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Th e ne w circular polarization analyser for th e Nasmyth- 1 focus of th e 6 m telescop e
V.D . Bychkov , S.N . Fabrika , D.N . Monin , G.G . Valyavi n Special Astrophysical Observatory of the Russian AS, Nizhnij Arkhyz 369167, Russia

Abstract . A circula r polarizatio n analyse r for th e Nasmyth- 1 focu s o f th e 6 m telescop e i s designed . Th e devic e widen s th e scop e o f polarimetri c investigation s o f uniqu e objects . Example s o f th e firs t advantageou s application s o f th e analyse r ar e given .

1.

Introductio n

On e o f th e mos t efficient method s o f studyin g stella r magnetis m i s obtainin g o f Zeema n spectra . I n "pho tographic " epoch , whe n photoemulsio n wa s th e mai n ligh t receiver , suc h spectr a wer e take n wit h th e cir cula r polarizatio n analyser s attache d t o th e entranc e o f high-dispersio n classica l spectrograph s (Babcock , 1952 ; Najdeno v an d Chountonov , 1976 ; Glagolevski j e t al. , 1976 ; Bychko v e t al. , 1988) . Thes e spectro graph s ha d ver y goo d positio n stabilit y whic h permit te d stud y an d allowanc e for al l instrumenta l effects appearin g i n measurements . Thi s i s o f specia l impor tanc e sinc e th e spatia l separatio n o f spectr a i n orthog ona l polarization s occur s perpendicularl y t o th e dis persio n direction , i.e . th e appearanc e o f th e secon d di mensio n cause s Zeema n spectrogram s t o b e processe d a s two-dimensiona l images . Th e ne w solid-bod y de tectors , wit h limite d linea r sizes , calle d for employ men t o f echell e spectrometer s producin g a larg e num be r o f geometrica l distortions , especiall y i n takin g Zeema n spectra . Fo r th e purpos e o f correc t separatio n o f informatio n an d instrumenta l effects i n Zeema n spectr a a ne w analyse r wa s designed . Th e genera l principl e o f th e analyse r i s th e possibilit y o f changin g th e sig n o f th e circula r polarizatio n wit h th e complet e preservatio n o f th e optica l pat h b y mean s o f turnin g a quarte r wav e phas e shiftin g plat e b y 90° .

2 . Th e circula r polarizatio n analyse r
Conside r th e performanc e o f th e circula r polarizatio n analyse r th e schemati c o f whic h i s displaye d i n Fig.l . A t th e entranc e o f th e analyse r / 4 (90° ) plate , a linea r phas e shiftin g element , labele d "in " i n th e figure , i s inserted . Th e azimut h o f th e axi s o f th e highes t spee d o f thi s elemen t make s a n angl e o f 45 ° wit h th e azimut h o f th e axi s o f th e highes t spee d (accordingl y a n angl e o f -45 ° wit h th e lowes t spee d axis ) o f a n Ice lan d spa r crysta l followin g th e plate . Th e Icelan d spa r crysta l i s intende d t o spatiall y separat e tw o mutuall y

orthogona l linearl y polarize d beam s o f ligh t produce d b y th e entranc e phas e shiftin g plate . Th e exi t phas e shiftin g plat e / 4 (90° ) ( "out " i n Fig.l ) transform s linearl y polarize d ligh t int o circularl y polarized . Th e exi t plat e i s oriente d i n th e sam e wa y a s th e entranc e on e t o eliminat e instrumenta l effects tha t aris e whe n th e ligh t i s reflecte d fro m th e diffractio n gratings . Th e element s incorporate d i n th e analyser s employe d pre viousl y wer e immovabl e an d use d t o stic k togethe r wit h th e Icelan d spa r crysta l afte r orientation . I n thi s cas e th e tw o outgoin g beam s o f ligh t ha d circula r po larizatio n o f definit e sign . Usin g suc h analyser s wit h echell e spectrometer s involve s difficultie s i n takin g ac coun t o f th e two-dimensiona l spectru m distortions . I n ou r cas e th e analyse r i s fitte d wit h a precisio n me chanica l uni t tha t turn s th e entranc e quarte r wav e plat e "in " b y 90° , whic h lead s t o a chang e o f sig n o f circula r polarization s o f th e outpu t beam s (denote d b y 1 an d 2 i n Fig.l) . Her e th e ligh t beams , havin g change d th e sig n o f polarization , ar e projecte d o n th e sam e element s o f th e detector . B y makin g tw o succes siv e exposure s o f th e objec t wit h differen t position s o f th e entranc e plate , obtai n image s o f Zeema n spectr a wit h equa l distortion s bu t havin g a n opposit e direc tio n o f th e Zeema n effect . Thi s metho d make s th e mos t correc t separatio n o f instrumenta l effects fro m th e measure d Zeema n effect. Th e parameter s o f th e analyse r ar e liste d i n Tabl e 1 .

© Specia l Astrophysica l Observator y o f th e Russia n AS , 200 0


25 9


26 0

Th e throughpu t o f th e analyse r wa s measure d fro m observation s o f th e sta r 5 3 Ca m wit h th e spec trograp h SP-12 4 i n th e Nasmyth- 1 focus . Th e smal l changin g throughpu t i n a wid e wavelengt h rang e i s a n outstandin g advantag e o f th e analyser . Th e analyse r wa s teste d i n observation s o f th e well-know n magneti c sta r 5 3 Cam . Th e result s sho w a goo d agreemen t wit h previou s behaviou r o f 5 3 Ca m magneti c field. Th e analyse r i s mad e s o tha t i t ca n readil y b e operate d a t th e tw o Nasmyt h foci o f th e 6 m telescop e i n conjunctio n wit h an y devices . Th e firs t astrophysica l observation s wit h th e anal yser , whic h ar e worth y o f note , ar e demonstrate d i n Fig.2 . Th e lowe r pane l o f thi s figur e display s th e spec tru m o f th e magneti c whit e dwar f obtaine d a s th e su m o f spectr a i n tw o circula r polarizations . Th e uppe r pane l show s th e distributio n o f circula r polarizatio n ove r th e spectru m o f thi s object . I n distributio n o f polarizatio n on e ca n wel l se e th e polarizatio n jump s i n th e wing s o f hydroge n lines . Th e magneti c fiel d es timate d fro m thes e dat a i s H = 2 5 ± 6 kG . Fig . 3 present s a magneti c curv e o f W D 0009+501 . Th e es timate s o f th e magneti c fiel d for thi s objec t hav e bee n mad e fro m 3 1 Zeema n spectra : 2 1 spectr a i n on e po sitio n o f plate , / 4 "in" , (filled circles ) an d 1 0 spectr a wit h th e plat e turne d b y 90 ° (ope n squares) .
e

acknowledge th e hel p of Makarov a N.A. , wh o furnished u s wit h a n Iceland spa r crystal , which mad e possibl e th e creatio n of th e analyser .

Reference s
Babcock H.W. , 1952, Astrophys . J . Suppl . Ser., 3 , 141 Bychkov V.D. , Glagolevskij Yu.V., Elki n V.G. , Nazarenk o A.F. , Najdenov I.D. , Chountono v G.A. , Romanyu k I.I., Shtol ' V.G. , 1988, in: "Magneti c Stars" , Yu.V. Glagolevskij an d I.M. Kopylov (eds.) , Leningrad , Nau ka, 12 Najdeno v I.D. , Chountono v G.A. , 1976, Soobshch.Spets . Astrofiz.Obs., 16 , 6 3 Glagolevskij Yu.V., Najdenov I.D. , Romanyu k I.I., Chuntono v G.A., 1976, Soobshch.Spets.Astrofiz.Obs. , 24 , 61 Glagolevskij Yu.V., Kozlova K.I. , Kumajgorodskay a R.I. , Lebedev V.S., Hajdenov I.D. , Romanyu k I.I., Chunakova N.M. , Chountono v G.A., 1977, Pis'm a Astron . Zh., 3 , 500 Schurcliff W.A. , 1962, "Polarized Light" , Harvar d Univ.Press , Cambridge , Massachusett s

Acknowledgements .

Th e

author s ar e pleased t o