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Back-and-forth spectropolarimetr y
G.A . Chountonov , V.A . Murzin , N.G . Ivashchenko , I.V . Afanasiev a Special Astrophysical Observatory of the Russian AS, Nizhnij Arkhyz 369167, Russia

Abstract . Th e spectru m i s alternativel y integrate d i n tw o differen t polarization s o n th e sam e pixel s o f th e CCD . Th e CC D i s rea d afte r a serie s o f integrations . Charge s ar e transferre d afte r eac h individua l exposur e synchronousl y wit h switchin g o f polarizatio n withou t readou t u p o r dow n alon g th e chip . I t help s t o diminis h th e error s cause d b y th e fluctuation s o f ligh t a t th e sli t o f th e spectrograph .

I n th e mod e o f back-and-fort h spectropolarime tr y a n electrooptica l modulato r o f ligh t lettin g pas s alternativel y righ t an d left circularl y polarize d ligh t i s installe d ahea d o f th e spectrograp h slit . A CC D detecto r i s mounte d o n th e spectrograp h camera . Th e essenc e o f th e recordin g mod e bein g discusse d i s tha t th e charg e pattern , accumulate d i n a give n stat e o f th e modulator , whic h correspond s t o th e por tio n o f th e spectru m o n th e casset e par t o f th e spec trograph , i s no t rea d afte r a shor t elementar y ex posur e tim e bu t transferre d t o a specifie d numbe r o f lines . The n a synchronou s switchin g o f th e eiec trooptica l modulato r stat e occurs , th e signa l i s be in g accumulate d durin g th e elementar y exposur e tim e wit h furthe r backwar d transfe r t o th e sam e numbe r o f lines . Th e syste m recover s th e origina l stat e an d th e proces s i s repeated . Afte r accumulatio n o f th e necessar y numbe r o f charge s th e proces s i s discontin ue d an d th e accumulate d imag e patter n i s digitized . Th e influenc e o f ligh t flu x fluctuation s o n th e spectro grap h sli t i s reduce d i n thi s mode , an d for recordin g o f spectr a i n left an d righ t circularl y polarize d light , th e sam e pixel s ar e used , i.e . th e necessit y for flat-fieldin g i s rule d out . Th e mod e o f iterativ e double-directe d charg e transfe r wa s implemente d o n th e basi s o f th e CC D syste m S100 0 b y mean s o f alterin g bot h hard - an d software . Th e controlle r scannin g generato r neede d t o b e modifie d becaus e previousl y i t di d no t revers e th e charg e transfe r fas t enoug h (thi s require d reloadin g fro m th e hos t compute r o f th e memor y storin g th e scan) . T o revers e th e charg e transfe r on e ha s t o re vers e th e tempora l scan s o f tw o paralle l phases . Wit h thi s i n vie w a circui t o f switchin g signal s o f tw o phase s wa s added . Th e programm e o f contro l o f th e mode s o f accu mulatio n an d readou t wa s adde d b y a modul e spec ifying parameter s o f th e mod e o f iterativ e double directe d charg e transfe r an d operatin g th e controller .

Th e followin g parameter s ca n b e specifie d b y th e pro gramme : · lengt h o va l 0.00 1 ... 6 5 · numbe r th e rang e 1 ... · numbe r 1 ... 65535 . f th e elementar y exposur e i n th e inter s; o f line s th e charg e i s transferre d t o i n 255 ; o f charg e transfe r cycle s i n th e rang e

Whil e th e adjustmen t o f th e mod e o f iterativ e charg e transfe r o n th e CC D syste m wit h th e matri x ISD17 A wa s performed , i t wa s foun d ou t tha t a ver y carefu l adjustmen t o f th e leve l o f signal s o f al l th e phase s i s neede d t o provid e for satisfactor y value s o f transfe r efficiency ove r th e fram e i n bot h direction s an d t o minimiz e th e spuriou s charg e generate d i n th e proces s o f transfer . Fo r thi s purpos e th e phas e forme r wa s supplemente d wit h circuit s for separat e ad justmen t o f th e uppe r an d lowe r level s (th e level s o f well s an d bariers ) o f bot h tw o paralle l an d tw o regis te r phase s (th e thir d phase s ar e virtual) . Afte r a carefu l adjustmen t o f th e mode s ther e stil l remaine d a regio n o f increase d charg e valu e i n th e uppe r left o f th e frame . Thi s effect is , apparently , du e t o a technologica l defec t o f th e matri x whic h doe s no t manifes t itsel f i n th e proces s o f on e single-directio n imaging . I n Fig . 1 i s displaye d th e dar k fiel d i n th e mod e o f cycli c imag e transfer . Fo r subsequen t recordin g o f spectra , w e chos e a homogeneou s portio n o f th e matri x i n it s lowe r part . I n th e uppe r par t o f Fig . 2 i s show n a portio n o f th e sola r spectru m derive d b y th e spectrograp h wit h a lo w dispersion . A ligh t fibr e wa s use d t o brin g radi atio n t o th e slit . I n th e lowe r par t o f Fig . 2 i s ex hibite d a vertica l sectio n o f th e image . On e ca n se e well th e recurrenc e o f th e detail s o n th e tw o spectr a i n section . Th e intensit y betwee n th e section s o f th e spectr a transvers e th e dispersio n rise s wit h decreasin g elementar y exposur e no t onl y becaus e th e inefficiency o f transfe r i s finite , bu t als o becaus e ther e i s n o snap -

© Special Astrophysical Observatory of the Russian AS, 2000


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actio n shutte r t o intercep t ligh t durin g th e tim e o f imag e shift . Nevertheles s th e efficiency i s hig h enoug h eve n for a n exposur e o f les s tha n 0. 2 s . Th e doubl e spectru m o f th e Su n i n Fig . 2 ha s bee n obtaine d wit h a n exposur e o f 0. 2 s afte r ever y shift , th e numbe r o f line s b y whic h th e spectru m wa s shifte d mad e 30 , th e numbe r o f th e shift s wa s 500 . A schemati c diagra m o f measurement s wit h th e BT A Mai n Stella r Spectrograp h (MSS ) (camer a 2 ) i s show n i n Fig . 3 . Th e ligh t fro m a sta r passe s throug h th e liqui d crysta l modulator , whic h ca n b e i n tw o state s creatin g phas e shift s o f 180 ° an d 0° , an d i s deflecte d b y Moone y rhom b t o th e spectrograp h slit . Th e Moone y rhom b i s i n th e capacit y o f th e fla t diago na l mirro r o f th e spectrograp h an d serve s a t th e sam e tim e a s th e achromati c quarter-wav e phas e retarder . Behin d th e spectrograp h sli t ther e i s a uni t o f tw o calcit e spa r plates . Wit h th e transfe r mod e operatin g for dat a accumulatio n i n th e spectrograp h camera , th e detecto r register s fou r spectr a i n th e left an d righ t circula r polarization s (Fig . 4 show s th e imag e cross sectio n a t th e inpu t o f radiatio n polarize d ove r th e circle) . Fig . 5 show s th e relationshi p betwee n th e ra ti o o f th e differenc e i n intensitie s right2-left 2 t o thei r su m (wit h bia s subtracted ) an d th e wavelengt h o f ra diation . I t i s eviden t fro m th e figur e tha t th e regio n o f achromatis m for th e liquid-crysta l modulato r take s abou t 1000 , whic h i s comparabl e wit h th e rang e o f achromatis m for th e crystal s DKDP . Th e contro l volt ag e for th e liquid-crysta l modulato r i s abou t 4. 7 V . Th e expositio n tim e wa s assume d t o b e fro m 5 t o 1 5

min , whic h i s typica l o f astronomica l observations . Acknowledgements . We than k Glagolevskij Yu.V. for supportin g th e work, Perepelitsi n E.I . for manufactur e of th e Mooney rhomb , Zhukovskij V.N . for platfor m design. Th e work was partl y funded throug h gran t 94-0206584 of th e RFBR .


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