ALowPowerCryogenic Shutter Mechanism forUseinInfrared Imagers D.ScottSchwinger* andClaefF.Hakun* Abstract Thispaperdiscusses therequirements, design, operation, andtestingoftheshutter mechanism forthe Infrared ArrayCamera (IRAC). Theshuttermovesamirrorpanelintooroutoftheincoming lightpath transitioning IRACbetween dataacquisition andcalibration modes. Themechanism features atorsion flexure suspension system, twolow-power rotaryactuators, abalanced shaft,andavariable reluctance position sensor. Eachoftheseitemsisdiscussed alongwithproblems encountered duringdevelopment andtheimplemented solutions. Introduction IRAC,developed byGoddard SpaceFlightCenter, underagreements withTheSmithsonian Astronomical Observatory andtheJetPropulsion Laboratory, is1of3scientific instruments onboardtheSpace Infrared Telescope Facility(SIRTF) tobelaunched inDecember of2001.SIRTFisthefourth"large observatory" performing imagery, photometry, andspectroscopy ofastronomical bodiesoverthespectral rangeof3.6to160l_m.TheIRACinstrument takesimages in4bandscentered onthefollowing wavelengths: 3.6,4.5,5.8,and8.0I_m.Themajorobjectives ofIRACaretoproduce thefollowing science datasets:1 •Deepconfusion-limited broadband (25%)surveys forhigh-redshift normalgalaxies •Largeareashallow surveys forbrowndwarfs •Imaging surveys ofstarclusters tosearchforbrowndwarfs •Surveys ofnearbystarsforbrowndwarfsandsuperplanets •Photometric observations ofselected ultraluminous galaxies andactivegalactic nuclei •Imaging surveys toidentify protoplanetary disksandyoungstellarobjects Duetoitsuseofstate-of-the-art largeformatinfrared detectors, IRACwillbemoresensitive andproduce morecompelling, higherresolution images thanpreviously possible withcryogenic imagers. 2 AtopviewoftheIRACinstrument isshowninFigure1.Apickoffmirrorreflects lightfromthefocalplane ofthespacecraft telescope offoftwomirrored surfaces atslightlydifferent angles. Theresultisa transmission oftwoseparate lightpathsintothetopandbottomcompartments oftheIRACstructure. Eachcompartment hasafullsetofrefractive opticsandtwonearinfrared detectors positioned almost symmetrically aboutthemid-plane ofthestructure. Following thelightfromthepickoffmirrorandintotheinstrument, itfirsttravelsthrough theaperture ofthe shutter. Theshutteristhefocusofthispaperandwillbeaddressed extensively later.Aftertheshutter, thelighttravelsthrough adoublet lenstoabeamsplitter. Herethebeamissplitintotwoseparate wavelengths. Thelongerwavelengths aretransmitted andtheshorterwavelengths reflected. Afterthe beamissplit,eachsubsequent beampasses through afilterandalyotstopandontoadetector array. Thereare4ofthesearrays,oneforeachofthestatedwavelengths, each256x256pixelsandcovering a5.12x5.12arcminfieldofview.Twodetectors areArsenic-doped Silicon(Si:As)IBCarraysandthe othertwoareIndiumAntimonide (InSb)arrays. Alongwiththeopticsandthedetectors, therearetwoonboardcalibration systems: atransmission calibrator andfourfloodcalibrators. Eachofthefourfloodcalibrators isattached toadetector mountand projectlightdirectly ontothearrays. Theintentistofloodthearraywithaknowninfrared sourceand confirm anexpected levelofresponse acrosstheentirearray.Thetransmission calibrator isdesigned to *NASAGoddard SpaceFlightCenter, Greenbelt, MD Proceedings ofthe34_Aerospace Mechanisms Symposium, Goddard SpaceFlightCenter,May10-12,2000