Table Of ContentAstronomical
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
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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.
