complicated gearconfiguration. Thetoroidalsteppermotorgavealsoagoodopportunitytoplacethepurge tube(ACT)oraslipring(TT)insideitshollowshaft. Materialshadtobenon-magnetic inbothhousingandbearings.Aluminumalloysareavoidedinthemotor bearingassemblybecauseoftheirhighcoefficientofthermalexpansion. AISI316steelashousingmaterial andCuBebearingswasfoundtobeagoodpairwithsimilarcoefficients ofthermalexpansion. The permanentmagnetofthemotorwasofcourseunavoidable. ReducingthemagneticemissionofSagem's motorwouldhaverequirednewmotordesignwithmajorcostpenalty.Toreducethemagneticdisturbance fieldoutsidethemotor,wmetal shieldsweremounted tobothhousing partsandtotheshaft.Alsoa compensation magnet wasmounted insidethehollowshafttocancelthefirstorderdipolefieldofthe stepper motor. Residual magnetic moment oftheshielded motorunithasbeenmeasured andfoundtobe compliant withrequired lessthan2.5nTat1m.The_-metal shielding reduces Sagem motor's maximum torqueto300mN-mat65mAcurrent(28V). Pointing direction oftheCAPSismeasured withaNovotechnik [12]resistive encoder mounted between the motorhousing andanon-rotating flange(called'flower'foritsshape). Twoadditional calibration positions for themotorposition areprovided bytwoABB'slimitswitches thatgiveasignalattheendsofthe+110° sweep. Themotioncontrolelectronics ontwoprintedcircuitboardsintheactuator housing andinthe CAPS-instrument DPUnominally drivethescaninawaythatthelimitswitches andthemechanical end stopsarenottouched. Inordertominimize torquedisturbances caused bytheactuator motion, theCoGofthepayload shouldbe ontheaxisofrotation. Evenininstalling spacecraft hardware, Murphy's lawmayturnouttobestillgoing strong: thepayload unitwasattached ontopofaactuator inawaythatmovedtheCoGfurtherawayfrom theaxisofrotation. Thiswascorrected, buttheincident couldbecounted asaconfiguration controlflaw. TheSEUL06050non-separable angular contact ballbearings wereselected. Theyaremanufactured by SNFA[13]ofberyllium copper, sincecommon bearing steelsaremagnetic. Bearing ringsare manufactured toHighPrecision ClassABEC7,andtheballsareofAFBMA-standard grade16.Their material isalsoBeCu.Thebearings weretoofinequalitywhencomparing theirfrictiontorquetothatofthe flexlink(fordataandpowertransmission through theactuator). Forwearconsiderations, thebearings were agoodchoice. Theflexlinkhastwelveshielded pairsofelectrical linesinalaminated Kaptonfoilflatcablemanufactured byGore(forACT).TTusesaslipringassembly because itscommanding unitislocated inaboxonthe spacecraft panel.Sometimes slipringsaretoldtooperate poorlyinthebeginning oftheirlife.Ifthestartupcommand mustgothrough ittoacomputer ontopoftheactuator, themission oftheexperiment canbe overbeforeitstarted. Bearings aremounted inanXconfiguration wheretheyarepushed intoseatsbyaxialpreload provided by theflexibleflangeofthehousing (Housing IIinFigure3).Thetwobearing flanges oftheunitarefitted together withthreealignment pinstoreachhighconcentricity requirements ofthebearings. Thethin bearing flangewitha~0.1mmgapbetween thehardstopandtheflangeallowsthermal expansion ofthe shaft.Thebearing housing machining totherequired tolerance hasproventobeaverydifficulttaskthatis tolerable onlyinsingleunitproduction. 255