干涉仪和平板玻璃的测试方法ThereisanincredibleamountofinterestintestingmirrorsusingInterferometersandwearealwaysbeingaskediftestingwithInterferometersisbetterthanaconventionalNulltest?Theanswerisaresounding"No"Interferometerscannotcompetewiththe"TimeHonoured"methodofaknifeedgeandthehumaneyeinaDoublePassNulltestforaccuracy.Iftheycould,wewouldbeusinganInterferometerinsteadofconventionalmethods.ButitisamazingthefaithplacedinanInterferometerresult!WeareoccasionallygettingchallengedaboutthespecificationofourmirrorsonthebasisofapoorInterferometertest.ThereseemstobeasemireligiousbeliefthattheInterferometerresultiscorrectandourmethodiswrong,-wheninrealityitistheoppositewayround!EvenfirmsintheopticsindustrywhoshouldknowbetterarebeingtakeninbyInterferometerresults.WearemakingsomeinformationavailableonthelevelofaccuracythatcanbeexpectedfromourtestscomparedwithInterferometertests."ConventionalMethods"-TheDoublePassNullTestWhatfollowsisadescriptionoftheDoublePassNullTestascarriedoutatOldhamOptical.Thisisthebasictestweuseonalllargeparabolicmirrors.Thedescriptionisverysimplifiedandisaimedatpeaktovalley(PV),measurement,butalltheDoublePassstrengthsarebroughtouttoillustratewhyitissuchagoodtestofamirror.Thistestisalsoknownas"Auto-collimation"andmostprofessionalmirrormakersagreewithusthatitisthedefinitivetestofaparabolicmirror.Thediagramadjacentshowsthebasicarrangementofaparabolicmirrorsetupundertestfacinganopticalflatthathasacentralhole.Apointlightsourceissetupnearthefocalpointofthemirrorandshinesthroughthecentralholeontothesurfaceoftheparabolicmirror.Thelightreflectsbackparalleltotheaxisofthesystemtotheopticalflatwhichreflectsitbackalongthesamepathtotheparabolicmirroragain.Itreflectsofftheparabolicmirrorasecondtimeandreturnstoafocusneartheoriginallightsource.Inpracticethelightsourcehastobesetupjustslightlyoffaxissothefocusofthereflectedlightcanbeaccessed.Aknifeedgeissetupattheexactpointoffocus.Theknifehasmicrometeradjustmentstoallowittobeadjustedslowlyandaccuratelyintothereturninglightcone.ThedetectorusedintheDoublePassNulltestisofcoursethe"Mk1Eyeball".Inourcasethepersonwieldingtheeyeballhasdevelopedtheskillfromcarryingoutthetestagreatnumberoftimes.Whileanamateursettingupthistestforthefirsttimewouldcertainlybenefitfrombeingledthroughthetestbyamoreexperiencedperson,-oncehehasbeenledthroughthetestonce,-hewouldprobablybeabletorepeatitonhisown.ThepointbeingmadehereabouttheDoublePassNullTestisthatifyouhaveaccesstoanopticalflat,-throughanAstronomygroupforinstance?-alltheotherequipmentiseasilymadeorreadilyavailableandthetestiseasytodo.Thenextdiagramisanenlargementofthelightrayspassingtheknifeedge.Ifthemirroristheperfectparabolicshapethenalltheraysoflightwillcometogetheratthefocalpoint.Iftheknifeedgeismovedonitsmicrometersitwillbepossibletofindasinglepositionatwhichallthelightraysarecutoffbythesmallestverticalmovement.TheobserverwouldseeaninstantaneousNull,(totalblockingofalllight),astheknifeedgeismovedintothebeam.(verticalmovementasshownonthediagram.)Inpractice,it'snotpossibletomakeanabsolutelyperfectmirror,althoughsomeofuscangetfairlyclose!Whentheparabolicsurfaceisnotabsolutelyperfectthelightrayscomingbackwillnotpassthroughonefixedpoint.Theywillrangearoundthenominalfocalpoint.Inthisnextdiagramtherangeisshownbythesolidanddottedlines.Sayinthisnextexamplethatlightraysfromthecentreofthemirrorarerep...
