Magnitude and Separation Limits

There are several factors that can affect the magnitude and separation limits (i.e. the faintest stars visible and the minimum separation attainable) for a pair of binoculars. For example, conventional hand-held 7 x 50 binoculars can resolve double stars separated by approximately 1', whereas image-stabilised binoculars in the 15 x 45 range can typically reduce this to

Table 3.1 Some fine binocular double stars.

RA

2000 Dec

Pair

Comp

Date

PA

Sep

Va

Vb

Constell.

0149.6

-1041

ENG8

AB

1991

250

184.0

4.66

6.72

Cet

0156.2

+3715

STFA4

AB

1991

298

199.5

5.69

5.89

And

0405.3

+2201

STT559

AB

1 993

2

173.9

5.90

7.94

Tau

0433.4

+4304

SHJ44

AB

1 993

1 97

120.3

6.09

6.80

Per

0506.1

+5858

STFA1 3

AB

1991

9

178.7

5.22

6.08

Cam

0530.2

-4705

DUN21

AD

1991

271

197.7

5.46

6.64

Pic

0535.4

-0525

STFA17

AB

1 995

314

134.6

4.90

5.00

Ori

0604.7

-4505

HJ3834

AC

1 950

321

196.7

6.00

6.34

Pup

0704.1

+2034

SHJ77

AC

1 994

348

100.3

3.80

7.56

Gem

0709.6

+2544

STTA83

AC

1 992

80

119.0

7.09

7.77

Gem

0750.9

+3136

FRK7

AB

1991

84

76.8

6.83

7.73

Gem

0814.0

-3619

DUN67

AB

1991

1 76

66.7

5.09

6.11

Pup

0855.2

-1814

S585

AB

1991

1 50

64.7

5.75

7.06

Hya

0929.1

-0246

HJ1167

AB

1935

4

65.8

4.60

7.18

Hya

0933.6

-4945

DUN79

AB

1 91 3

32

135.4

7.37

7.50

Vel

1228.9

+2555

STFA21

AB

1991

250

145.2

5.29

6.63

Com

1235.7

-1201

STF1659

AE

1991

275

155.7

7.99

6.70

Crv

1252.2

+ 1704

STFA23

AB

1991

51

196.1

6.32

6.93

Com

1313.5

+6717

STFA25

AB

1 995

296

179.0

6.52

6.96

Dra

1327.1

+6444

STTA123

AB

1 994

1 48

69.6

6.64

7.01

Dra

1350.4

+2117

S656

AB

1 993

209

85.5

6.82

7.29

Boo

1416.1

+5643

STF1831

AC

1991

40

109.9

6.68

7.07

UMa

1520.1

+6023

STTA138

AB

1991

1 96

151.7

7.62

7.74

Dra

1536.0

+3948

STT298

AC

1991

328

122.0

6.78

7.65

Boo

1620.3

-7842

BSO22

AB

1991

10

103.2

4.68

5.27

Aps

1636.2

+5255

STFA30

AC

1 994

1 93

88.9

5.07

5.53

Dra

1732.2

+5511

STFA35

AB

1 992

311

61.7

4.86

4.89

Dra.

2013.6

+4644

STFA50

AC

1 993

1 73

107.2

3.80

7.01

Cyg

AB

1 993

323

336.5

3.80

4.80

Cyg

2028.2

+8125

STH7

AC

1991

282

196.8

5.38

6.55

Dra

2037.5

+3134

STFA53

AB

1 993

1 76

181.1

6.32

6.51

Cyg

2110.5

+4742

STTA215

AC

2000

189

135.7

6.40

7.29

Cyg

2113.5

+0713

S781

AB-D

1991

352

183.9

7.15

7.24

Equ

2143.4

+3817

S799

AB

1991

60

150.0

5.69

6.99

Cyg

2144.1

+2845

STF2822

AD

1991

46

198.3

4.49

6.89

Cyg

around 15'' (Sue French, private communication). On the negative side, a bright moon or artificial lighting can create the all too familiar sky-glow that renders faint stars invisible, whilst the presence of atmospheric pollution, cloud or haze can also impair observation. This is most obvious when attempting to study objects located at low altitude; incoming light is more readily absorbed by the thicker layer of atmosphere which may, in severe cases, reduce the apparent brightness of a star by several magnitudes.

Table 3.2. Descriptions of some double and multiple stars from Table 3.1.

Pair

Notes

ENG8 x Cet. A white and pale yellow double located SW of the orange 4th mag. star Z Cet.

STFA4 56 And. Pale yellow, pale blue. Lies on the southern border of the open cluster NGC 752.

STT559 39 Tau. Easy white and bluish-white double. East of the yellow 4th mag. star 37 Tau.

SHJ44 57 Per. Superb, bluish-white pair in a field sparkling with many faint stars.

STFA13 11,12 Cam. Bluish-white, pale yellow. Fine pair. A curved chain of four stars following.

DUN21 Orange, blue. Spectacular. Forms a right-angled triangle with two 7th mag. stars.

STFA17 01 - 02 Ori. Two silvery white, 5th mag. twins enveloped by the Orion Nebula.

HJ3834 A neat white pair in a curved E to W chain. The white 4th mag. n Col. lies NW.

SHJ77 Z Gem. An unequal, yellow and bluish-white couple on a rich background. Tiny comes.

STTA83 A faint, white double in a dense region near the open cluster NGC 2331.

FRK7 A splendid, white pair, 3° E of P Gem. Preceding a dense field.

DUN67 This bluish-white double forms a parallelogram with three other faint stars. Fine area.

S585 A pleasant, pale-yellow pair located south of a W-shaped formation of stars.

HJ1167 T1 Hya. White, bluish-white. Unequal. Easily found south of a group resembling Sagitta.

DUN79 An easily resolved pure white couple. The 4th. mag. M Vel. lies N.

STFA21 17 Com. A beautiful, blue pair situated in the Coma Berenices cluster.

STF1659 This white double lies at the NE end of a chain of three tiny stars.

STFA23 32, 33 Com. Pale orange and bluish-white. Lovely contrast. S of the Coma Cluster.

STFA25 A superb orange pair, easily resolved. Situated 3° from STTA123 (see below).

STTA123 Both components are pale yellow. Located in a small arc of fainter stars.

S656 This neat white pair closely follows the yellow 5th mag. star 6 Boo.

STF1831 A splendid, bluish-white double in a field densely populated with tiny stars.

STTA138 This delicate, white pair follows the pale yellow 3rd. mag star i Dra.

STT298 Both pale yellow. Fine field with 53 Boo (white) and u Boo (orange) to the NE.

BSO22 A beautiful, golden yellow pair, almost equal in brightness and easy to resolve.

STFA30 Grand, bluish-white pair preceded by a five-star group shaped like a capital X.

STFA35 v Dra. An exquisite double, comprising two pure white 5th mag. stars.

STFA50 31, o1 Cyg. Gold, green, blue. A magnificent triple star on the fringes of the Milky Way.

STH7 75 Dra. Both stars orange. A fine, bright pair located in a rich area of sky.

STFA53 48 Cyg. Two pure white "twins" set in a superb region of the Milky Way.

STTA215 Both stars white. Rich area. The orange, 5th mag. 63 Cyg. lies W.

S781 This equal, bluish-white pair is situated near the centre of the Equuleus quadrilateral.

S799 79 Cyg. Both components white. The SE member of a circlet of six stars.

STF2822 | Cyg. White and bluish-white. Unequal but easy. Set against a rich stellar background.

These "minus points" afflict all visual observers, but should not discourage perusal of the heavens. On a clear, dark night there is much that we can see and do.

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