9.25 (235mm)
12"
1.6157
.5
9.25 (235mm)
14"
2.1991
.85
9.25 (235mm)
16"
2.8723
1.15
9.25 (235mm)
17.5"
3.4361
1.35
9.25 (235mm)
18"
3.6352
1.4
9.25 (235mm)
20"
4.4879
1.65
9.25 (235mm)
24"
6.4626
2.05

Table for a 10" (254mm) telescope

Present Scope
New Scope
Stars will appear this many times brighter
Stars will appear this many magnitudes brighter
10" (254mm)
60mm
0.0576
3.1 fainter
10" (254mm)
90mm
 0.1225
2.3 fainter
10" (254mm)
4"
 0.1600
2.0 fainter
10" (254mm)
4.5"
 0.2025
1.7 fainter
10" (254mm)
5"
 0.2500
1.5 fainter
10" (254mm)
6"
 0.3456
1.15 fainter
10" (254mm)
7"
 0.4704
0.85 fainter
10" (254mm)
8"
 0.6144
0.55 fainter
10" (254mm)
9.25"
 0.8214
0.25 fainter
10" (254mm)
11"
1.1616
0.15
10" (254mm)
12"
 1.3824
0.35
10" (254mm)
14"
 1.8816
0.65
10" (254mm)
16"
 2.4576
0.95
10" (254mm)
17.5"
 2.9400
1.15
10" (254mm)
18"
 3.1104
1.25
10" (254mm)
20"
 3.8400
1.45
10" (254mm)
24"
 5.5296
1.85



Table for an 11" (279mm) telescope

Present Scope
New Scope
Stars will appear this many times brighter
Stars will appear this many magnitudes brighter
11" (279mm)
60mm
0.0476
3.3 fainter
11" (279mm)
90mm
0.1012
2.5 fainter
11" (279mm)
4"
0.1322
2.2 fainter
11" (279mm)
4.5"
0.1674
1.95 fainter
11" (279mm)
5"
0.2066
1.75 fainter
11" (279mm)
6"
0.2856
1.35 fainter
11" (279mm)
7"
0.3888
1.05 fainter
11" (279mm)
8"
0.5078
0.75 fainter
11" (279mm)
9.25"
0.6788
0.45 fainter
11" (279mm)
10"
0.7934
0.25 fainter
11" (279mm)
12"
1.1425
0.15
11" (279mm)
14"
1.5550
0.45
11" (279mm)
16"
2.0311
0.8
11" (279mm)
17.5"
2.4298
0.95
11" (279mm)
18"
2.5706
1.0
11" (279mm)
20"
3.1736
1.25
11" (279mm)
24"
4.5699
1.65


Table for a 12" (305mm) telescope

Present Scope
New Scope
Stars will appear this many times brighter
Stars will appear this many magnitudes brighter
12" (305mm)
60mm
0.0400
3.5 fainter
12" (305mm)
90mm
0.0851
2.7 fainter
12" (305mm)
4"
0.1111
2.4 fainter
12" (305mm)
4.5"
0.1406
2.15 fainter
12" (305mm)
5"
0.1736
1.9 fainter
12" (305mm)
6"
0.2400
1.55 fainter
12" (305mm)
7"
0.3267
1.25 fainter
12" (305mm)
8"
0.4267
0.95 fainter
12" (305mm)
9.25"
0.5704
0.6 fainter
12" (305mm)
10"
0.6667
0.45 fainter
12" (305mm)
11"
0.8067
0.25 fainter
12" (305mm)
14"
1.3067
0.3
12" (305mm)
16"
1.7067
0.55
12" (305mm)
17.5"
2.0417
0.8
12" (305mm)
18"
2.1600
0.85
12" (305mm)
20"
2.6667
1.05
12" (305mm)
24"
3.8400
1.45


Table for a 14" (355mm) telescope

Present Scope
New Scope
Stars will appear this many times brighter
Stars will appear this many magnitudes brighter
14" (355mm)
60mm
0.0294
3.85 fainter
14" (355mm)
90mm
0.0625
3.05 fainter
14" (355mm)
4"
0.0816
2.7 fainter
14" (355mm)
4.5"
0.1033
2.5 fainter
14" (355mm)
5"
0.1276
2.25 fainter
14" (355mm)
6"
0.1763
1.9 fainter
14" (355mm)
7"
0.2400
1.55 fainter
14" (355mm)
8"
0.3135
1.25 fainter
14" (355mm)
9.25"
0.4191
0.95 fainter
14" (355mm)
10"
0.4898
0.75 fainter
14" (355mm)
11"
0.5927
0.55 fainter
14" (355mm)
12"
0.7053
0.4 fainter
14" (355mm)
16"
1.2539
0.25
14" (355mm)
17.5"
1.5000
0.45
14" (355mm)
18"
1.5869
0.5
14" (355mm)
20"
1.9592
0.7
14" (355mm)
24"
2.8212
1.15


Table for a 16" (406mm) telescope

Present Scope
New Scope
Stars will appear this many times brighter
Stars will appear this many magnitudes brighter
16" (406mm)
60mm
0.0225
4.1 fainter
16" (406mm)
90mm
0.0479
3.3 fainter
16" (406mm)
4"
0.0625
3.05 fainter
16" (406mm)
4.5"
0.0791
2.8 fainter
16" (406mm)
5"
0.0977
2.5 fainter
16" (406mm)
6"
0.1350
2.2 fainter
16" (406mm)
7"
0.1838
1.85 fainter
16" (406mm)
8"
0.2400
1.55 fainter
16" (406mm)
9.25"
0.3209
1.25 fainter
16" (406mm)
10"
0.3750
1.15 fainter
16" (406mm)
11"
0.4538
0.85 fainter
16" (406mm)
12"
0.5400
0.65 fainter
16" (406mm)
14"
0.7350
0.35 fainter
16" (406mm)
17.5"
1.1484
0.15
16" (406mm)
18"
1.2150
0.2
16" (406mm)
20"
1.5000
0.45
16" (406mm)
24"
2.1600
0.85

Table for a 17.5" (444mm) telescope

Present Scope
New Scope
Stars will appear this many times brighter
Stars will appear this many magnitudes brighter
17.5" (444mm)
60mm
0.0188
4.35 fainter
17.5" (444mm)
90mm
0.0400
3.5 fainter
17.5" (444mm)
4"
0.0522
3.2 fainter
17.5" (444mm)
4.5"
0.0661
2.95 fainter
17.5" (444mm)
5"
0.0816
2.75 fainter
17.5" (444mm)
6"
0.1128
2.35 fainter
17.5" (444mm)
7"
0.1536
2.05 fainter
17.5" (444mm)
8"
0.2006
1.75 fainter
17.5" (444mm)
9.25"
0.2682
1.35 fainter
17.5" (444mm)
10"
0.3135
1.25 fainter
17.5" (444mm)
11"
0.3793
1.05 fainter
17.5" (444mm)
12"
0.4514
0.8 fainter
17.5" (444mm)
14"
0.6144
0.55 fainter
17.5" (444mm)
16"
0.8025
0.25 fainter
17.5" (444mm)
18"
1.0156
0.05
17.5" (444mm)
20"
1.2539
0.25
17.5" (444mm)
24"
1.8056
0.65


Table for an 18" (457mm) telescope

Present Scope
New Scope
Stars will appear this many times brighter
Stars will appear this many magnitudes brighter
18" (457mm)
60mm
0.0178
4.4 fainter
18" (457mm)
90mm
0.0378
3.55 fainter
18" (457mm)
4"
0.0494
3.25 fainter
18" (457mm)
4.5"
0.0625
3.05 fainter
18" (457mm)
5"
0.0772
2.75 fainter
18" (457mm)
6"
0.1067
2.4 fainter
18" (457mm)
7"
0.1452
2.1 fainter
18" (457mm)
8"
0.1896
1.85 fainter
18" (457mm)
9.25"
0.2535
1.5 fainter
18" (457mm)
10"
0.2963
1.3 fainter
18" (457mm)
11"
0.3585
1.15 fainter
18" (457mm)
12"
0.4267
0.95 fainter
18" (457mm)
14"
0.5807
0.6 fainter
18" (457mm)
16"
0.7585
0.3 fainter
18" (457mm)
17.5"
0.9074
0.1 fainter
18" (457mm)
20"
1.1852
0.2
18" (457mm)
24"
1.7067
0.6



Table for a 20" (508mm) telescope

Present Scope
New Scope
Stars will appear this many times brighter
Stars will appear this many magnitudes brighter
20" (508mm)
60mm
0.0144
4.6 fainter
20" (508mm)
90mm
0.0306
3.8 fainter
20" (508mm)
4"
0.0400
3.5 fainter
20" (508mm)
4.5"
0.0506
3.2 fainter
20" (508mm)
5"
0.0625
3.0 fainter
20" (508mm)
6"
0.0864
2.75 fainter
20" (508mm)
7"
0.1176
2.35 fainter
20" (508mm)
8"
0.1536
2.05 fainter
20" (508mm)
9.25"
0.2054
1.7 fainter
20" (508mm)
10"
0.2400
1.55 fainter
20" (508mm)
11"
0.2904
1.3 fainter
20" (508mm)
12"
0.3456
1.15 fainter
20" (508mm)
14"
0.4704
0.8 fainter
20" (508mm)
16"
0.6144
0.55 fainter
20" (508mm)
17.5"
0.7350
0.35 fainter
20" (508mm)
18"
0.7776
0.3 fainter
20" (508mm)
24"
1.3824
0.35


Table for a 24" (610mm) telescope

Present Scope
New Scope
Stars will appear this many times brighter
Stars will appear this many magnitudes brighter
24" (610mm)
60mm
0.0100
4.9 fainter
24" (610mm)
90mm
0.0213
4.2 fainter
24" (610mm)
4"
0.0278
3.9 fainter
24" (610mm)
4.5"
0.0352
3.65 fainter
24" (610mm)
5"
0.0434
3.4 fainter
24" (610mm)
6"
0.0600
3.05 fainter
24" (610mm)
7"
0.0817
2.75 fainter
24" (610mm)
8"
0.1067
2.45 fainter
24" (610mm)
9.25"
0.1426
2.1 fainter
24" (610mm)
10"
0.1667
1.95 fainter
24" (610mm)
11"
0.2017
1.75 fainter
24" (610mm)
12"
0.2400
1.55 fainter
24" (610mm)
14"
0.3267
1.25 fainter
24" (610mm)
16"
0.4267
0.95 fainter
24" (610mm)
17.5"
0.5104
0.75 fainter
24" (610mm)
18"
0.5400
0.65 fainter
24" (610mm)
20"
0.6667
0.45 fainter

Star Magnitude Comparision Table

If you are not sure of (for example) what "3 magnitudes" looks like, you can look at some naked eye stars to get an approximate idea. Use the table below to locate a pair of stars with the approximate brightness difference you are looking to verify:

Magnitude Difference
Compare these two stars to get an approximate idea of the difference
.5
Procyon and Betelgeuse or Pollux and Bellatrix
1
Vega and Spica or Regulus and Denebola
1.5
Sirius and Capella
2
Rigel and Alphard
2.5
Rigel and Mintaka (northernmost Orion Belt star)
3
Rigel and Zeta Taurus
3.5
Sirius and Alhena (gamma Gemini)
4
Rigel and Zeta Gemini
4.5
Sirius and Beta Canis Minor
5
Sirius and Zeta Gemini

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