Using Notch-Filter Masks for High Contrast Imaging

arXiv:astro-ph/0402654v127Feb2004UsingNotchFilterMasksForHighContrastImagingofExtrasolarPlanetsJohnH.Debes1,JianGe1,MarcJ.Kuchner2MichaelRogosky3ABSTRACTWepresentthefirstlaboratoryexperimentsusinganotch-filtermask,acoro-nagraphicimagemaskthatcanproduceinfinitedynamicrangeinanidealLyotcoronagraphaccordingtoscalardiffractiontheory.Wefabricatedthefirstnotch-filtermaskprototypewith.25μmprecisionusingane-beamlithographymachine.Ourinitialopticaltestsshowthattheprototypemasksgeneratecontrastlevelsof10−5at3λ/Dand10−6at∼8λ/D,withathroughputof27%.Wespeculateonthe“as-is”performanceofsuchamaskintheHubbleSpaceTelescope.Subjectheadings:circumstellarmatter—instrumentation:adaptiveoptics—methods:laboratory—planetarysystems1.IntroductionDirectlyimagingextrasolarterrestrialplanetsinreflectedlightrequiresfacingtheex-tremelyhighpredictedcontrastratiosbetweenplanetsandtheirhoststars,e.g.,∼10−10foranEarthanalogorbitingasolartypestaratquadrature.Aplanet-findingcoronagraphmustrealizethiscontrastwithinafewdiffractionwidths(λ/D,whereλisthewavelengthoflight,andDisthelongaxisoftheprimarymirror)ofthestellarimage.Thoughseveralcoronagraphdesignscanachievethiscontrastaccordingtoscalardiffractiontheory(Kuchner&Spergel2003a),substantialworkonmaskdesignandlaboratoryinvestigationprobablyremainsbeforethiscontrastcanbeachievedinpractice.Somecoronagraphdesignsuseimage-planemaskstoabsorbon-axislightanddiffractitaway(Malbet1996;Sivaramakrishnanetal.2001;Kuchner&Traub2002).Otherdesigns1DepartmentofAstronomy&Astrophysics,PennsylvaniaStateUniversity,UniversityPark,PA168022HubbleFellow,DepartmentofAstrophysicalSciences,PrincetonUniversity,Princeton,NJ085443PennsylvaniaStateUniversityNanofabricationFacility,UniversityPark,PA16802–2–useshapedorapodizedpupilswhichbenefitfromcombiningapertureshapeandthepupilintensitydistributiontoremovethewingsofacircularaperture’sPSF(Kasdinetal.2003;Vanderbeietal.2003;Debesetal.2002,2003;Geetal.2002).Imagemasksoffertheadvantagesthattheyexplicitlyremovestarlightfromthebeam,andthattheycanprovidehighcontrastatsmallanglesfromtheopticalaxis,givensufficientcontroloverlow-spatialfrequencymodes.Scatteredlight,wavefronterrors,andmaskconstructionerrorscanalldegradethecontrastofacoronagraph.Forexample,foranycoronagraphicimagemask,mid-spatialfrequencyintensityerrorsnearthecenterofthemaskmustbe.10−9(Kuchner&Traub2002).Someoftheseerrorscanbecontrolledusingactiveoptics,butthesecorrectionswillnecessarilyapplyonlyoveralimitedrangeofwavelengths.Notch-filtermasksofferapromisingchoiceforplanet-findingcoronagraphs(Kuchner&Spergel2003b).Theseimagemasksabsorbmostofthelightfromanon-axispointsource,anddiffractalloftheremainderontoamatchedLyotstop.WhileLyotcoronagraphswithGaussianimagemasksmusthaveathroughputof.1/2toreach10−10contrast,linearnotch-filtermaskshaveunlimiteddynamicrangeaccordingtoscalardiffractiontheory,andathroughputof∼(1−ǫ),whereǫistypically∼0.3−0.5.Othercoronagraphdesignsbesidesnotchfiltermaskscancreateperfectsubtractionofon-axislight.However,thosedesignsbasedonmaskswithoddsymmetry(Rouanetal.2000;Riaudetal.2001)orinterferometricallysynthesizedmaskswithoddsymmetry(Baudozetal.2000a,b)createnullsthatdegradeasθ2,whereθistheanglefromtheopticalaxis.Thisrapiddegradationmeansthatthefinitesizeofarealstarcausesthecoronagraphtoleaklightatlevelsunsuitableforterrestrialplanetdetection.Otherdesigns,likethedualphasecoronagraphicmaskwithanapodizedentrancepupil(Soummeretal.2003a,b),producetheneedednulldepth,butmustusemaskswithspecialchromaticbehaviorandrequireprecise,achromaticapertureapodization.Notch-filtermasksareintrinsicallyachromaticandlikethedualphasecoronagraph,theycreatenullsoforderθ4orslower(Kuchner&Traub2002).Notch-filtermaskscanbedesignedsuchthattheyarebinary–regionsofthemaskareeitheropaqueortransparent.Thisisagreatadvantageasintensityerrorsarenotanissuesolongasthemaskissufficientlyopaque,leadingtomanufacturingconstraintsthatareordersofmagnitudesmaller.However,theshapeofthemaskmustbepreciselyreproduced,tothelevelofλf#/3600foracontrastof10−10withinthesearcharea.Foraninstrumentwithf/100andworkingλ∼.66μm,thiscorrespondstoatoleranceontheorderof20nm.Nanofabricationtechniquesarerequiredtoreachthisprecision.Aspartofajointuniversity-industrystudypartlyfundedbyBallAeorospaceandin–3–collaborationwiththePennStateNanofabricationfacility(Nanofab),wehavefabricatedaprototypenotch-filtermaskandtesteditinanexperimentalsetup.WediscussbrieflythemaskfabricationprocessinSection2,describemethodsformodelingperformanceinSection3,reviewtheexperimentsandresultsinSection4,anddiscusswaysofimprovingperformanceinSection5.2.MaskDesignandFabricationFollowingtheprescriptionlaidoutinKuchner&Spergel(2003b),wedesignedanotch-filtermaskbasedona1−sinc2bandlimitedfunction.Themaskiscomprisedofaverticallyrepeatingpatternofopaquecurvesdescribedby:ˆMBL(x)=±.5λminD1−sinc2πǫxD2λmax(1)whereǫisusedtodeterminethehalfpowerofthemask,theeffectivedistancewhereacompanioncouldreasonablybedetected;λminandλmaxaretheminimumandmaximumwavelengthsofthespectralbandofinterest.Inrealitythemaskisconstructedandthereforesampled