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Astronomical Measurement A Concise Guide Andy Lawrence Astronomical Measurement Andy Lawrence Astronomical Measurement A Concise Guide 123 AndyLawrence InstituteforAstronomy UniversityofEdinburgh RoyalObservatory Edinburgh,UnitedKingdom ISBN978-3-642-39834-6 ISBN978-3-642-39835-3(eBook) DOI10.1007/978-3-642-39835-3 SpringerHeidelbergNewYorkDordrechtLondon LibraryofCongressControlNumber:2013954550 ©Springer-VerlagBerlinHeidelberg2014 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpartof thematerialisconcerned,specificallytherightsoftranslation,reprinting,reuseofillustrations,recitation, broadcasting,reproductiononmicrofilmsorinanyotherphysicalway,andtransmissionorinformation storageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilarmethodology nowknownorhereafterdeveloped.Exemptedfromthislegalreservationarebriefexcerptsinconnection with reviews or scholarly analysis or material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. 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Coverdesign:JimWilkie Coverfigure:AcollageofthePanSTARRSdome(CreditRobRatkowskiCopyrightPS1SC,usedwith permission.Variantsandhigh-resversionsavailableathttp://ps1sc.org/Photo Gallery.shtml).Pictureof detector and JohnTonry,Institute forAstronomy, University ofHawaii (Credit :Richard Wainscoat, Institute forAstronomy, University ofHawaii, usedwithpermission ofWainscoat and Tonry),Trifid Nebula (Credit : Eugene Magnier (UH IfA), Peter Draper & Nigel Metcalfe (Durham University) CopyrightPS1SC,usedwithpermission) Printedonacid-freepaper SpringerispartofSpringerScience+BusinessMedia(www.springer.com) Thosewho havehandledscienceshavebeen eithermen ofexperiment ormen of dogmas. Themen ofexperiment areliketheant,they onlycollect anduse;thereasonersresemble spiders,who makecobwebs out oftheirown substance.Butthebeetakes amiddlecourse: itgathersitsmaterialfromtheflowersof the gardenand ofthefield,buttransformsand digestsit byapower of itsown. Not unlike thisisthetruebusinessofphilosophy;forit neitherreliessolelyorchieflyonthepowers ofthemind,nordoes ittakethematter which itgathersfromnaturalhistoryand mechanicalexperiments andlayitup inthe memorywhole,as itfindsit,but laysitup in theunderstandingalteredand digested. Thereforefromacloser andpurer league between thesetwo faculties,theexperimental andtherational(suchas hasnever yet been made),much maybehoped. (Book1, Aphorism95 from“The New Organon,orTrueDirectionsConcerningthe InterpretationofNature” byFrancisBacon) Preface Thisbookisaboutthecollection,imaging,detection,andcharacterisationofsignals comingfromobjectsabovetheatmosphere,andabouthowtheatmospheremakes those jobs harder. It has grown out of a course given to fourth year Astrophysics studentsattheInstituteforAstronomyoftheUniversityofEdinburgh. Thosesignalscouldbeintheformoflight,particles,orevengravitationalwaves. Most of the time we are concernedwith light, i.e. electromagneticradiation. This radiation can cover a wide range of frequencies, from radio waves to gamma- rays. The problems and the technologies involved can be quite different at these differentfrequencies.Sometimes,astronomersarespecialistsdefinedbythevarious technical skills required when working at these different frequencies – i.e. radio astronomers,optical astronomers,X-ray astronomers.However, the universe does notrespectthese divisions;forexamplegalaxiesemit stronglyat bothopticaland infrared wavelengths, and significantly at radio and X-ray wavelengths. To solve astronomicalproblems,agoodscientistthereforeneedsanunderstandingofawide rangeoftechnicalissues. Aims andApproach The prime aim of this book is to provide a broad introduction to the techniques and technologies involved in astronomical measurement, at a level appropriate to a senior undergraduate or beginning graduate student. It should also be a useful overview for working astronomers, or for other kinds of scientist interested to knowhowastronomyisactuallydone.Theemphasisisonphysicalprinciplesand concepts,ratherthantechnicaldetails. The book tries to cover at least briefly all the major types of astronomy. It is tempting to describe in turn how to do radio astronomy, then how to do optical astronomy,andso on. However,this would feellike severaldifferentbooksglued together, and would miss the multiwavelength spirit of modern astronomy. More vii viii Preface importantly,althoughdifferentfrequenciespresentdifferentproblems,thereisalso much in common between these areas. My approach is therefore orthogonal to the traditional approach. I will examine in turn the principles involved in various problems–imaging,detection,spectroscopyetc.–andwithineachstageconsider boththegenericissuesandthefrequencydependentissues.PersonallyIfindthisa moreilluminatingapproach.Ihopethereaderagrees. As well as providing a modern multiwavelength perspective, I have also tried to supply what I think of as the “missing middle”. For many of the topics treatedinthisbook–forexampleadaptiveoptics,radiointerferometers,andinfra- red detectors – one can find both short conceptual descriptions in introductory astronomytextbooks,andthoroughanddetailedmaterialinresearchpapersorvery advancedspecialisedprofessionalbooks.However,theformeroftendoesn’treally prepareyouforthelatter...Myintentionwastoexplainthingswhichageneralbook doesn’thave spacefor,butwhich advancedtreatmentstendto assume are already obvious. I hit this problem often in my own reading, so I have really written the bookformyself–Ihopeyoufinditusefultoo! WhatThisBookIs Not I do not provide a complete description of all the technical details involved in astronomicalmeasurement–thebookisintendedtobeashortintroduction.Inany one of the various areas I cover, there are much more comprehensive treatments available,someofwhicharelistedattheendofeachchapter. Thisbookisnotintendedasanoverallintroductiontoobservationalastrophysics. It therefore contains almost nothing on matters such as the observed properties of stars, how we correct for interstellar extinction, how we deduce velocities or temperaturesanddensitiesfromobservedquantities,andsoon.Itisalsoverybrief on many practical issues such as the definitions of various co-ordinate systems, standard magnitude systems, astrometric systems, and so on. Those issues are discussed, but at a minimum level, so that we can concentrate on the physical principlesratherthanthearbitrarydetails. There are two specific areas which are central to the practice of astronomical measurement, but which I do not treat here. One such area is astronomical computing and data analysis software; the other is statistics. In both cases this is because it would take too much space to do them justice, which would remove somefocusfromthebook.However,somebasicknowledgeofstatisticsisassumed inafewplaces,soIhaveaddedanAppendixwithwhatIhopeisausefulsummary ofkeytechniquesandapproaches. Preface ix An Overview oftheBook Thebookisinfivebroadchapters,followedbytwoappendices.Chapter1,Signal, Distortion, and Noise describes the most general aspects of making astronomical measurements.Theopeningsectionsummarisesthequantitieswhicharethetarget ofourmeasurements–fluxes,positions,andsoon.Followingthis,wetakeavery genericlookattheproblemsinvolvedinmakingastronomicalmeasurements–how light is lost, distorted, and smeared, and how noise makes measurement harder. Everymeasurementisactuallyachainofevents,withlossesanddistortionsateach stage. In Chap.2, The atmosphere and how to avoid it, we will look at what the atmosphere does to light before we can even start to make our measurements – absorbingandblurringsignals,andaddingabackgroundlightthatmakesithardto detectfaintobjects.Onesolutionistoriseabovetheatmosphereanddoastronomy fromspace,butthisbringsitsownproblems. Next,inChap.3,Mappingthesky,welookathowwedeterminethebrightnessof theskyasafunctionofangularposition.Insometypesofastronomywearelucky enough to have telescopes which collect and focus light, and form images of the sky;inothercasesitisnotpossibletoformanimageandsoweneedmoreindirect waysofmeasuringthedirectionalityofsignals–forexampleinterferometryinradio astronomy,orcodedmasksingamma-rayastronomy. In Chap.4, Detection, we look at how the interaction of light with physical devices,suchasaCCDcameraoraradioantenna,producesasignalwecanactually read out e.g. a voltage, or a current. We will examine the physicsof detection at different wavelengths, what determines the efficiency of detectors and their noise characteristics,howtinysignalsareamplifiedintosomethingmeasurable,andhow componentsareassembledintopracticalworkingdevices.Thisisanenormousarea whichwewillbecuttingveryshort! Finally,inChap.5,Spectroscopy,welookatmethodsusedtoanalysethestrength ofthesignalasafunctionoflightfrequency.Thisworksverydifferentlyatdifferent freqencies. At high frequency, we can estimate the energy of individual photons. Atmiddlingfreqencywetendtodispersethelight,i.e.physicallyseparatedifferent wavelengths.At low frequencieswe typicallyuse Fouriertransformtechniquesto deducespectralpropertiesfromtemporalfluctuations.However,newtechnological developmentsinsuperconductingdetectorsmayleadtoindividualphotondetection becomingthenormacrossalargefractionoftheelectromagneticspectrum. These five chapters are followed by two appendices. The first Appendix is on Statistics.Observationalastronomyreliescruciallyonanunderstandingofstatistics. This is a huge subject which is treated in many other textbooks. In this book, I assumesomeknowledgeofbasicstatistics;howeverforthosereaderswhoneedto brush up, Appendix A summarises the key things we need to know. The second AppendixisonBasicOrbitalMechanicsandprovidesalittlemoreinformationas backgroundforthematerialononspaceastronomyinChap.3.