Polarisation in the galaxy and the large Magellanic Cloud

POLARIS.i.TIOlJ lIT ~HE G. LA.:A.""Y

um

TH.w IJI ,GE l!"_G£LLAFIC CLOUD

1: AT UIAJ112 VI'V lL4..TEAtT 1965

} .. thesis subrni tted for the De,;ree of Doctor of Philosophy

The original \Iork reported in this thesis is that of the candidate alone.

i}.";/l~~

(IT. VISV.'JT.ATI \IT)

"

I

T ;BLE OF CCJTENTS

clmowledbements iv

Ch~.pter I

InJeroduction . . . . . . . . . . . . . . .. . . . . Ch~~ter II

The (101).ble channel :>ol~rilleter

1. O~tical ley- out . . . . ... . . . . 17 2. Detc..i ls of con' truction.. .. .. .. . .. . .. . .. .. .. .. 18 3. hdjustment of the optical com~on~nts ... . .. ... . 4. Depol rise::· . . . · . . . . 5. C.-,libra tor ... . . .. . . .

-u. S lertion of ffiulti:!!li ers . . . ... . . .. . 7. Electl'onic eql.J.ipment . . . . ... . .. . . .. .. . 8 . .. ountin...., the ~oL..rimete::.:' on th~ telescope . . . . .

S

.

Checkin~ the focus of the F8bry lens . . . . . ... . . 10. Tilting the pol'rime~er . . . .

.

...

.

Chcpter III

Obs rvational techflique end l'eduction of data

1. Lethod of measuring the poL ris~ tion . . . . .. . . 2. Reduction of datG-. ... . ... . . ·. ·· · · 3. Elliotic8l polaris tion ... ....... ··· ··· · 4. Che c_{kinb the func tion of the Fabry} lens ... .. . . 5. Di cu,)sion of SOUl ces of erl'or 3 'lJ1..d tl1t-ir

elinlinf'tion . . . .... . .. ... ... . . . J . Findi lC the osi tion ,-.n...;le of tnL e 11..1.8 ~orial

north- south \i th respect to position' 11. le sc le ensraved in the pol'rimeter . . ... .. .. ... .

21 24

29

31 33 35 36 37

3b

.1.~ _{, )} 45 45

46

I

f

,:

ii

(continued) TABLE OF COlTTENTS P. GE

Chapter IV Instrumental pol arisation

1. Definition . . . 54 2. Polari sation inherent to mirror •...•...• 55

3

.

The evaluation of t e mirror polarisation .••.. 57 4. Observational data for unpolarised stars •....• 58

5. Errors... 59

6. .1ean instrumental polari sation... 61 Chapter V

Observational data for wavelength dependence of polarisation

1. Selection of stars .•...•...••. 63 2. Selection of fil ters ..•... 63

3

.

Stability of the instnunent vrith filters ...• .. 66 4. Observations of polari sation ot U,B,V,R

'lild IR i'or programme stars. . . 68 5. Intrinsic polarisation ... ... ... ... 70 6. Discussion of errors (internal) •... .... .... 70 7. Duplicate observations plot . . . 75 8. Calculetion of external errors ... . ··· 75 9. Comparison of OUI errors \ii th those of other

obse:r"'Vers. . . . . . . . . . . . • . . . ... . ... . . . . . .. . 78 10. Photometric [m,d spectral data for :,rocraLllIle

stars.... . ... . .... . . . . . . . .. .. . . . . 78 Chapter VI

Uavelength dependence of polarisation in southern Milky ay ,

1. Observations by other observers ..•... • 81 2. Theoretical results for the wavelength

i i i

(continued) TABLE OF CONTElTT

3. Our observations of v/avelength dependence

of polarisation. . . 87 4. Polarisation and extinction by c...ust particles 88 5. :Tormalisation of P

t . . . .. . . .. 89 6. Alignment parame t er. . . . . . . . . . . 93

7

.

Correlation between P

/

A

and the \lavelenoth

de}endence of pol~risXti~n... 99 8. Colour-orientation dependence .. . . .... .. . ... 102 9. Correlation vIi th colour exces.J.. . . 104 10. Interpl'etation of our results.. .. . . . .. . ... 105 11. Conclusion.. ... . . .... .. . . .. . 107

Chapter VII

Interstellzr polarisati on in the Large Eagellanic Cloud

1. Previous observations . . . .. . . .... 111 2. Sin,;l e- channel observe:.. tions. . . . . . . . . . . . . 112

3

.

Double-chamlel observations .. .. . ... .... . . 114 4. ; avelength de}endence of polarisation .... . ..• 116 5. Discussion ... . .. . .. .. . . ... . ... ··· · ····• 117 6. Conclusion .... . . . .. .... ... . .. ... . . · · · ·· 124

Chapter III Pol~risation in Eta Carinae

4-iv

~ CnTO~7LEDGEI1El TS

I am greatly indebted to Professor B.J. Bok for his const[nt hel and encoure.gemcmt in the

course of this study and to Professor S.C.B. Gascoi.;ne ~ho has ~iven so freely of his tine not only in the desi~n and construction of the double channel

:901arimeter but also in advice and guidance throughout the investication.

Dr. T. Walraven acted as adviser durin~ 1963-64. Dr. T. "Jalr::ven has always been more than tSenerous in advisin . me in the pro bl ens connected Vii th the making the depol' l~ser, adjustment of the optical components in the polarimeter and electronic probl ems. I am

deeply indebted to him for the unfailing support,

he Gave, durinG this period.

I 'Iould like to express my sincere thanks to

Drs. A.'N. Rodgers, B. Westerlund and L. Searle for the Dl..lmelOUS discussions I had \ i th them.

I am thankf'ul to r.Trs. P. Kennedy for the cl"ssification of the sgectrograLlS.

I Gra.tefully acknowledge The Australian National

University Scho18Tship Hhich made possible my

CI PTER I

IlP.J:RODUC TIOn

In 1946 Chand~asekh r showed from 2. theoretical

invcstic;ation thc.t early B st2.rs should display as

much as 11.7% linear polc:...risation at the limbs of

their discs because of Thompson sC2.tterin~ of free

electrons in their atmospheres. Bina~J stars \nth primaries of early type snd secondaries of late type

\"fere found to be the best c2.ndid2tes for testing this theory, and the ... "'olarisation \fas predicted to appear

only immediately before and 2fter eclipses. But

photoelectric observ tions by Hall (1949) end by

Filtner (1949 a,b) of the binary system C Ce~hei showed that the light from C Cephei is polarised

5%

,

which value rem8.ined unchanged at different phases

of the eclipse.

Subsecuently, Hall and Hiltner observec other

eclipsing systems as well as single stars and found that

the light from many sinsle stars is elso linearly

pol~rised . This led to the discovery that the polarisation they observed is not of stellar orisin but of interstellar

-2

-Hiltner (1951,1954,1956) 2nd Hall (1958) extended

their observations ino.ependently to more than 4000 stars,

distributed all along the northern ~Iilky aYe Most of these are early type stars having distances 100C to

3000 narsecs. The equipment used by Hiltner ,as a

d.c single-channel polarimeter using polaroid filter

as analyser. Hall usect 811. a. c. t echnique with a

rotatabl e plane parallel calcite plate as analyser.

In the Southern hemisphere E.V.P. Smith (1956) observed

200 st ars for polarisation.

The above surveys aJ.1.d the observations of others

(T-Iall and Serkovrski 1963) have shown the following:

1.i The polarisation is independent of spectral type;

ii. The l~rgest amount of polarisation is observed

nearer the plane of the gal~xy and the polc.risation decreases rapidly with increasing g~18ctic

1 ".ti tude;

iii. For a star's li~ht to be ~olarised, it is a necessary but not ~lfficient condition that

it should have undergone interstell ar absorption.

The polarisation is correlated "/i th colour excess

in such a way that the ratio p/~-V does not

exceed the value 0.195 (Hiltner 1956, Scbmidt 1958).

-3

-of the electric vector shows a striking tendency to

lie parallel to the plane over a long range of galactic

longitude. On the other hand, there are regions where

the electric vectors are randomly oriented and where

the amounts of polari sation are smaller.

The above observational evidence has ~de it clear that the pol~risation observed in the continuum of the stars' radiation is of interstellar origin. Its

correlation with colour excess shows that both interstellar

polarisation and absorption are complementary manifestations

of the interstellar particles.

An attempt made by Thiessen (1961) to interpret

the polarisation as caused by synchrotron radiation

in stellar atmosphere has been shown by Behr (1961) to be

based on a faulty selection of data.

To explain the interstellar pol''''risation by

interstellar grains, it i s essential that the grains

should be elongated which implies that a large scale

mechanism must be present which orients these grains

in space. For quantitative treatment, extinction

cross sections for particles shaped as long cylinders

have been calculated for both dielectric and etallic

particles (Van de Hulst 1950, 1957), (Smith <.lIld

-4

-laboratory extinction cross sections for oblate s Jheroids

have been obtained by Gr eenberg (1960,1963), and the

results show that the polcrisation measurements obtained

for completely orient ed prolate and oblate spheroids

are nearly the same as those for completely oriented

infinite cylinders obtained by theoretical calcul&tions.

Platt (1956, 1960) has suguested that the lLrge

unsaturated molecules of sizes of the order of 20

muy 0roduce l ol~risation because these are likely

o

to be elongated since the electric charges tend to be

at the ends of the long axis of the particles. These

molecules are varamagnetic.

The suggestion that interutellar polari sation may

be causeC by the graphite flakes, which are supposed

to condense at the surfaces of the cool N stars and

to be ejected into the interstellar medium by radiation

pressure, has been put forward (Hoyle and' ickramDsinghe

1962) . Once the graphite flill~es reach interstellar space, they will be easily covered by a thin manti e

of ice, (' ickramasinghe 1963, 1965). For gra~hite,

the conductivity perpendiculo.r to the basal plane i s

about 100 times l ess than that po.rallel to the basal

-5-Of the ali0nment forces proposed, the Davis

Greenstein mechanism (1951) is the one most generally accepted and is 2.lso one vlhich explains the observations closely. This mechanism aSSlrnes a magnetic field

p~rallel to the plane of the Galaxy. Davis and

Greenstein suggested that the field i s nearly uniform along a spiral arm. Chandrasekhar and Fermi (1953) had deduced from independent considerations, that the equilibrium of the gas in the spiral arms of the

galaxy, requires a magnetic field of the order of 10-

5

gauss to be embedded in the s)iral arm along its axis.

According to Davis and Greenstein, the elongated interstellar dust particles, assumed to be paramagnetic, are in a state of rapid rotation, of the order of a million radicffis per second, resulting from collisions with interstellar gas atoms. In a ID.-:.gnetic field, part of the rotational energy is dissipated by

pararn~.gnetic absorption in such a way as to align the short axi s \fi th the direction of the mcgnetic field. As the field i tself i s generally p2rallel to the

gaL.-.ctic plone, the p:::.rticles \lill be aligned lith their short axes parallel to the galLctic plane. When \Ie

-6

-line of sight. This direction, "Thich is the di rection

of the electric vector, indicates the direction of

the projection of the magnetic field on the plane of the sky.

:fuen the line of sight is perpendicular to the

ma0netic field, i .e. perpendicular to the axis of

the spiral arm, one can see much alignment and hence high polarisation because the particles al'e in rapid

rotation around the short axis. When the line of

sight is p; rallel to the magnetic field i .e. along the spiral arm, one sees little alignment and less

pol arisation. The si tua tion vlill be between these tvlO extremes when the line of sight makes an an.:;le

with the magnetic lines of force. This model explains

the longituoinal variation of the observed polarisation

in our Gal axy, but it appears likely that important

systematic deviations from this general picture

exist (Hall 1958) . The mean direction of the magnetic

field derived from polarisation data is in good

agreement with the direction of the spiral arm delineated

from O,B stars Llld other Population I objects both in

the northern and southern hemispheres. Behr (1959)

observed 250 stars, most of them nearer the sun than

250 p .rsecs and his observations suggest that the

-7-In their survey of linear pol risetion of u'-'.l'lctic

radio emission G.t ~·08 11c , I~athe;;son and t ilne (1965)

found thc.t the pol ris eel re .... iol1s ~:re confined to 2..

lide band \lhich is ')er end~ icul l' to our 0 ' alcctic

plane. They also found that the direction of the

elec~ric vectors of intrinsic polar~sation is pe~pendicular

to the gal2.cti c plane. _"l.ccol'clinS to t:le s~"nchrotron

theory, this direction is perpenclicul" to the maenetic

field. As the distance to the re ... ions '.There ~he radio

emission ori -inates is about 250 ::>0.1'8 cs, -'.;he gredicted

direction of the ma....,netic field in the vicinity of the

sun, is frOIa 2300 and 50u , which e;rees r 11 vi th

Behr 's results.

The :D~vis-Greenstein rlech' 1i r::J. C2.n be applied to

al l ty:!)es of t>rains. A stron~ rna 'netic field of the

order of 3 x 10-5C~u::::s i s rec·uired for ali..;nine the parema 'netic dielectric )a1'ticle . ~he ..;netic field

reQuired vro'\.1.1d be less or I e:!.'ri tes , iron l'!'1ins , GraI:L1i te

f'l "ke J cU1d In. tt p< rticl es (~Tall and Serkowski 1963,

ickramasin..;he 1963).

The theoretical maximum value of the ratio

1

-8

-prol te spheroids obtained from micro 'rave measurements is 0.09 (Greenberg et al 1963) for a refractive

index 1.33 - 0.05i (dirty ice) Ylhile for gra hite, metallic, ferrite and Platt particles the maximum

PIAv

i s very much larger; these values should be

compared to the observed maximum of 00065. ( _v is

assumed to be equal to 3 x E_B__V)'

Thus in the case of dielectric ice particles alone,

the difference betVleen the theoretical ms.ximum value of

PIAv

and the observed msximum is small indicating

that, if the ice particl es vrere considered as

interstellar crains, they should be nearly com91etely aligned. This rigorous condition does not arise for

other types of p rticl eso

The first observations of wavelength de endence

of polaris8tion by Hall and Hiltner had shorm th3t the

polarisation is nearly constr.nt with wavelength from

ul traviolet to visual becominG smaller towards the

red, Yli th a st eady decrease towards loncer wavelen[, thso

Subse~uent observations by Behr, Treanor, and Gehrels

have shown that the v.,ravelength dependence of polarisation

also decreases towards the ultraviolet and that

there are cha.nc;es in wavelensth dependence of po12.risation

-9-wavelength dependence of pol arisation for perfectly

orientec infinite cylinders of refractive index

1.3

for the Oort-Van de Hulst size distribution, have been made and the computed wavelength dependence curve agrees well lith the ob serve ~ one.

Davis (1959) has given a qualitative interpretation

of Behr's wavelength dependence of polari sation

observations in terms of differences of grain alignment. He also pointed out that any chan~e in the size of

the particles or the refractive index of the particle will shift the wavelength at 'Thich the maximUL1

polarisation occurs. Of course a knowledGe of the

alignment force is essential for the study of these chanGes. Thus, in principle, the study of wavel ength

dependence of polarisation is a method of determining whether or not the dust grains varJ from region to region in size or chemical properties.

Scope~f the present investigation

1. avelenGth deQendence of polarisation in the Southern

Jllilky '!N..: One of the problems v~e have undertaken is

the study of the Havelength dependence of po12risation for stars in different regions of the Southern IIilky .ay.

A double channel polarimeter was constructed at

-10

-fainter stars could be studied without loss of accuracy,

-llie sgectrum from ultraviolet to infrared was divided

into 5 broad bands by filters (U,B,V,R,IR).

26 stars were chosen for observation from those

observed by E.V.P. Smith (1955) in her "Survey of interstellar polarisation for the Southern Milky 7lay".

These have been selected in such a way that they are distributed from Vela-Puppis to Sagittarius.

Throughout most of the galactic plane, the dust clouds near the sun produce much of the observed

polarisation. In the southern hemisphere, the

Vela-Puppis nebulpe, Coal Seck and Ophiuchus complex come under this category. In Vela-Puppis the observed polarisation may be due to the dust associated with H II regions and overlying dark nebulae (Whiteoak 1962).

Nevertheless, in certain directions more remote clouds

contribute to the effect, such as the Carina region,

and also there are cases where we are not able to fix the position of the clouds, such as in the direction of

the galactic centre. Further, according to current ideas on spiral structure, the clouds in Vela-Puppis,

the Coal Sack and in Carina lie in our own spiral arm

-11

-selected, may be located ei ther in our own spiral arm, or in the inner part of the Sagittarius arm. Thus, by detailed studies of our selected stars, we may hope to investigate the differences in optical properties and the alignment forces , if any, of the medium over

different regions of the southern l,'iilky ay.

2. Interstellar polarisation in the Large ~.~agellanic

Cloud: Polarisation observations in our Galaxy can be

made only to distances of a fevl kiloparsecsj further,

beyond a certain distance their interpretation becomes ambiguous because of the presence, in the line of

sight, of several clouds of dust having alignments

in different directions. Thus, observations in our

Galaxy reveal the direction of magnetic field only to a limited distance. Therefore it i s im:;>ortant to

observe external galaxies to understand the behaviour

of the lUc.gnetic fi elds on a Galoctic scale.

The problems connectec. \lith the measurement of

)ol arisation in external galaxies are:

i . If we wish to have a reasonable eX.gectation of high 1!ol r.risation which can be me8sured with high percentcge

accuracy then we should select galaxies ri ch in cosmic

dust. The best candidates for such observations will

-

12-~as and dust on which magnetic fields can act (I.:Leinel 1953) . ii. To make polarisation measurements as in our orm

Galaxy we should be abl e to measure individual stars.

All external ualaxies except the I,Iagellanic Clouds

are so f~r away that it is impossible to isolate

individ~ml stars vath existing telesco~es. The next

best method is to use bi.:; focal plane diaphrag'ms

and t len measure t_le integrated effect of all stars

contained in the c1iaphro.gm. Here we run into the

problem that \Ie receive not on1 stellar radiation,

port of which has been extin:,uL.hed by the dust ~ articles, but 21so radiation from the nucleus of the salaxy

vihich has been scattered into the beaJU b:r _{the dust}

particles. The intensity of the latter component

varies accordinc to the part of the ~a13xy selected

by the diaphraGm. Such measurements have been nBue

by Ohman (194·2) end A. Elvius (1951,1956). The

interpretation of the observations i s further complicated

since the radiation scattered from the bri[,ht nucleus is Dolarised neroendicular to the direction of the

"- ~

-~oll.riso.tion arisinG fro~ the absorpti on of the

articles.

iii. The aSJect from v/hich polorisation can best be

-13-perpendicular to the equatorial plc,ne of the ,-,alaxy.

The \/hole of the gelactic plane, in which the mac-net'-' ic

fj. eld is contained, if:; then perpendicular to the line

of sid~t and maximum alignment and polarisation can be

ex]ectedo

The ealaxy r131 'lualifies ·r.i..th regard to points

one and three but its distance is so Great that

individual stars cannot readily be observed. As a

ne rest approximation, Hiltner (1958) observed 21

.::;lobul -.r clusters brighter than 17th masni tude "lith

.. he 82-inch l:cDonald telescope 'r.i.. th the aid of a

photoelectric single-channel ~olarimeter. The

results sho\l that the electric vectors have a

tendency to be preferentially aligned in a direction

parallel to the m&jor axis of 131 . The maxim~un

polarisation found is 0.061 mag and the average ratio

of polarisation to absorption is 0.030 . Until now,

no individual star has been observed for polarisation

in any external ~alaxyo

The Large MaGellanic Cloud is at a distance of

les8 than 1/10th of the distance of M31 and provides

an unparalleled opportunity for examinine the

distribution of the maGnetic field in an external galaxy

-

14

-Even though the Larue rIagellanic Cloud is classified as an irregular galaxy, observations ShOT that it is a highly flattened rotating system and t!lere is evidence

that i t is a barred spiral with, perhaps, faint outer spiral strLlcture (de Vaucouleurs

1955)

.

The line of

sic;ht makes Em angle between

75

0 and

63

0 \lith the plane

of the Cloud according to de Vaucouleurs

(

1

955), 45

0

according to Westerlund

(1963)

End

55

0 according to

Hindman et 0.1

(19

63

)

.

But the dust content which is

essential for the observation of polarisation is

-)robably much lm:er in the L2_rce Eagel lanic Cloud

(T. 7alrdven ~ J.H. 7alraven

1

96

3,

Feast et al

19

60

than in the Gal C".xy.

Polarisation observations VTere lTI.2.de for

30

reasonably certain stcl lc:-.r members of the Large LIabel lcmic Cloud.

3. Polarisation in Et2. Carinae in U,B,V,R,end IR: The

object Eta Co.rinLe is situated inside the dominatin~

Eta Carinae nebulae. Large 2JIlOunt of ;:012risation was discovered by Thackeray

(19

56

)

in the halo around the

nucleus of Eta Carinae and the same was confirmed later

by 'fes elink

(1962).

~his special nat-ure of the

poL.risation in Eta Carinae has pro.lJ)ted us to include

Eta Carinae in our ~)rogram:1I1e of vravelength de)cndence

CHltIJTER II

T}IE DOUBLE Cill lTl!EL POLA.RII':ETER

~'raximum polarisation observed in stellar radiation

is

10%

(Hiltner

1956)

rnd more often polarisation is only

a fevi percent. Thus the polarisation measures involve

measurements of .small differences of intensity and

differences of the order of hundredth of a percent

become important in the study of wavelength dependence

of polarisation.

The methods used to measure polerisation h've

recently been adequately SUIilll'laris eO. b:r _{Hall and Serkowski}

(1963)

and Hiltner

(1962)

.

The chief sources of error in

polorimetry are photon or shot noise, scintillation

noise, variations in atmos0heric transparency and the

polarisation of the telescope l'lirrors. The first

is inherent in the si.r-.;nal and the second and the third

can be l[.rgely eliminated by sui table design as in

either the A.C or the double channel D.C polarimeter. The polarisation produced by the telesco)e mirrors

constitutes an insiduous dource of error and it can

be eliminated only by rotating the whole telescope,

-1.6-Polarisation measures are absolute in character unlike magnitude and multicolour observations which are

expressed to a group of standard stars. Thus the instrumental characteristic can be carried from one

observation to another in photometry while in polarimetry, it is necessary to remove all instrumental characteristics

because of its absoluteness both in the amount of polarisation and the position angle. To achieve the above requirements, the need for care in the design and construction of the polarimeter cannot be

overemphasized.

The D.C double channel polarimeter gives a

somewhat better signal to noise ratio and since there was a good deal of experience in this observatory in the measurement of small direct currents and since most of the electronic components needed were already

available, it was decided to construct a double channel polarimeter on the same lines as Hiltner's (1962)

and Behr' s (1956).

Prof. Gascoigne designed the polarimeter and it was constructed in the Observatory workshop. A

calcite Wollaston pri sm was purchased for the analyser from the Karl Lambrecht Company. The prism is 24 rom

FIGURE 1

THE

SCI-!E~,

TIC DIAGR. : OF THE DOUBLE m:.:.l1:.TEL POL!

-~

I

Ld~ER

--

_f/IB BEAM

QUARTER YlAVE

APERTURE - ___ _

--==-+-"7 APERTURE VI EYlER

I I ",.----fA BRY LENS

WOLLASTON

P"SM_~

\

CHHODE

~D-+---1~

lJ

~

ij

0

i

111 \ 1 M, \

r

I

MIRRORS LENS FILTER CATHODE L

-1

FI L TER LENS

r

-17-and the end-on faces are perpendicular to the geometrical

axis of the prism. The deviation of the ordinary ray

from the central axis of the crystal changes with

wavelength from 110 to 80 from ultraviolet to infrared.

For the extraordinary ray, the change is from 100 to

o

7.8 • The reflection loss inside the ~ollaston, for

the ordinary ray is 0.003% , and for the extraordinary

ray is 0.8% •

1. Optical lay-out: Figure 1 represents the schematic

optical lay-out of the polarimeter used at the Cassegrain focus of the 50-inch telescope. The Fi re is drawn for a focal ratio of 18:1 .

i. provision for A /4 plate which c~n be brought into

the beam at any time.

ii. Provision for mounting a depolariser which cru~ be

brought into the beam whenever ~anted.

iii. Aperture rvheel having apertures of different sizes.

iv. A prism which deflects the star field into an

eye-piece. The prism can be removed from the beam

during exposure.

v. The first Fabry lens - focal length is 41.6-inches

vi. The calcite ollaston prism

vii. The ordinary and extraordinary rays are reflected

perpendicular to the geometrical axis of the crystal

FIGURE 2

MIRROR-CONDENSING~LE~iSi1IT-~.u..- ~

rJ4:

l

caoeox

I"

t.OM1NG FACE/.~ I

Il..OO< '

----

--,

I

I

r I

I

~IRROR

ONDENSING LENS

WHEEL

BEARING

~COLDBOX

L 1 I, MOUNTING FACE

"

WOlLASTON II ' "":

~

!

~

!

!n~

,

I

CALCITE WOLLASTON

MOJNT₂rNG

I

'

r

rn

___

~_

lr

r!

~

-

2:

I

'

~:~~t!NeIEWER

II.OCK ___ I r-'l I 1 . 1 ' I _ I _APERTURE WHEEL

m

'

, ,

OEPOLARISER

- I . ' BEARING

"/4 PLATE

-

18-viii. The condensing lens which forms an image of the primary mirror in the cathode: focal length

=

5.9-inches (One for each channel)

ix. Filter wheel (One for each channel) x. Cold box (one for each channel)

2. p-~tails of construction: A primary requirement is that the entire optical train should be accurately rotatable about the geometrical axis of the crystal. The axis of the polarimeter must therefore be made to coincide as accurately as possible with the axis of the prism. Since only a small relative movement of one of the optical components may introduce a

considerable error into the measurement, the rigidity of the whole system is of first importance.

The instrument was based on an aluminium block 1, 6.37 inches diameter and 3.9 inches thick (Fi re 2). It was turned on a precision lathe and a hol e of

1.25 inches diameter was bored in the centre of the block for mounting the 1ollaston prism. The two bearings were also turned at the same time. This

block rotates inside a steel tube of diameter 7.62 inches, length 5.87 inches and wall thickness 0.25 inches.

-19-To the front end of block 1 , another aluminium block 2 , diameter 6.60 _{inches and thickness 1.93 inches}

has been _attached. It houses two brass wheels of

diamter 3.6 inches having holes of 1.125 inches

diameter. It also houses the aperture wheel of

0.125 inches thickness and diamter 3.6 inches. The turning controls of all three wheels have been brought to the back of the polarimeter. All three wheels are

eccentrically fixed.

To the back of block 1 an aluminium block 3 of

4.87 inches thickness in barrel shape (8.5 inches long and diamter 8 inches) has been attached. This

houses the two mirrors (cut at the angle 400 50')

which deflect the ordinary and extraordinary rays to the condensing lenses. After the condensing lens

a filter wheel is mounted, which can take seven

one-inch filters. At each end of the block, a five inches square plate is attached with a hinged plate

which can take the cold boxes. As a whole, the

polarimeter weighs 60 lbs. without the aol d boxes.

2.1 ~perture wheel: This wheel consists of six holes of diameter 6.4 , 4.0 , 2.5 , 1.6 , 1.0 and 0.3 rom

which correspond to 57 , 36 , 22 , 13 , 9 , 3 seconds

-20-Each hole can be brought into the exact centre of the block. Special precautions have been taken to see no error occurs in the centring of the apertures.

To avoid any spurious polarisation due to scattering,

the diaphragms are tapered behind the apertures.

2.2 Wollaston ~rism: The ~ollaston prism and the first Fabry lens (a plano convex lens of 41.6 inches

focal l ength and 1 inch aperture) are put together

inside a brass tube and screwed in, at one end to

form one unit. The inner diameter of the tube is

such that it will hold the prism vdth no movement. The outer diameter of the tube fits exactly inside

the central hole made for the ollaston prism in the

aluminium block 1 •

2.3 Mirrors M1 and M₂: Mirrors ] 1 and H2 are plane

mirrors ground at our optical workshop. The surfaces

are aluminised. They are mounted in small brass blocks and introduced into position from the back of the polarimeter. Provision for tilting these mirrors, for pushing them in and out and for rotating

them have been provided by eccentric and push-pull

screws.

2.4 Condensing lenses: The condensing lenses are of

-21-(ouble conve:: lens

'it'l r ... C.L't11.;.'8 C.u7 inche8. T:lis co:.werucs th ben 1

to t'lE: cat~loc1e [.l1.C~ bas been )laced. 2."~ 8. dist~.ncc

0::.' ( . ,-,3 inch8 S fro "lJ'18 j.rst F .by] lens. This lens

con1)inecJ. Hi th tl.8 .oL.'st F 'or./ lens forms an e _ui vL1ent

f'v' teIll of' foc 1 1en ... th 6.1 inc cs. '.;he i"'12.Ge of -'-;l-:.e

o jcctiV8 i::. fo_'mea c.t a dis'~cnce of 5. 1 incher>, ,-:my

::1'0111 the condcnsin~ lens. The size of the iTIl2.C e of th~ 50-inch mirror on the cathode is 8.5 IJIll •

3. : djus,,:,c!I!e}~t. _o_f _ t.he. _0. )ti.c_B:..~ c.o.r~one.~t..'~: To djust the

no-:i t ion of t 1e ',011 r ston 'Jrism, the fo110 rin~ 2.rr&1:.:e cent

\1 S set 1.111 in the labor tory:

Li ,ht from a cource i -renderer. 9201'£'.11e1 ~)Y .ue~ 1S of

~ lens. The 01 rimeter is mounted rouch t 1 t i t s

'_.0"" face is per~)endicu1:.. r to the )ara11el be .Tr in

"1 fo1lO'.rin..; manner: plane Hir~o1' is fL~ed 'x:1'<..11el

.~() 'h~ _ront face of the polr l'i!"letel' and the t i l +

0''> t 1e f)' mt face is ~ djus ted <'0 tha.t tll rl f1 "'ctec

H, ht from the Ill' ne l.1irror forLls ~n imace in the

30urce itself. ~:he 01 l'iraetei' front f.J.ce is then

pel'pendi cu1 l ' to . h ral:e1 becun an the )l8one "irror

i r moved \;-1. thout disturbinG the pol\.. rill ter. The

1011'~ston l.:mi t io int'L'oduce 1 in its .,lace in block 1

,:.no. rotated until orc1in~ ry and c:rtraordinary i q~es

- 22

-mirrors I.11 ' M2 are introduced. At this position,

the . ollaston unit is permenantly screwed into the

aluminium block.

3.1 fo.djust~ent o_i_the mirro~(l11 an4. M₂): The

polarimeter was fixed on a sturdy t~ble, on four metnllic blocks, such that the rotntion axis is

perpendicular to the surface of the table. The

aperture of the polarimeter was illuminated by a

distant point source of light. The mirrors 1:1_{1 '}

end the two condensing l enses are 91aced in position.

The tilt of 1111 and 12 is adjusted to get the light from 'lo11aston prism roughly in the centre of the hole of

the cold box face. A tub e with an eyepiece at one end, is fixed to the centr21 hole of the cold box fqce, throu[':h which the ordinary and extraordinary rays emer,("e from the polarimeter. The focus of the evep1ece i s kept 1n the position of the cathode. The Fabry lens tor'ether wit the condensin,3 lens of the polarimeter form an imafe of the liGht source

in the eyepiece. The polarimeter is then rotated while

observing how the image moves in the eyepiece. The position of the source and the tilt of the prism which

directs the lir'ht into the polarimeter are adjust ed until the image ceases to shift in the eyepiece "Then

-23-the beam entering -23-the polarimeter is made to coincide

with the rotation axis of the polarimeter.

The image of the source can be brought to the

centre of the crosswires in the eyepiece for each

channel by adjusting the tilt and position of the

mirrors M1 or M2 •

It is also essential to see that the ray which

emerges from the polarimeter and passes to the cold box

is perpendicular to the rotation axis, so that the

light falls on the cathode perpendicularly. In order

to check this, the eyepiece tube is removed from the

central hole of the cold box face, and a plane mirror

is stuck to the hole such that its surface is parallel

to the cold box face. If the image of the source can

be seen through the hole at the same time as the

observer's eye can be seen in the plane mirror, then

the light ray is perpendicular to the cold box face.

The mirrors M1 and M2 are adjusted until this condition

is satisfied. The above adjustments were done with a

visual filter.

3.2 Mounting the quarter-wave plate for the measurement

o!.~llip.tical polarisation: A

1\

/4 mica sheet, for

visual light is mounted in the first wheel of the

-24-inserting it in between the crossed polaroids whose axes are known. When the mica is brought into the beam, the fast axis makes an angle of 450 with the

principal plane of the Wollaston prism. This is checked

experimentally in the following Vlay:

In the laboratory, the aperture of the polarimeter is illuminated by a fully polarised beam. The polarimeter

i s rotated until no light comes through one of the channels, say channel 1 • The mica sheet is brought

into the beam in front of the aperture of the

pol arimeter. If the angle between the fast axis of the mica and the principal plane is not 450 , the light

can be seen coming through channel 1. In this case the mica i s rotated until again no light comes through

channel 1 • The angl e between the fast axis of the

i\

/4 plate and the principal plane of the lollaston

prism is t hen 450 •

4. Depolariser: The readings taken with the depolariser

in the beam indicat e the zero level from which the measurements of polarisation are made. Hence, it is

-25

-Billings (1951) analysed the Lyot type depolarisers

mathematically and showed that the efficient depolariser

should consist of 2 x-cut quartz or calcite plates

(optic axis parallel to the surface) having thicknesses

in the r tio 1:2 . These two plates should be put

together so that the fast axes of the two pl otes make

an angle of 450 •

The depolarisation failure dp because of the error

in the ratio of the thicknesses,

dp

₌

p

₌

dx

₌

x

₌

dp

=

p

2-

₃ dx x depolarisation failure

amount of polarisation measured

error in the thickness of the thick plate thickness of the thinner pl ate.

The alignment of the fast axes is also important.

A small error (. in this alignment leads to a depolarisation

failure

dp

=

lL::..-

sin 2 E: )_ )

P(1 -

T1

+ sin2()

It is important to note that the depolarisation failure

is a function of the amount of polarisation measured.

Natural light is unpolarised because the direction

of polcrisation and the amplitude vary erratically

and rapidly with time, and in any sufficient interval

-26

-light of broad band of wavelength is passed through

a Lyot type depol .riser, the light of different wavelengths

undergoes dofferent amount of retardation and the light

emerging has all possible directions of polari sation.

Thus it i s important to bear in mind that the natural light i s depolarised because of integration over an

interval of time while in the depolariser the depolarisation

takes pl ace because of integration over a range of

wavel ene ths •

£' 2(n - n )t

u~ __ e 0

{ \ -

/\

0-

_A

₌

_{angular retardation}

n ,n

=

refractive indices for ordinary and extraordinary

e 0 _rays

t = thickness

The greater the retardation introduced, the greater

the possibility of depolarisation. The retardati on is proportional to the thickness of the pl ate for a

particular wavelength and particular material. A

calcite plate gives ten times the retardation of a quartz pl ate of the same thickness. The range of wavel engths analysed should be sufficiently brocd.

If two quartz plates having thicknesses of 1 rom and

2 Iron are combined to form a depolariser and light of

o

band width of 100 A is passed through, the failure

-27-Making a depolariser in the observatory optical shop:

The quart z crystals were polished and those having no

air bubbles inside and with no colour were selected.

An optical arrangement consisting of a light source followed by two crossed polaroids was set up on a

horizontal pl ane. One of the polaroids pl ane of

vibration was fixed in such a way as to be perpendicular

to the hori zontal plane. The crystals were placed

one after another in between the crossed polaroids and

the position of the opt ic axis was determined. The

surfaces were ground so as to become parallel to the

optic axis. This was achieved by grinding the surfaces

little by little and t esting the crystal s in the above

arrangement until a uniform illumination was obtained

vlhel1 the crystal was placed lengthwise with respect

to the line of propagation of light. At the same time

the thicknesses were also reduced to

5

mID for one quartz plat e and 10 mID for another quartz plate.

4.2 To make the angl e between the optic axes of these

two plates 450 : The pl tes were kept in between the

crossed polaroids so that the surfaces were perpendicular

to the line of propagation of light. One of the sides

was then ground to make it parallel to the optic aXis>,

as in the previous method. The other side of the plate

-28-side as one. The optic axis was parallel to the length

of this plate. (length of the pl ate

=

20 mm : breadth is

equal to 14 mm.)

The second pl ate was kept in between the crossed

polaroids in exactly the same way as the first one,

but this time, one of the sides was ground in such a

way that it makes 450 with optic axis. Then the other sides of the block are ground to make a rectangular

plate n th thi s as one side.

Now these two rectangular plates are placed one

over the other with a strip of thin paper along the

edge of one of the plates, coming in between them.

They can be aligned so that all sides are parallel to

each other and taped all around so that they will not

move separately between one another. This constitutes

the depolariser.

4

.

3

Testing the thickness ratio: The plates of 5 mm

and 10 mm are first checked by a micrometer to an

accuracy of 0.02 mm . To check the ratio of the

thicknesses 1:2 the following experimental arrangement

has been set up: A light source followed by two crossed

polaroids, followed by a prism spectroscope, is arranged

on an optical bench. The

5

mm and 10 mm plat es are

-29

-the fast axes make an angle of 450 \uth the plane of vibration of the polaroid and the location of one of the fringes is noted by means of cross hairs in the

e~,repiece of the spectrograph. From the number of fringes seen in aparticular wavelength interval,

thickness x of the plate can be calcul~ted.

AI

,,~ 1

=

,\

1-

1\,.,

,

-2-( n-~---n ) .~ e 0 x

x

=

thickness of the plate

n

=

refractive index for ordinary ray o

ne

=

refractive index for the extraordinary ray

A

l

A~= favelengths of two adjacent fringes

The thicker the plate the more crowded are the fringes. If the plate thicknesses are exactly 1:2 ,

at a narrow portion of the spectrum, the 10 mm plate

should show exactly double the number of fringes as seen with the 5 rom plates.

5. Calibrator: The polarimeter is attached to the Johnson photometer off-set guiding box. To check the stability of the polarimeter's performance from time to time, provision to inject a known amount of

polarisation by a tilted glass plate has been provided.

As it i s necessary to keep the tilt of the glass plate with respect to light beam constant, two glass plat es

"'--~2

---\

\

\

\

\

\

\

QUARTZ

\

PLA"- ES \

I

/"ll-I

/ FIGUP.E 3

C

AL

C

I

E

PRISM

-30-clear that a

1 + a2

=

a , which can be made to remain constant by attaching the two glass plates to a unit

which is shaped as in the Figure. The advantage of

this system is that minor changes occurring in _{a 1 due}

either to the tilt of the incident beam or of the

whole unit, will be neutralised by the changes in _{a 2}

in the opposite direction thus keeping polarisation

produced by the unit constant.

Fused quartz pl ane parallel plates of thickness

of 1/32 inches are used for making the unit as in

Fi ure 3 . The angle a is put as 500 and the

polarisation introduced by the calibrator can be

calculated by the Fresnel formulae.

. 2C- _r)

I1

=

Sln _{. 2("} _1 -Sln 1 + r)

I2 tan

2

li

- r)

=

tan2(i + r)

I1 - I2 P

₌

₁

(I 1 I ) x 100

-

- 2

i

=

the all3_{l e} of incidence in this case 25

0

r

=

the angle of r efraction P = polarisation in percent.

The calibrator unit i s screwed into the bottom

plate of the Johnson photometer head. This can be

-

31-to ensure that the dust cannot accumulate on the glass

surface, when not in use.

A calcite crystal which can be brought in and out of

the beam is al so mounted in the Johnson photometer head

and allows one to obtain compl ete polari sed light for

checking the efficiency of the depolariser .

Care is taken to see that all the optical surfaces

are cleaned free of dust before mounting. The entrance

hol e and the two exit holes of the cold box end of the

polarimeter are always closed whenever the polarimeter

is not in use. After observations, the instrument is

kept inside a box whi ch is l eft in a warm place.

6. Sel ection of multipliers: For accurate double channel

polarimetry good signal to noise ratio and seeing

compensation are essential . ith this in view, and al so

so as to be able to centre the Fabry image of the

50-inch mirror in the most sensitive portion of the

cathode surface, 12 cells of EMIT 95248 (cathode is

Cs Sb) and 5 cells RCA 7102 (cathode is S1 surface)

were tested so that pairs of tubES having equal and

uniform sensi tivity could be selected.

A point source of light i s focused on the cathode

FIGURE 4

c!

r:::FCJDBS OF l'='_TCh"'ED EI I 9 5248 ~UBES

The lines of equal sensi ti vi ty has been dravm

on the cathode surface. The sensitivity at the

point on the cathode, whl.re 'lIe get m ximum

cold bo,x

entrance

hole

o

6

FIGURE

4

9?ometrica

1

cenlre

of

the cothode

1

2mm

FIGUrtE 5

The lines of equal sensitivity hE's been

drarm on the cathode surface. 'i:he sensi tivi ty

8t the point on the cathode ~here we bet

geometrical centre of the cathode

20

!

40

/

60

/

-'

0

/

C

oldbc5x

e

ntron

ce ho

I

e

-32

-image in the cathode is moved to different known points

on the cathode surface by moving the point source of

light in directions X and Y. Thus the sensitivity at

diff erent points at the cathode surface has been

measured by making horizontal and vertical scans and

the lines of equal sensitivity drawn on the cathode

surface.

Two pairs of tubes of each type having nearly

equal and uniform sensitive surfaces were selected for use. (Fig. 4 and 5) It was found that the centre of

maximum sensitivity shifted from the geometrical centre

of the cathode surface for all Efi1I tubes by about the

srune amount.

6.1 Magnetic and electrical shields for the multipliers:

When the polarimeter i s rotated, the effect of the

earth's magnetic field will affect the flow of electrons

and this leads to a change in the sensitivity of the

cells when they are kept in different positions. This

effect is compari tively larger in end-on tubes. In

the case of RCA 7102 , the electrons emitted from the

photo cathode travel one inch before they reach the

first dynode and the effect of the magnetic field

at this stage of the electron flow is great .

-33-permeability, have been fitted over the glass surfaces

of both the RCA 7102 and the ENI 9524S tubes. Further,

electrical shield has also been provided for all the

cells by connecting the cathode potential to it.

In addition to the above in the case of red

cells RCA 7102 , an aquadag coating was applied to

the outer glass envelope of the multiplier and connected

to the cathode potential. This prevented the inside

wall of the tube charging to a potential near that

of the anode especially when operating near maximum

vol tage, vlhich is apt to lead to an internal discharge,

thus increasing the noise. It was found impossible

to put the cell into regular operation without this

aquadag coating.

7. ~le<?tronic equipment: Until November 1963, the

signals from both the multipliers were fed into two

G.R. amplifiers, two integrators, and two Broln

recorders. A single stable John Flulce power pack was

u~ed for the supply of high tension to both the tubes.

"Provision was made to put resistances in series with

one of the dynode chains in steps of 100 K , so that

hi8h t ension in one of the multipliers can be changed

\nth r espect to onother. Much trouble was experienced

TO G.R. AMPLIFIFIER 2

I

E

~

. / " ' _

-Ry

Ry

I

E

TO G. R. AMPLI FI ER I

SCHEMATIC CIRCUIT DIAGRAM OF

DOUBLE CHANNEL INTEGRATOR

M

FROM THE ANODE OF

T

TIMER THE MUL TIPLlE R

I

TO I NPUT TERMINAL OF

Ry

RELAY

G.R

E

TO E TERMINAL OF G.R

V

BATTERY

P

PUSH BUTTON FOR

C

CONDENSER

I _{INTEGRA TION}

~

PUSH BUTTON FO R

R

RESISTANCE 10 OHMS 4

DISCHARGING THE CONDENSER

-3

4

-As it 12S felt that in general, the more instruments used the more complications, it was decided to modify

the G.R. amplifier itself as the condenser-integrator, thus eliminati ng the separat e integrator s in both the channel s .

The circuit diagram of the integrator is Given in

Fi re 6 0 Only matched polyst rene condensers have

been used. The condensers provided are 1 , 0.1 , 0.01

microfarad. Reed relays have been used. {hen the G.R. is connected to the circuit, the switch of the input resistance RA of the G.R. is pl aced in the infinity position as the condenser is coming in the

pla_{ce of RA} •

If any variable leakage of input to ground ha.ppens

in the ceble or anywhere else before the input goes to G.R., it Ylill lJroduce 8 high di sturbing current if

the G.R. \lorks in the E-mode. If the G.R. i s worked

in I-mode the effect of this l eakage is reduced by a factor equal to the amplification of the factor

of the amplifier i.e. 106 times. Hence, it was

decided to use the G.R. in I-mode.

7.1 The _e):_e:.ctJ:.:.o}lic gyteII!.._used from December

1

9~3:

Signal s from both the multipliers were fed to G.R.

FIGURE 7

MULIPLI

S

1

G.R

AMP.

MODI

FlED

AS INTEGRATOR

DOUBLE CHANNE

BROW

RECORDER

~---

~

~

~

~

---I

r

DIGITAL VOL

TME TE

R

.. ~

IBM

TYPEWR

I

TE

R

-35-i s used for both the integrators. The integration

time can be fixed any "There between 0 to 60 seconds.

Two buttons, one to integrate and another to discharge

the condenser are kept at the eye-end of the telescope.

The outputs from the integrators are fed into a

double channel Brown recorder and a digital volt meter

and the printer. Figure

7

is a block diagram of the

recording system.

Another Brovm recorder in connected in between the

E-terminals of the G.R.s to measure the differences

between the two channels. A potentiometer to shift the

zero of the recorder to the centre of the chart has

been provided. A resistance box is also put in series

\lith the recorder so that the difference between the

two G.R.s can be enlarged five times. s we experienced

difficulty in putting this part of it into r egular

operation, we did not measure the differences for our

observations.

8. The mounting of the polarimeter on the telescope:

The polarimeter's outer steel case is synwetrically

bolted to the base plate of the Johnson photometer

guiding head by means of four 3/8 inches screws such

FIGURE 8

The photograph dericts the polcrimeter along with the cold boxes mounted at the Cassegrain focus of the 50-inch telescope. The section at the

-3

6

-are in the same line. The Johnson photometer head is

connected to a round steel fl ange which is in turn

fixed to the 50-inch mirror cell by six screws. The

fl exure of the whole set up has been checked and was

found to be l ess than 1/1000 inches when the telescope

is at an angle of 500 from zenith. The flexure between

the Johnson photometer head and the polarimeter was

found to be very small. The play in the bearing

is so low that it carmot be measured by a dial gauoe.

Figure 8 is a photograph of the polarimeter as attached

to the Cassegrain focus of the 50-inch reflector.

9

.

Checking the focus of the Fabry l ens: The focusing

of the objective mirror on the cathode is necessary

because the movement of the star's image due to bad

seeing or bad guiding in the aperture of the polarimeter

will move the Fabry image on the cathode if the cathode

is not in focus. The locati on of the Fabry image of

the 50-inch mirror in the polarimeter cathode ends

was found by using a knife-edge at the location of

the cathode positions. As the focus varies from filter

to fil ter, the average focus of blue and visual and

red filters was taken and found to be 3.75 inches

away from the face of the cold box end of the polarimeter.

The cold box is so arranged that the cathode surface

-37-10. Tilting the polarimeter: If the rotation axis of the polarimeter i s not pointing towards the centre of the telescope, the images in the photocathodes will move in circles over the surface of the cathode, when

the polarimeter is rotated to different angles. Further there is a possibility of light being cut by

the mirrors M1 and M2 if the alignment is out by

much. A very bright star is focussed in the aperture of the polarimeter and the polarimeter then rotat ed

to different angl es while the movement of the Fabry images in the cathode end is observed with respect to the crosswire of an eyepiece placed in the position

of the cathode. From the movement of the images the necessary tilt is calculated, and is realized by introducing packing between the mirror cell and the iron ring which holds the Johnson photometer head.

This is repeated if necessary, until the images at the cathode end do not move when the polarimeter is rotated to different angl es. Later this packing has

been replaced permanently by an aluminium ring

CIL'. .. PTJr' I II

OBSERVATIOYAL T3CHlTI.UE lID RIIDUCTIOlT OF DATA

Hal l (1949) and Hiltner (1949) found that the libht from the distant star is )artially plane )olarised. Hal l and TIikesel l (1949), Dombrovski (1953) and Serkowski

( 1961, 1965) made ob serva tions to detect the circulcrly polrrised comlonent in the radiation of these stars

wi thout success. Thus the measurement of interstellar

)01 risation reduces to the measurement of )artially

plene pol&risation and the star l ight C&ll b~ considered

r.S incoherent superposition of two beams, one un .olo.rised

and the other com-)letely linearly _)ol"ri sed.

1. l~~thod of.. measurin&....£.ol<Jisatio:q: Suppose the

intensity of the star l i.::ht is I and the real degree

of _)ol '"1risation is P and that the electric vector of the incident polarised li~ht makes an an~le a (measured

~Arom north through east)

I

=

(1-P)I + PI

::'I

=

nol 'rised com"[)onent

( 1-P)I = un,}olarised comTlOnent

\1hen I (star l ight ) ::lasses through the ~ollaston it is divided into ordinary Lnd extraordinary rays ~olcrised

-39-If the el ectric vector of ₁₁ makes an angle b (measured

from north through east) , from the lew of Malus, we get

11 = "2( I 1-P) + PI cos2(b-a)

I _sin2

(b-a) 12 = "2( 1-P) + PI

If P

b is the degree of polarisation measured at the orientation of the polari meter, b

11 - 12

P

=

=

P cos 2(b-a)

b 11 + 12

Now 11 and 12 can be revrri tten in terms of I and _Pb

Vhen ₁₁ and

m'b and m2 _b

11

=

~

(1

+ _{Pb )}

12

=

~(

1

- _{Pb )}

12 fcl.ll on the multinliers,

are recorded.

m' _b = S' _b G' _b I₂ ( 1 + Pb )

2 _{S2 G2 I( 1}

-Pb )

m b = _{b b 2}

the signals

...•. ( 1 )

•.... ( 2 )

m'b ' S'b ' and G'b are the output signals of the light

whose electric vector makes an angle b with the north

pole, the f actor S'b r e)resenting the noise due to

scintill~tion and G'b the sensitivity factor of the

2

multiplier of one channel. m b is the output signal

-

40

-with the north pole, the factor S2b representing

seeinc noise and G2

b the sensitivity of the multiplier for the second channel.

Hiltner (1951) proved that the scintillation noise

is the same for different planes of vibration,

S' _b

₌

if the sensitive surfaces of both the multipliers are

unifo rm an d equal.

P

b is calculated by the ratio method. In this

2

method, the signals of the two multipliers (m'b ' m b)

are measured and taking the ratio, I and SIb are

elimin,-"t ed.

The ratio

in front of

signals m3 _b

Then, G' b G2 b m' 2b m b

=

G't(1 + _{Pb )}

G2

b( 1 - Pb)

•••• • (3)

is evaluDted by introducin~ a de90lariser

the 1ollaston in the beam and the two

4 _{are r ecorded.}

,

and m b

m3 _b = S" _b

_-2

I G' _b

m4 = S"

I

G2 since P = 0

b b 2 b

m3 _b G' _{b ••.. • ( 4)}

K2 m4 = G2 =

-4

1-3 / 4

by sub sti tu ting m b L1 b in place of in (3) ,

m' b -2-m b From (3) &- (4 ) ,

K1

K2

=

=

=

1 + P

b -:;-::-P

b

K1 - K2

~+ K2

P

b

=

P cos 2(b-a)

The observations of P

b are repeated at different

angl es band P and a are evaluated by least squares.

1.1 The observation procedure: To obtain the maximum

accuracy, the follmving observing nrocedure vIas ado')ted:

The pol~rimeter i s set at 00 and the star is centred

in the suitabl e diaphragm. The required filters are

brought into the beam in both channels. The drive

rate is checked and if necessary, adjusted. The

[,'Uiding vj_ewer is removed. from the beam Dnd the light

is allowed to fallon the cathodes. The signals from

both the nrulti pliers are fed to seuarate G.R. amJlifiers

Hhich are modified as intebrators, ru1d then to a

D.C voltmeter and printer.

A nroper condensor is chosen and a rou[;h intebration

-

42-is made and the readings are noted in the amplifier.

Then the sensitivity step of the amplifier, the

integration time and the voltage of the powerpack

are adjusted so that the integration signal in one of

the amplifiers reads approximat ely 75 divisions. The

second amplifier sensitivity step i s then turned to

the same reading as the first. As the two multipliers

have nearly the same sensitivity, the readin~ in this

amplifier will be near to 75 divisions. If it is not,

the individual voltage of both the multipliers is then

adjusted until both the amplifiers read about 75 divisions.

This adjusted sensitivity, integration time and the

voltage are allowed to remain the same throughout

the set of observations.

The observations at each angl e consist of one

integration with and without the depolariser. t 00 one

observation is made and m' , m2 , m3 , m4 are measured. o

Then, the polarimeter is rotated to 45 ,another

observati on i s made, and further observations continue

in teps of 450 upto 3600 After the last observation,

the sky reading is noted. If the sky reading is

substantial, the sky polarisation i s al so measured

ex ctly as for the star. This constitutes a set.

The star cent ring i s checked after each integration.