Spectroscopy for Amateur Astronomers: Recording, Processing, Analysis and Interpretation

165 Pages·2017·18.94 MB·English

by  Marc F. M. Trypsteen

Spectroscopy for Amateur Astronomers
Recording,Processing,Analysis and Interpretation
Thisaccessibleguidepresentstheastrophysicalconceptsbehindastronomicalspectroscopy,coveringboththe
theory and the practical elements of recording, processing, analyzing and interpreting your spectra. It covers
astronomicalobjects,suchasstars,planets,nebulae,novae,supernovae,andeventssuchaseclipsesandcomet
passages.Suitableforanyonewithonlyalittlebackgroundknowledgeandaccesstoamateur-levelequipment,
the guide’s many illustrations, sketches and figures will help you understand and practice this scientifically
important and growing field of amateur astronomy, up to the level of pro–am collaborations. Accessible to
non-academics, it benefits many groups, from novices and learners in astronomy clubs, to advanced students
and teachers of astrophysics. This volume is the perfect companion to the Spectral Atlas for Amateur
Astronomers, which provides detailed commented spectral profiles of more than 100 astronomical objects.
Marc F.M. Trypsteen is a Belgian pharmacist and astronomer, with a background in analytical chemistry and
spectroscopy. He has lectured on astronomical spectroscopy at university, high schools, observatories
and astronomy clubs. He reviewed the Spectral Atlas in detail and co-authored Spectroscopy for Amateur
Astronomers. He is co-founder of the Astro Event Group, Belgium and also of the Astropolis Space Science
Center in Ostend, where, in addition to outreach activities, he is responsible for the section on spectroscopy
education and research.
Richard Walker has been an avid astronomer since he was aged 12. He spent his career in civil engineering,
planninglargeprojectssuchaspowerplants,damsandtunnels.Nowretired,inthelast10yearshehasfocused
increasingly on stellar astronomy and on the indispensable key to this topic – spectroscopy. He undertook
alargeobservingprojecttorecordanddocumentthespectraofthemostimportantastronomicalobjects,and
chose to share this gathered information for the benefit of other amateurs worldwide. The Spectral Atlas
and Spectroscopy for Amateur Astronomers are the fruits of his labor. He lives near Zürich, in Switzerland.

Spectroscopy for Amateur
Astronomers
Recording, Processing, Analysis and Interpretation
Marc F. M. Trypsteen
Amateurastronomer,Belgium
Richard Walker
Amateurastronomer,Switzerland
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Firstpublished2017
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LibraryofCongressCataloging-in-PublicationData
Names:Trypsteen,MarcF.M.|Walker,Richard,1951-
Title:Spectroscopyforamateurastronomers:recording,processing,analysis,and
interpretation/MarcF.M.Trypsteen,amateurastronomer,Belgium,RichardWalker,
amateurastronomer,Switzerland.
Description:Cambridge:CambridgeUniversityPress,2017.|Includesbibliographical
referencesandindex.
Identifiers:LCCN2016036436|ISBN9781107166189(Hardback:alk.paper)
Subjects:LCSH:Stars–Spectra.|Spectrumanalysis.|Amateurastronomy.
Classification:LCCQB871.T792017|DDC522/.67–dc23
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ISBN978-1-107-16618-9Hardback
Alsoavailableaspartoftwo-volumeset:ISBN978-1-316-64256-6Twohardbackbooks
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CONTENTS
Preface xi 2.3 Definitions and Notation 9
Acknowledgements xiii 2.3.1 IonizationStageversusDegreeofIonization 9
2.3.2 AstrophysicalNotationfortheIonizationStage 9
1 2.4 The Hydrogen Spectral Series 9
2.4.1 ThePhotonEnergyattheWavelengthsofthe
Physical Basics of Spectroscopy 1 H-BalmerSeries 9
2.4.2 ThePhotonEnergyofElectronTransitions 10
1.1 Photons: Carriers of Information andEnergy 1
2.4.3 QuantifiedOrbitalEnergyLevelDiagramofthe
1.1.1 Photons:CarriersofInformation 1
H-BalmerSeries 10
1.1.2 TheWave–ParticleDuality 1
2.4.4 TheLymanLimitofHydrogen 10
1.1.3 TheQuantizationofElectromagnetic
2.4.5 TheBalmerandRydbergEquations 10
Radiation 2
2.4.6 TheBalmer,PaschenandBracketContinua 11
1.1.4 Photons:CarriersofEnergy 2
2.5 Formation and Typology of Astronomical Spectra 11
1.1.5 OtherPropertiesofPhotons 2
2.5.1 Overview 11
1.2 The Electromagnetic Spectrum 2
2.5.2 TheThreeBasicTypesofSpectrainthe
1.2.1 TheUsableSpectralRangeforAmateurs 2
ContextoftheSun 13
1.2.2 TheSelectionoftheSpectralRange 2
2.5.3 MolecularAbsorptionBandSpectrum 13
1.2.3 TerminologyoftheSpectroscopic
2.5.4 MolecularAbsorptionBandwithInversely
Wavebands 3
RunningIntensityGradient 13
1.3 Wavelengthand Energy 3
2.5.5 CompositeorIntegratedSpectrum 13
1.3.1 PreliminaryRemarks 3 2.5.6 ReflectanceSpectrum 14
1.3.2 UnitsforEnergyandWavelengthAppliedin
Spectroscopy 3
1.3.3 Planck’sEnergyEquation 4 3
1.4 The Continuum and BlackbodyRadiation 4
Quantum Mechanical Aspects of Spectroscopy 15
1.4.1 TheBlackbodyasaPhysicalModelforStellar
Radiation 4 3.1 Quantum MechanicalView of Transition
1.4.2 Planck’sRadiationLawandCourseofthe Probabilities 15
ContinuumLevel 4
3.2 The Energy Levels of Multi-Electron Atoms 16
1.4.3 Wien’sDisplacementLaw 5
3.2.1 TheQuantumNumbersn,Ɩ,m,sandParity
1.4.4 EffectiveTemperatureT andthe
eff OperatorP 16
Stefan–BoltzmannLaw 5
3.2.2 DipoleTransitions,LaporteRuleandForbidden
Transitions 17
2
3.2.3 GrotrianDiagramsofAstrophysicalInterest 18
3.2.4 SpectroscopicNotation:TheTermSymbol 18
Electron Transitions and Formation of the
3.2.5 TheHyperfineStructure 19
Spectra 7
3.3 The Schrödinger Equation 20
2.1 Simple Textbook Example: The Hydrogen Atom 7 3.3.1 PreliminaryRemarks 20
2.2 Transition Types andProbabilities 8 3.3.2 DeBroglie’sElectronWaveModel 20
2.2.1 Absorption 8 3.3.3 TheSchrödingerEquationandAstronomical
2.2.2 Emission 8 Spectroscopy 21
2.2.3 Ionization 8 3.3.4 Time-independentand
2.2.4 Recombination 8 Time-dependentForms 21
2.2.5 ElectronsintheFree–FreeMode 9 3.3.5 Hydrogen 22
vi Contents
4 5.6 Czerny–Turner Spectrograph 46
5.6.1 Overview 46
Types and Function of Dispersive Elements 24 5.6.2 ApplicationofCzerny–Turner
4.1 Physical Principle of Dispersion 24 SpectrographsforAmateurs 47
4.1.1 PreliminaryRemarks 24 5.7 Spectral Information and Required Resolution 47
4.1.2 DispersionbyRefraction 24
4.1.3 DispersionbyDiffraction 26 6
4.2 TheDispersive PrincipleofPrism Spectrographs 26
4.2.1 SpecificGlassTypesfor Recording of the Spectra 49
PrismSpectrographs 27 6.1 VisualObservation of Spectra 49
4.2.2 MinimumAngleofDeviationandAngular 6.1.1 ScotopicorNighttimeVision 49
Dispersion 27 6.1.2 PhotopicorDaytimeVision 49
4.2.3 ResolvingPower:AMeasureofPerformance 30
6.2 RecordingofSpectrawithElectronicImageSensors 50
4.2.4 PracticalApplicationsofPrismsToday 30
6.3 The RecordingSystem: Telescope,
4.3 TheDispersive PrincipleofGrating
Spectrograph and Camera 51
Spectrographs 30
6.3.1 PreliminaryRemarks 51
4.3.1 TheGratingEquation 30
6.3.2 LimitingMagnitudeoftheSystem 51
4.3.2 ManufacturingProcessandPerformance
6.3.3 ExposureTimesforGratingSpectrographs 51
Parameters 32
6.3.4 PixelSizeandSamplingwithSlitSpectroscopy 53
4.3.3 AngularDispersion 34
6.3.5 DeterminationoftheSamplingbyaRecorded
4.3.4 GratingDiffractionEfficiency 35
SlitImage 53
6.3.6 AnalyticalDeterminationoftheSampling 54
5 6.3.7 InterferenceFringesandReflectionGhosts 54
6.4 Recordingof Echelle Spectra 54
Types and Function of Spectrographs 36
6.4.1 PreliminaryRemarks 54
5.1 Slitless Spectrographs with 6.4.2 SpecialFeaturesofEchelleSpectra 55
Transmission Grating 36 6.4.3 TheOrientationoftheSpectralImage 55
5.1.1 AvailableTransmissionGratings 36 6.4.4 FocusingoftheSpectralImage 56
5.2 Slit Spectrographswith Reflection or 6.4.5 ExposureTimesforEchelleSpectrographs 56
Transmission Grating 37 6.5 Influences of Mount andGuiding 56
5.2.1 TheClassicalConcept 37 6.5.1 MechanicalandStructuralProblemswithSmall
5.2.2 TheLittrowDesign 37 Mounts 56
5.2.3 ConfigurationsandOptionsforthe 6.5.2 TheOptionofFiberCoupling 56
DevelopmentofSlitSpectrographs 37 6.5.3 ImpactoftheSpectrographLoadtoSmallMounts 57
5.2.4 AnamorphicMagnification 39 6.5.4 TheMountingAngleofSpectrographandCameras 57
5.2.5 SpectrographThroughputandEtendue 40 6.5.5 DocumentationoftheSetup 58
6.5.6 PositioningofFaintObjectsontheSlit 58
5.3 Commercial Slit Spectrographs for Amateur
6.5.7 SpecificRequirementstotheGuidingQuality 58
Applications 41
6.5.8 LoadDistributionandAutoguidingProcesswith
5.3.1 ALPY,ShelyakInstruments 41
SmallMounts 59
5.3.2 DADOS,BaaderPlanetarium 41
6.5.9 Autoguiding:InteractionofHardwareand
5.3.3 LhiresIII,ShelyakInstruments 41
SoftwareComponents 59
5.3.4 LISA,ShelyakInstruments 41
6.5.10 SpectroscopicAspectsoftheGuideStar 59
5.3.5 Minispec,AstroSpectroscopyInstruments 41
6.5.11 RecordingofCloseBinaryStarComponents 60
5.3.6 SpectraL200,JTWAstronomy 41
5.3.7 StarlightXpressSX,StarlightXpress 41
7
5.4 Echelle Spectrograph 43
5.4.1 Overview 43 Processing of Recorded Spectra 61
5.4.2 BasicDesignsofEchelleSpectrographs 44
7.1 Available Software for Data Reduction 61
5.5 Commercial Echelle Spectrographs for Amateur
7.1.1 SoftwarePackagesfortheAmateur 61
Applications 45
7.1.2 ProfessionalSoftwarePackages 62
5.5.1 BACHESSpectrograph,BaaderPlanetarium 45
7.1.3 Spectroscopy@Cyberspace:TheVirtual
5.5.2 eShel,ShelyakInstruments 45
Observatory 62
5.5.3 SQUES,Eagleowloptics 45
Contents vii
7.2 From the Recorded Spectrum to theCalibrated 9.1.3 PeakIntensityI andEnergyFluxFofa
P
Intensity Profile 63 SpectralLine 76
7.2.1 PreliminaryRemarks 63 9.1.4 PeakIntensityandEnergyFluxofan
AbsorptionLine 76
7.3 Removal of Light Pollution and Airglow 63
9.1.5 PeakIntensityandEnergyFluxofan
7.3.1 ObjectsofPoint-shapedAppearance 63
EmissionLine 77
7.3.2 ObjectsAppearingas2D 63
9.1.6 EnergyFluxoftheContinuum 77
7.4 Removal of Remaining Hot Pixels and Cosmics 63
9.1.7 SuperpositionofEmissionand
7.5 Dark-frames and Flat-fielding 65 AbsorptionLines 77
7.5.1 Dark-frames 65 9.1.8 TheContinuum-RelatedPeakIntensityP 78
7.5.2 Flat-fields 65
9.1.9 TheContinuum-RelatedEnergyFlux:
7.6 Processing of Echelle Spectra 65 EquivalentWidthEW 78
7.6.1 TheProcessingofanEntireEchelleSpectrum 65 9.1.10 NormalizedEquivalentWidthWλ 79
7.6.2 TheProcessingofIndividualOrders 66 9.1.11 FullWidthatHalfMaximumHeight(FWHM) 79
7.6.3 SubtractionoftheSkyBackgroundandLight 9.1.12 HalfWidthatHalfDepth(HWHDorHWHM) 79
Pollution 66 9.1.13 FullWidthatZeroIntensity(FWZI) 79
9.1.14 HalfWidthatZeroIntensity(HWZI) 80
8 9.1.15 MeasurementofAsymmetry 80
9.1.16 InfluenceoftheSpectrographResolution
Calibration of the Spectra 67 ontheFWHMandEWValues 80
9.1.17 AdditionalMeasurementOptions 81
8.1 Calibration of theWavelength 67
8.1.1 PreliminaryRemarks 67 9.2 Shape and Intensity of Spectral Lines 81
8.1.2 RelativeCalibrationBasedonKnownLines 67 9.2.1 TheShapeofAbsorptionLines 81
8.1.3 AbsoluteCalibrationwithLightSources 67 9.2.2 TheShapeofEmissionLines 81
8.1.4 AbsoluteCalibrationbyUnshifted 9.2.3 TheInformationContentoftheLineShape 81
WavelengthsofAtmosphericH OLines 67 9.2.4 Blends 81
2
8.1.5 LinearandNonlinearCalibration 67 9.3 Identification of Spectral Lines 82
8.1.6 PracticalAspectstoMinimizeSourcesofErrors 69 9.3.1 TaskandRequirements 82
8.1.7 HeliocentricandGeocentricCorrections 69 9.3.2 PracticalProblemsandStrategies
8.1.8 TheSelectionoftheCalibrationLightSource 69 forSolvingThem 82
8.1.9 TheFeedingoftheCalibrationLight 69 9.3.3 ToolsfortheIdentificationofSpectralLines 82
8.2 Calibration of theSpectral Flux Density(Intensity) 70 9.4 Temperature Related Appearance of Elements and
8.2.1 PreliminaryRemarks 70 Molecules in theSpectra 83
8.2.2 SelectiveAttenuationoftheContinuumIntensity 70
8.2.3 ProportionalAttenuationoftheSpectralLines 71 10
8.2.4 InformationContentofthe
Pseudo-Continuum 71 Temperature and Luminosity 85
8.2.5 ProportionalProceduresforthe
10.1 Information ContentoftheSpectral
RelativeFluxCalibration 72 Classification 85
8.2.6 RectificationoftheContinuumIntensity 72
10.1.1 TheHertzsprung–RussellDiagram 85
8.2.7 RelativeFluxCalibrationbyaSynthetic
10.1.2 InformationContent 85
Continuum 73
10.1.3 SpectralClass,StellarMassand
8.2.8 RelativeFluxCalibrationbyRecorded
LifeExpectancy 85
StandardStars 74
10.1.4 TheEvolutionoftheSunintheHRD 86
8.2.9 AbsoluteFluxCalibration 74
10.1.5 TheEvolutionofMassiveStars 87
8.2.10 TasksandRequiredCalibrationProcedures 75
10.2 Measurement of the Stellar Effective
Temperature T 87
9 eff
10.2.1 Introduction 87
Analysis of the Spectra 76 10.2.2 TemperatureEstimationbytheSpectralClass 87
10.2.3 TemperatureEstimationbyApplyingWien’s
9.1 Measurementof SpectralLines 76 DisplacementLaw 88
9.1.1 MeasurementoftheWavelengthinaSpectral 10.2.4 TemperatureDeterminationBased
Profile 76 onIndividualLines 89
9.1.2 IntensityMeasurementinaSpectralProfile 76 10.2.5 The“BalmerThermometer” 89
viii Contents
10.3 SpectroscopicDistance Measurement 90 12.1.7 TheRotationalVelocityofStars 106
10.3.1 OptionsforSpectroscopicDistance 12.1.8 EmpiricalEquationsforvsiniinFunction
Measurement 90 ofFWHM 107
10.3.2 TermandPrincipleofSpectroscopicParallax 91 12.1.9 CalibrationEquationsbyF.Fekel 107
10.3.3 SpectralClassandAbsoluteMagnitude 91 12.1.10 SuitableMetalLinesfortheFWHM
10.3.4 Wilson–BappuEffect 91 Measurement 107
10.3.5 AbsoluteVisualMagnitudeIndicatorby 12.1.11 RotationalVelocityofCircumstellar
Millward–Walker 92 DisksaroundBeStars 108
10.3.6 DistanceModulusandEstimationofthe 12.1.12 EmpiricalEquationsfortheRotational
Distance 92 VelocityoftheDisk 108
12.1.13 DistributionoftheRotational
11 VelocitywithintheDisk 108
12.1.14 AnalysisofDoublePeakProfiles 108
Expansion and Contraction 93 12.1.15 TheOuterDiskRadiusRs 109
12.2 EstimationofOrbitalElementsinBinarySystems 109
11.1 Radial Velocityand Expansion of the
12.2.1 Introduction 109
SpacetimeLattice 93
12.2.2 TermsandDefinitions 110
11.1.1 TheRadialVelocity 93
12.2.3 SomeBasicsofCelestialMechanics 110
11.1.2 TheClassicalDopplerEffect 93
12.2.4 SpatialOrientationoftheOrbitPlane 110
11.1.3 TheSpectroscopicDopplerEquation 94
12.2.5 AnalysisoftheDopplerShiftΔλin
11.1.4 Thez-Value:AFundamentalMeasureof
SB2Systems 111
ModernCosmology 94
12.2.6 TheCalculationoftheIndividualRadial
11.1.5 TheRelativisticDopplerEquation 94
VelocitiesofM andM 112
11.1.6 MeasurementoftheDopplerShiftand 1 2
12.2.7 TheEstimationofSomeOrbital
DeterminationofRadialVelocity 95
ParametersinSB2Systems 112
11.1.7 RadialVelocitiesofNearbyStars 95
11.1.8 RelativeDopplerShiftwithina
SpectralProfile 95 13
11.1.9 RadialVelocityofGalaxies 95
Gravity, Magnetic Fields and Element
11.1.10 TheExpansionoftheSpacetimeLattice 95
11.1.11 TheApparentDilemmaatz>1 96 Abundance 114
11.1.12 Thez-Value:ConsideredasaMeasurefor 13.1 Measurement of the SurfaceGravity 114
thePast 96 13.1.1 Overview 114
11.1.13 MessierGalaxies:RadialVelocityand 13.1.2 MethodBasedontheWilson–BappuEffect 114
CosmologicalSpacetimeExpansion 97 13.1.3 FurtherSurfaceGravityIndicators 114
11.1.14 TheRedshiftofQuasar3C273 97 13.2 Measurement of StellarMagnetic Fields 116
11.1.15 TheGravitationalRedshiftorEinsteinShift 97 13.2.1 Overview 116
11.1.16 AgeEstimationoftheUniverse 97 13.2.2 TheZeemanEffect 116
11.2 Measurementof Expansion and Contraction 99 13.2.3 SpectralLineswithStrongLandéFactors 117
11.2.1 PCygniProfiles 99 13.2.4 PossibleApplicationsforAmateurs 117
11.2.2 InversePCygniProfiles 100 13.3 Abundance of Elements 117
11.2.3 BroadeningoftheEmissionLines 100 13.3.1 AstrophysicalDefinitionof
11.2.4 SplittingoftheEmissionLines 101 ElementAbundance 117
13.3.2 AstrophysicalDefinitionofMetallicity
12 Z(MetalAbundance) 117
13.3.3 QuantitativeDeterminationof
Rotation and Orbital Elements 103 theAbundance 118
12.1 Measurementof Rotational Velocity 103 13.3.4 RelativeAbundanceofStarsofSimilar
12.1.1 TermsandDefinitions 103 SpectralClass 118
12.1.2 TheRotationalVelocityof
Apparent3DObjects 103 14
12.1.3 TheRotationalVelocityofLargePlanets 104
12.1.4 TheRotationalVelocityofGalaxies 104 Analysis of Emission Nebulae 120
12.1.5 ExampleofaLightReflectingObject 104 14.1 The Balmer Decrement 120
12.1.6 ExampleofaSelf-LuminousCelestialBody 105 14.1.1 Introduction 120
Contents ix
14.1.2 DefinitionoftheBalmerDecrement 120 14.2.11 CoolingMechanismbyForbidden
14.1.3 TheoreticalBalmerDecrementforEmission Transitions 127
Nebulae 120 14.2.12 InfluencesofT,N andTransition
e e
14.1.4 BalmerDecrementsatStellarandother ProbabilityAontheCoolingMechanism 127
AstronomicalObjects 121 14.2.13 EstimationofT andN bytheOIIIand
e e
14.1.5 ApplicationsoftheBalmerDecrementinthe NIIMethods 128
AmateurSector 121 14.2.14 EstimationoftheElectronDensitybythe
14.1.6 MeasurementoftheBalmerDecrementby SIIandOIIRatio 129
Amateurs 122
14.1.7 SpectroscopicEstimationofInterstellar 15
Extinction 122
14.1.8 ExtinctionCorrectionbytheMeasured Amateurs and Astronomical Science 130
BalmerDecrement 122
15.1 Participation in AstronomicalResearch 130
14.1.9 BalmerDecrementandColorExcess 122
15.1.1 AstronomicalSpectroscopyandthe
14.2 Plasma Diagnostics for Emission Nebulae 123 Pro–AmCulture 130
14.2.1 PreliminaryRemarks 123 15.1.2 TheStructureofaPro–AmCollaboration 131
14.2.2 ThePhotoionizationinEmissionNebulae 123
15.2 Observation Campaigns for Amateurs 132
14.2.3 KineticEnergyandMaxwellianVelocity
15.3 Contributions by Amateurs 133
DistributionofElectrons 123
14.2.4 SignificantProcesseswithinIonizedNebular 15.3.1 Short-termCampaign(Months) 133
Plasmas 123 15.3.2 Long-termCampaign(Years) 133
14.2.5 RecombinationProcess(CaseA) 123
14.2.6 LineEmissionbyElectronTransitions Appendix A Abbreviations, Acronyms and Common Units 134
(CaseB) 124 Appendix B Absolute Magnitudes ofMain Sequence Stars 137
14.2.7 LineEmissionbyCollisionExcitation Appendix C The Solar Echelle Spectrum: An Aid to
(CaseC) 124
Orientation 139
14.2.8 LineEmissionbyDirectAbsorption
Appendix D Flip Mirror and Calibration Light Source 142
(CaseD) 124
14.2.9 LineEmissionbyForbiddenTransitions 125 Referencesand Further Reading 143
14.2.10 SchemeoftheFluorescenceProcessesin Index 148
EmissionNebulae 126