Maqn_tic Desiqn Thegimbalisdrivenbyquasi-Helmholz coils.Thisisacoilsetthatgenerates areasonably gooduniform magnetic fieldbetween theircenters. AHelmholz coilhasaspecific dimensional relationship thatis closely, butnotexactly, followed bytheactualgimbaldesignduetootheropticalordimensional constraints. Thefieldinandaround thecenterhassufficient uniformity toaccomplish thegoals. Fortestingpurposes, thesurrounding areasmustbefreeofstrayfieldscaused byelectrical equipment or magnetic materials. Oneoftheearliest indications ofthisrequirement wasthecompassing ofanearly prototype totheassembler's beltbuckle. Onetestistoposition themirrorwithacurrent profile,turnoff thecurrent allowing themirrortoswingfreelyasdictated bytheEarth'sfieldandgravity, turnonthe currents tothesamevalues, andhavethemirrorrepeatitsprevious position towithin4milliradians. Earlyonintheprogram wefoundthatthemirrorwouldtakeextraordinarily large,stableoffsetsfromthe original settings. Wefoundthateventhoughthesmallsprings inthebearings weremadeofberyllium copperastheyshouldhavebeen,theactualBeCugradeusedcontained traceamounts (0.02% by weight) ofironwhich,ofcourse, causes themtobemagnetic. Thiswasfoundbyremoving themain magnet fromthesystem andusingittosearchformagnetic material inthevicinity. Thermal Considerations Thegimbalissubjected toalargetemperature deltainbothstorage andoperation. Thegimbalbearings aredesigned tomaintain consistent dragbehavior andarelargelyimmune tothermal effectsintheir operation. Thiswasaccomplished byspringloading ofthebearing. Specific Details physical Optical Path:Thegimbalisrequired tohaveaclearaperture of17mmapproaching fromeitherside andexitingfromfrontorbackwithafieldofregardof+15°inbothelevation andazimuth. As mentioned earlier,thislimitsavailable spaceforgimbalstructure toa3-mmwidezonealongthe edgesofthegimbalenvelope. MirrorConstruction: Themirrorisconstructed fromthesamealloyasthegimbalframetomaintain cohesive thermal behavior. Beryllium wasnotselected duetoverypoorreflectivity ofpureberyllium atthedesired wavelength. Goldplatingoftheberyllium mirrorwasnotpossible duetotherequired substrate nickelplatingwhichwouldcausemagnetization problems. Themirrorismachined froma solidbillet,including features toacceptthemagnet andpivots. Themachined mirroristhenthermally stabilized andthereflective surface isfinalmachined viadiamond turning. Adielectric oxidecoating protects thediamond turnedsurface. Thefabrication ofthemirrortotherequired flatness (;V4) provedtobeoneofthebiggerchallenges oftheproject. Balance vs.Weightlessness: Themirroraswellastheoutergimbalisfinelybalanced toallow Earth-bound testing. Thebalancing isaccomplished through theuseofseveral adjustable copper screws attached tothemirrorandthegimbalstructure. Inordertofacilitate Earth-bound testing, the innergimbalisbalanced towithinbearing frictionlevelsoflessthan5xE-5N-m. Windings Form: Thewindings ofallsixcoilsarerectangular inshapetofollowtheperimeter ofthe gimbalclosely. FEAanalysis oftherectangular coilspredicted arelatively large,uniform fieldinthe center. Bycontrast, thesmaller roundcoils,although lighter,werefoundtogenerate weaker fields withlesslinearity inthecenterregionresulting innonlinear behavior andwerediscarded. •Materials: Allmajorcomponents ofthegimbalareconstructed fromthesamealuminum alloy. Components ofthejeweled bearing assemblies arefabricated fromaBeCualloy.Balance screws 191