Tools of Radio Astronomy

611 Pages·2013·9.61 MB·English

by  Thomas L. Wilson

Astronomy and Astrophysics Library
Thomas L. Wilson
Kristen Rohlfs
Susanne Hüttemeister
Tools of Radio
Astronomy
Sixth Edition
ASTRONOMY AND
ASTROPHYSICS LIBRARY
SeriesEditors: G.Börner,Garching,Germany
A.Burkert,München,Germany
W.B.Burton,Charlottesville,VA,USAand
Leiden,TheNetherlands
A.Coustenis,Meudon,France
M.A.Dopita,Canberra,Australia
B.Leibundgut,Garching,Germany
G.Meynet,Versoix,Switzerland
P.Schneider,Bonn,Germany
V.Trimble,LCOGT,Irvine,CA,USA

Thomas L. Wilson Kristen Rohlfs
(cid:2)
Susanne Hüttemeister
Tools of Radio Astronomy
Sixth Edition
123
ThomasL.Wilson KristenRohlfs
NavalResearchLaboratory Inst.Astrophysik
Washington,DC Ruhr-University-Bochum
USA Bochum
Germany
SusanneHüttemeister
ZeissPlanetariumBochum
Bochum
Germany
ISSN0941-7834
ISBN978-3-642-39949-7 ISBN978-3-642-39950-3(eBook)
DOI10.1007/978-3-642-39950-3
SpringerHeidelbergNewYorkDordrechtLondon
LibraryofCongressControlNumber:2013955089
(cid:2)c Springer-VerlagBerlinHeidelberg2013
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Coverillustration:IntheforegroundaredipoleantennasofLongWavelengthArraystation1(LWA1).
Inthebackgroundaresomeofthetwenty-eight25-meterparaboloidsoftheJansky-VeryLargeArray
(JVLA), of the National Radio Astronomy Observatory (NRAO). The LWA1 consists of 256 dual
polarization, active dipole antennas (designed by the Naval Research Laboratory, NRL), operating
between 10 MHz and 80 MHz. LWA1 was conceived by the NRL, designed and constructed in
collaboration with university partners, and operated bythe University ofNew Mexico. TheJVLAis
beingequippedwithreceiversforoperationatfrequencies<1GHzaspartofaprojectinvolvingNRL
andtheNRAO,andmayeventuallybeabletooperateinconjunctionwithLWA1.(PhotocourtesyofJ.
Monkiewicz,SchoolofEarthandSpaceExploration,ArizonaStateUniversity.)
Printedonacid-freepaper
SpringerispartofSpringerScience+BusinessMedia(www.springer.com)
Preface to the Sixth Edition
Thisbookintroducesthetoolsneededtopursueresearchinradioastronomy.These
consistof:
1. ThebasicprinciplesinChaps.1–4and6
2. ThepropertiesandusesofvarioustypesofreceiversandantennasinChaps.5,7,
8,and9
3. The analysisof mechanismsresponsibleforbroadbandcontinuumandspectral
lineradiationinChaps.10–16
Our aim is to help users to understand and apply well-established results. In
addition,thisbookusesrelationsandexpressionsintheformnormallyencountered
inradioastronomypublicationssothatthereadercanconnectthesewithconcepts
foundinbasicphysicsandchemistrytexts.
There havebeenmanymajornew instrumentaldevelopmentssince the appear-
ance of the fifth edition. One example is the practical use of Focal Plane Arrays
of receivers. Another is the Expanded Very Large Array (now the Jansky-VLA)
and, more generally, interferometry at frequencies below (cid:2) D 1GHz and above
(cid:2) D 200GHz. For this reason, Chap.9 has been expanded relative to the 5th
edition.Overthepast30years,themostdramaticimprovementsininterferometric
aperturesynthesisimagingarisefromtheuseoffastercomputersandsophisticated
imagecorrectionalgorithms.Inmodernsystems, computersoft-andfirmwarehas
become more and more integrated with receiver hardware, so are an integral part
ofradioastronomicalsystems.Otheradditionsaretheinclusionoftheconventional
derivationsof receivernoise (AppendixC forChap.4) and the radiationfield of a
filled aperture (Appendix E for Chap.6). Our descriptions of a specific topic are
dispersed because some aspects are related to the theory of receivers (Chap.4),
actual receiver systems (Chap.5), measurements with such systems (Chaps.7
and8),andtheinterpretationofresults(Chap.10andfollowing).Tomitigatethis,
wehaveindexedtermsextensively,soaspectsofadeviceorconceptcanbefound.
Itisassumedthatthereadershavea thoroughknowledgeofphysics.However,
difficulties have often arisen when the instrumental topics were discussed. Often
there is a difference between how a subject is treated in basic physics books and
v
vi PrefacetotheSixthEdition
the way it is presented using the language of radio astronomy. Our presentation
is meant to bridge this gap, but in all cases, we try to use concepts familiar to
physicists. A challenge encounteredwhen writing a textbook is that of consistent
designations, symbols, and units. Since the astronomical community prefers their
traditionalmixedsetof units, we use theGaussianCGS system, augmentedwhen
necessary with SI or MKS units. Where needed, we give the relations in their
respective units in the equations. As to nomenclature, in the text and index, we
referto single radiotelescopesas“antennas”and arraysof antennaswith coupled
outputsas“interferometers”;themorespecializedinstruments,suchasthoseused
tostudythecosmicmicrowavebackground,arereferredtoas“facilities.”
Foreachchapter,alistofreferencesisgiven.Usuallythislisthastwoparts:(1)
generalreferencesthatarereviewarticlesandbooksthatcoverthegeneralaspects
andwhichoftengiveanoverviewofthesubjectscoveredand(2)specialreferences
for specific topics. We favor more recent and more general references in which
citationstoearlierworkcanbefound.Finally,manyofthemostrecentdescriptions
of facilities are to be found in the Internet. Since the exact locations may change
with time, we have tried to give unique names for facilities, so the web sites can
befoundusingsearchengines.Inanumberofcases,resultsaregiventoillustrate
aspectsofthetheoriesorto makethe approachplausible.The bookisintendedto
aidinapplyingtheprinciplesofradioastronomy,butitisnotintendedtobeareview
oftheentirefieldofradioastronomy.
We have made a number of improvements and corrections to the 5th edition;
manyof theseare dueto J. W. M. Baars, T. Clarke(NRL),J. J. Condon(NRAO),
Fujun Du (Canada), J. Helmboldt (NRL), R. Mauersberger (ALMA), J. Moran
(Harvard-CfA), P. Ray (NRL), E. Polisensky (NRL), H. Sommer (ESO), and
K.Weiler(NRL).B.Jiang(Peking)andS.Truskin(RussianAcademyofSciences)
havesuggestedimprovementsandalsotranslatedthefiftheditionintoChineseand
Russian,respectively.
Washington,USAandBochum,Germany T.L.Wilson
June2013 K.Rohlfs
S.Hüttemeister
Contents
1 RadioAstronomicalFundamentals....................................... 1
1.1 OntheRoleofRadioAstronomyinAstrophysics................ 1
1.2 TheRadioWindow ................................................. 3
1.3 DiscoveriesinRadioAstronomy................................... 5
1.3.1 ASelectedListofFacilities............................. 6
1.4 SomeBasicDefinitions............................................. 7
1.5 RadiativeTransfer .................................................. 10
1.6 BlackBodyRadiationandtheBrightnessTemperature .......... 13
1.7 EmissivityandReflectivitiesofSurfaces.......................... 18
1.8 TheNyquistTheoremandtheNoiseTemperature................ 19
Problems ..................................................................... 20
References.................................................................... 23
2 ElectromagneticWavePropagationFundamentals..................... 25
2.1 Maxwell’sEquations................................................ 25
2.2 PlaneWavesinNonconductingMedia............................. 26
2.3 WavePacketsandtheGroupVelocity ............................. 30
2.4 PlaneWavesinConductingMedia................................. 32
2.5 TheDispersionMeasureofaTenuousPlasma.................... 33
Problems ..................................................................... 37
References.................................................................... 39
3 WavePolarization .......................................................... 41
3.1 VectorWaves........................................................ 41
3.2 ThePoincaréSphereandtheStokesParameters.................. 46
3.3 Quasi-monochromaticPlaneWaves ............................... 50
3.4 TheStokesParametersforQuasi-monochromaticWaves ........ 51
3.5 FaradayRotation.................................................... 52
Problems ..................................................................... 57
References.................................................................... 59
vii
viii Contents
4 SignalProcessingandReceivers:Theory................................ 61
4.1 SignalProcessingandStationaryStochasticProcesses........... 61
4.1.1 ProbabilityDensity,ExpectationValues
andErgodicity............................................ 61
4.1.2 AutocorrelationandPowerSpectrum................... 63
4.1.3 LinearSystems........................................... 66
4.1.4 Filters..................................................... 68
4.1.5 DigitizationandSampling............................... 68
4.1.6 GaussianRandomVariables............................. 71
4.1.7 SquareLawDetectors ................................... 72
4.1.8 ReceiverCalibrationProcedure......................... 73
4.2 LimitingReceiverSensitivity ...................................... 74
4.2.1 NoiseUncertaintiesDuetoRandomProcesses ........ 76
4.2.2 TheMinimumNoiseforaCoherentSystem........... 78
4.2.3 ReceiverStability........................................ 79
Problems ..................................................................... 85
References.................................................................... 89
5 PracticalReceiverSystems................................................ 91
5.1 HistoricalIntroduction.............................................. 91
5.1.1 IncoherentRadiometers ................................. 91
5.1.2 CoherentRadiometers................................... 91
5.1.3 BolometerRadiometers.................................. 92
5.1.4 TheNoiseEquivalentPowerofaBolometer........... 94
5.1.5 CurrentlyUsedBolometerSystems..................... 95
5.2 CoherentReceivers ................................................. 98
5.2.1 BasicComponents:PassiveDevices.................... 98
5.2.2 BasicComponents:ActiveDevices..................... 99
5.2.3 SemiconductorJunctions................................ 105
5.2.4 PracticalHEMTDevices................................ 107
5.2.5 SuperconductingMixers................................. 109
5.2.6 HotElectronBolometers................................ 111
5.3 SummaryoftheCoherentFrontEndsPresentlyinUse........... 113
5.3.1 SinglePixelReceiverSystems.......................... 113
5.3.2 MultibeamSystems...................................... 114
5.4 Back Ends:CorrelationReceivers,Polarimeters
andSpectrometers .................................................. 115
5.4.1 CorrelationReceiversandPolarimeters ................ 115
5.4.2 Spectrometers............................................ 118
5.4.3 FourierandAutocorrelationSpectrometers ............ 119
5.4.4 PulsarBackEnds ........................................ 130
Problems ..................................................................... 132
References.................................................................... 134
Contents ix
6 FundamentalsofAntennaTheory........................................ 137
6.1 ElectromagneticPotentials......................................... 137
6.2 Green’sFunctionfortheWaveEquation .......................... 139
6.3 TheHertzDipole.................................................... 143
6.4 TheReciprocityTheorem .......................................... 148
6.5 ArraysofDipoles................................................... 148
6.6 RadiationFieldsofFilledAntennas ............................... 152
6.6.1 TwoDimensionalFarField ............................. 152
6.6.2 ThreeDimensionalFarField............................ 154
6.6.3 CircularApertures....................................... 157
6.6.4 AntennaGradingRelatedtoPowerPatterns ........... 159
6.7 Summary............................................................ 161
Problems ..................................................................... 162
References.................................................................... 164
7 PracticalAspectsofFilledApertureAntennas.......................... 165
7.1 DescriptiveAntennaParameters ................................... 165
7.1.1 ThePowerPatternP.#;'/.............................. 165
7.1.2 TheMainBeamSolidAngle............................ 166
7.1.3 EffectiveArea............................................ 168
7.1.4 AntennaTemperature.................................... 172
7.2 PrimaryFeeds....................................................... 174
7.2.1 PrimeFocusFeeds:DipoleandReflector .............. 175
7.2.2 HornFeedsUsedToday................................. 175
7.3 ReflectorSystems................................................... 177
7.3.1 AntennaBlockage ....................................... 179
7.3.2 FieldofView............................................. 181
7.4 AntennaToleranceTheory ......................................... 183
7.5 ThePracticalDesignofParabolicReflectors...................... 188
7.5.1 GeneralConsiderations.................................. 188
7.5.2 SpecificAntennas........................................ 190
7.6 Summary............................................................ 194
Problems ..................................................................... 195
References.................................................................... 197
8 SingleDishObservationalMethods ...................................... 199
8.1 TheEarth’sAtmosphere............................................ 199
8.2 CalibrationProcedures.............................................. 204
8.2.1 General................................................... 204
8.2.2 CompactSources ........................................ 205
8.2.3 ExtendedSources........................................ 207
8.2.4 ExtremelyExtendedSources............................ 208
8.2.5 CalibrationofMeterWavelengthInstruments.......... 208
8.2.6 CalibrationofCentimeterWavelengthTelescopes..... 209