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"Stellar magnetism" International Conference Proceedings, 1992, 190-193.

CIRCULA R POLARIZATIO N ANALYSE R O F HYDROGEN-LIN E MAGNETOMETE R O F TH E 6 M TELESCOP E

V.G.SHTOL'* , V.I . POLYAKOV** ,

A.I . KONOJKO** , N.A . VIKUL'EV *

* Special Astrophysical Observatory of the Russian AS, Nizhnij Arkhyz 357147, Russia ** Institute of Electronics of Belorussian AS, Minsk 220841, Belorussia

ABSTRACT. zation scope gion are

In

this of

paper the The

the

test

results

obtained of

with to the

the analyzer

circular the 6 in m

polaritelerethe

analyzer 4000-5000

hydrogen-line spectral

magnetometer transmission

attached

presented.

is 93%.

The circular polarization analyzer of the hydrogen-line magnetometer attached to the 6 m telescope (Shtol', 1991) is designed to measure magnetic fields of faint objects such as white dwarfs, polars, etc. The optical scheme of the analyzer is shown in Fig. 1.

Fig.1.Optical scheme of the circular polarization analyzer. 1. Negative lens 2. Ring electrodes 3. Electrooptical phase element 4. Positive lens 5. Polarization splitter 6. Magnetometer entrance slit (height).

190


The analyzer consists of an electrooptical phase element with ring electrodes (Pilipovitch et al. , 1987) and a polarization splitter. A distinctive feature of the analyzer is the high light transmission in the working region of the spectrum (40005000 ). The high light transmission (93 %) was attained due to the use of specially selected optical materials, small number of optical surfaces and their coating. The characteristic of the spectral transmission of the analyzer is presented in Fig. 2.

Fig. 2.

Characteristics

of

the

spectral

transmission of the circular polarization analyzer.

The light transmission of the polarimetrical analyzer of the hydrogen-line magne tometer, constructed on the basis of the electrooptical phase element with semitransparent electrodes, is about 50%. At the observations of the 6 star 33 Gem at the spectral band 9
s

with the circu

lar polarization analyzer the count rate was 2*105 counts/s, and with the polarimeter analyzer with semi-transparent electrodes it was 1.1*10 counts/s. The count rate ra tio, equal to 1.82, is sufficiently close to that of the light transmissions of the analyzers - 1.86. The values of count rates were reduced to one image by the techni que presented by (Shtol', 1992). The main difference between the phase element with ring electrodes (ERE) and phase element with semi-transparent electrodes (ESE) is the dependence of the thickness of the ESE (along the optical path) upon the aperture of the parallel light beam (the diameter of ring electrodes) formed by the entrance lens of the ESE (see Fig. 1) . When the thickness of the ERE increases, it is required that the beam should be parallel to a high degree since the S/N ratio and modulation degree are dependent on its high parallelism (Shtol', 1992). The way the analyzer is mounted inside the magnetometer housing and its adjustment are also complicated by this condition. One of the systematic errors of the hydrogen-line magnetometer (Shtol', 1992) is connected with the modulation degree, while the modulation degree depends on the star

191


image siz e and the siz e of the entranc e slit of the magnetomete r (parallel light beam characteristics , performe d laborator y telescope . passin g throug h ERE) . I n orde r t o determin e thes e dependencie s w e fro m a n artificia l the prim e focu s star of the under 6 m

the measurement s o f condition s and from

the modulatio n degre e the star 33 Ge m in

Fig.3. The dependence of the modulation degree M on the magnetometer entrance slit size L. 1 - measurements of an artificial star 2 - on the 6 m telescope after the corrections for the seeing 3 - the same before corrections.

Characteristic s

of

the

modulatio n

degre e

dependenc e

on

the

siz e

of

the

magnetometer entrance slit are presented in Fig.3, where 1 is the dependence of the modulation degree equal ) M on the slit the size L (the width an and the height sta r of wit h the the slit are

whe n

measurin g

modulatio n

from

artificia l

polaroid,

cuttin g of f on e bea m (installed after the polarizatio n splitte r of the analyzer) ; 2 the sam e on the 6 m telescop e for the slit widt h L= 3 mm correction s for the image befor e the corrections . The modulation degree of the polarimetric analyzer with ESE does not change with size , at its fixe d height , after

whil e measurin g it wa s equa l to 5";

3 - the same

variation of the slit size and for one hydrogen line (after the corrections for the seeing) it is 92%. For two hydrogen and lines it becomes less and depends on

the energy distribution in the star spectrum and on the characteristic of the magnetometer spectral transmission. The modulation degree reduction is caused by its de-

pendence on the wavelength and on the value of the control voltage, supplied to the electrooptical phase element. From Fig.3 it is seen, that the modulation degree va-

ries by 7% when the slit varies from its minimum width (0.5 mm) to maximum one (3.6 mm), not at the fixed at height of 3 mm. The 7% modulation oriented degree mainly is variation, on the actually, for this is

important

faint so

objects

observations,

search for

their very

magnetic purpose.

fields,

this

circular

polarization

analyzer

designed

Besides that, on February 6, 1991 from 5 51 to 6 14 , we carried out observations

h

m

h

m

192


of

CVn with the known magnetic field, the value B=-990±90 Gs was obtained.

ACKNOWLEDGEMENTS

In conclusio n the author s wis h to thank S.N. designing the analyzer , B. D. Bychkov , A.N.

Fabrik a for hi s usefu l

suppor t

in

Belo v and V. S.

Marko v fo r their hel p in

testing the equipmen t on the 6 m telescope and als o Yu. L. Manilo v and V.N . Zhukovski j for man y usefu l discussion s durin g the preparatio n of this paper .

REFERENCES

Pilipovich V.A., Polyako v V.I. , Konojk o A.I. : ta, 1 , 182.

1987,

in: Pribory i technika experimen-

Shtol' V.G.: 1991 , Astrofiz. issled.(Izv. SAO), 33 , 176. Shtol' V.G.: 1992 , in: Stellar magnetism, Intern. Conf. Proceed. , eds. : Glagolevski j

Yu.V. , Romanyu k I.I. , St.-Petersburg , Nauka , 194, (this issue) .

193