sendaknownsourcethrough alloftheopticsandontothedetector arrays. Thetransmission system consists ofacalibration spherethatproduces auniform sourceofknownintensity. Whentheshutterisin theclosedposition, thelightfromthetransmission calibrator isrelayed through theopticaltraintothe detector arrays. Withthecalibrator system offandtheshutterclosed, adarklevelcalibration ofthe detectors canbeobtained. IRAC,alongwiththeotherinstruments aboard SIRTF, isacryogenic instrument thermally coupled toa superfluid Heliumdewar. Therefore, thebasetemperature oftheinstrument isexpected tobe1.4Kelvin. Themission lifetime requirement issetat2.5yearswithagoalof5.Atthattime,thecryogen willbe expended andtheinstrument chamber isexpected toriseto30Kelvin. Twoofthedetectors, theInSb arrays,arestillexpected toworkinthepost-cryogen statefurtherextending theusefullifeofthe instrument. Theshuttermechanism isexpected tocontinue towork,aswell. TheShutter Mechanism Therequirements, design, operation, andtestingoftheIRACshuttermechanism aredescribed. The shutterhasbeendeveloped overathree-year period. Duringthattime,twoETUswerebuiltandtested. Oneoftheseunitsremains intheIRACInstrument Demonstration Module (IDM)whichwasbuiltfor engineering testingandproofofconcept. Inaddition, threeflightunitshavebeendesigned andbuiltand arecurrently invarious stagesoftest.Twooftheassembled flightunitshavebeentestedand characterized atliquidheliumtemperatures andoneoftheseunitshasbeendelivered totheIRACflight coldassembly. Thetestdatatakenincludes actuator characterization, position sensorcharacterization, repeatability ofmirrorposition, opticalmetrology, andexcessactuator torque. Functional Description TheIRACshutter(Figure 2),approximately 0.15m(5.9in)inlength,isdesigned tomovea79.8mmby 37.1mm(3.14inx1.46in)mirrortotwopositions identified asopenandclosed. Themotionisrotational andsweeps atotalarcof38degrees (Figure 3).Whentheshutter isopened, themirrorisstowed outof theincoming lightpath.Therefore, theinstrument isabletoreceive lightfromthefocalplaneofthe spacecraft telescope andtakeimages oftheoutside sky.Whentheshutterisclosed, theoutside lightis tobeattenuated by1.0E06providing adarkenvironment intheinstrument forinfrared detector calibration. Toachieve this,themirroristranslated directly intothelightpathcompletely fillingthe aperture andinterfacing withabaffleintheIRAChousing. Theresultisthelightpathbeingcompletely blocked withverylittleleakage around themirror. Theinsideportionofthemirroritselfprovides a reflective surface offofwhichthetransmission calibration sourceisreflected. Thesourcethenpasses directly through theopticalpathoftheinstrument andontotheinfrared detectors. R_quirem_nt_3, 4 Spacecraft: •Stiffness mustassurefundamental frequency >50Hz Mustsurvive launchloadsgenerated byaDeltaIIHlaunchvehicle Uncompensated momentum shallnotexceed 200E-06 N-m-s Shalloperate at1.4Kelvinformission lifetime and28Kelvinforextended mission Mechanism: •Massnottoexceed 1.6kg •Shalltranslate mirrortotwopositions: OpenandClosed, with0.5degrepeatability •Timetocloseshallnotexceed 10seconds •Shallbefailsafe open •Lifetime operations shallbe20,000cycles •Nomicrophonic transmission beyond 0.5second transient •Maximum powerdissipation shallnotexceed 0.5J/100s(5mWaverage) •Attenuate incoming photonradiation by>1E06 •Redundancy: Thereshallbenoelectrical singlepointfailures Minimize mechanical singlepointfailures