AtTheUniversityofChicago,wearedesigningafar-infrared polarimeter tocoverawiderangeof wavelengths. Thetelescopethatthispolarimeter isintendedtoworkinconjunction withisthe Stratospheric Observatory forInfraredAstronomy (SOFIA)(Ref.1).Attheheartofthepolarimeter, a rotatingcarousel-style stagewillbeusedtoputanyoneoffivepolarizinghalf-waveplatesintotheoptical path.Itmustthenbepossibletorotatetheselectedhalf-waveplate(HWP)aboutitsownaxistoconduct thepolarization studies.ItdoesnotappearpracticaltoproducetherequiredHWProtationonacarouselstylestageusingshaftsfromexternalmotors.Wehavethereforecarriedoutinvestigations onaprototype moduleinwhichacryogenicmotorislocatedimmediately adjacenttotherotatingHWP,eliminatingthe needforprotrudingshaftsandexternalmotors.ThisprototypemoduleiscomposedoftheHWPholder withitsbearing,motor,gearing,andhousing(showninFigure1)andisdesignedtoholdalens,a spectralfilter,andapupilstopattachedtothehousing.Themodule,withitsaccompanying optical elements, isdesignedtobeeasilyreplaceable, allowingdifferentoptionsforwavelengths and magnification. Thefinalinstrument willhavefivesuchmodulesonacarousel-style stage.Suchanarrangement envisioned formeasuringmultipleFIRpassbands isshowninFigure2.Forasenseofscale,eachHWP hasaclearapertureofapproximately 75mmdiameter. __._.__--z-_..::._... .../_ sj_ ....... ---..:::::.. 2-_;_W:7--i;--..... .... ":_h-:..... , '" _- /_'7_ _re,.R,_ ....;, j. ...\. /. /',.':_,'.'X"h-.:_" .,/......<<>-_-......._>--....../ -.___, "-.'.._"-_._S-" .,'_/ "_ /" o"_'.. ,.",X.", '_,...."-.. °.'_¢." - ¢.-"-_-.'- "..".",V'.. i _-_i-_'z..... .-"t-"_eit_,* "'t...'....... :orous_.I 'if---........--..,.\_, _ .....,"::...._..,.'>/., ./,,.:,,£ _ "-,''-G_" --. ',._-. _S'_ ." /-_ ',', 'I/It_I{t_+.%',"l"Jfct",",'"--../;.C"-. ',''"'..\_',";'-_,'/'".'-_"f",t"---.,',..'.,,.,..'.c....:_-_ ',,."--_-.>i'...zx>'--..-_--:.... ',,I '111_/ !4_l'----f,....-.....-:<:-",-- ,',,__------_ _,.,.,,_:.:__ t_ 2,1 i.", e_".. ":'- "-> ' x,v,,_, L_,,/o<>_--.-.-.,, "....<i!.,......'......".-............. i'__i!<77'_.:::.,,>,,,_..../....' -';.......... './1!/ff_D7,"<";'>_i<',,. /,........ ,-,,,,_ //' I,/_'_,,,'.. I!'_'1"i11 '. //_.._'-,o,f.',-_is',:,",_,;',"_::.----.__.,"...._J . I[_ "'"-:__ .,..,., .._. tl:t.... s,.([.,._.,,.,-........ /.,,/ "..!_°",)..',_.-...)_//ifI"_::,,........-_.---,,' --..,",X'_F_>'°itiit_> ..""'-...%.. +'i_...... -" ....._ ..----.___-" //,, / ''"'-.'---d-" /,,'ii,".-" Figure2.Carousel-style stagesupporting multiple HWPmodules Inthispaper,wedescribe thedesign andperformance oftheprototype HWPmodule, outline the difficulties thathadtobeovercome, andrecommend improvements tofuturemodules. Module Design Specific objectives ofthedesign effortweretoproduce aprototype HWPmodule thatwascompact, lightweight, relatively inexpensive, andreliable, particularly sinceitwouldnotbeaccessible during operation. Themodule hadtooperate atbothroomtemperature andnearthetemperature ofliquid helium(near4K).Additionally, ameansofsensing therotational position ofthehalf-wave platehadtobe included. Itwasdesired thatsuchposition information beindependent ofcounting stepcommands sentto thecryogenic motor.During normal operation, theHWPwillbestepped onceaminute in30-degree increments thattakeapproximately 5seconds, corresponding toadutycycleofapproximately 8%. NASA/CP--2002-211506 28