Position Angle

The measurement of position angle is easiest to make and is usually done first since the measurement of separation depends on the separation wires being perpendicular to the line joining the two stars (Figure 15.1). Position angle is defined as 0° when the companion is due north of the primary, 90° when it is due east and so on. The orientation of the position angle wire can be determined on the sky by several methods; the most common is to set the telescope on an equatorial star, allow the star to drift across the field and rotate the micrometer until the star drifts exactly along the position angle wire. Repeat at the end of the night and the mean of the two values will give the correction to be applied to all readings of position angle made during the night. If for instance at the start of the night the reading is 89°2 and at the end it is 88°8 then the mean value of 89°0 means that +1°0 needs to be added to each mean position angle taken during the night. Even if the micrometer remains on the telescope it is worth going through this procedure each night.

The measurement of position angle involves setting the PA wire to lie across the centre of the images of each star. It may be difficult to see a faint star under the wire but an alternative of setting the wire tangen-tially to the two star images is not to be recommended. Another possibility is to use the fixed and movable separation wires set slightly apart, turning them until the line between the stars is parallel to the wires. In this case the exact angle between these wires and the position angle wire needs to be known but once established should remain fixed until the threads need to be replaced.

If using the single position angle wire, it may be necessary instead to turn down the illumination so that the companion can be seen. Several measures of angle should be made depending on the brightness and separation of the pair but it is good practice to move the wire well away from the last determination before making the next measure. This should mean that the readings will be more independent.

It is as well if you are familiar with the position of the cardinal points for the telescope in use. The final position angle, being the mean of each independent setting, may need to be corrected by 180° depending on the quadrant in which the fainter star lies. Remember that in Schmidt-Cassegrain telescopes the cardinal points are a mirror reflection of those in Newtonians and refractors. The use of star diagonals will also add a mirror inversion.

As mentioned above, pairs of accurately known separation and position angle can also be used to calibrate the position angle circle on the sky and a list of some bright ones is given in Table 15.1.

Table 15.1. A list of bright calibration pairs

Pair

RA

Dec

Mags

PA

Sep

PA

Sep

PA

Sep

(2000)

(2000)

2000

2000

2005

2005

2010

2010

ß Tuc

00 31.5

-62 57

4.4

4.5

168.4

26.98

168.3

26.97

168.2

26.97

Z Psc

01 13.7

+07 35

5.2

6.4

62.9

22.81

62.8

22.77

62.8

22.73

e Pic

05 24.8

-52 19

6.3

6.9

287.7

38.14

287.7

38.14

287.6

38.14

S Ori

05 32.0

-00 18

2.2

6.8

0.2

52.45

0.1

52.45

0.1

52.45

Y Vel

08 09.5

-47 20

1.8

4.3

220.4

41.22

220.4

41.22

220.4

41.21

i Cnc

08 46.7

+28 46

4.0

6.6

307.4

30.39

307.4

30.38

307.5

30.37

£1627

12 18.2

-03 57

6.6

7.1

195.6

20.00

195.6

20.00

195.6

19.99

24 CBe

12 35.1

+ 18 23

5.0

6.6

270.3

20.18

270.2

20.18

270.2

20.18

a CVn

12 56.0

+38 19

2.9

5.6

228.5

19.34

228.5

19.32

228.5

19.30

Z UMa

13 23.9

+54 56

2.2

3.9

152.3

14.44

152.5

14.45

152.6

14.45

K Lup

15 11.9

-48 44

3.9

5.7

143.1

26.45

143.1

26.42

143.1

26.40

v Dra

17 32.2

+55 11

4.9

4.9

311.0

62.07

311.0

62.08

310.9

62.09

e Ser

18 56.2

+04 12

4.6

5.0

103.6

22.38

103.6

22.40

103.6

22.42

16 Cyg

19 41.8

+50 32

6.0

6.3

133.3

39.56

133.3

39.62

133.2

39.69

o Cap

20 29.9

-18 35

5.9

6.7

238.4

21.86

238.4

21.86

238.4

21.85

ß PsA

22 31.5

-32 21

4.3,

7.1

172.2

30.37

172.2

30.38

172.2

30.39

P Tuc Both stars are close pairs in

a large telescope.

Z Psc The compan

ion is a close pair in

a large

telescope

0 Mus The primary

is a close

pair in a large telescope.

8 Ori The primary

is a close

pair in a large telescope.

Was this article helpful?

0 0
Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook


Post a comment