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January 2012 A1 Observational Methods

Observational Methods

Val OShea

[email protected]

Rm 329a Kelvin Bldg.

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Observational Methods 9 lectures

Electromagnetic (EM) radiation frequency and wavelength; flux;atmospheric windows; different spectral regions used in astronomy;magnitude system.Telescopes and their Purpose telescopes as flux collectors; signalstrength and conversion to photon flux;, telescope efficiency, limits ofsource detection; signal-to-noise ratio, angular resolving power.

Optical telescopes refractors and reflectors, telescope mounts,visual use of telescope; magnifying power; effects of Earthsatmosphere; active and adaptive optics.Detectors photomultipliers; CCDs, detector linearity, sources ofnoise, signal-to-noise ratio. Spectrometers - basic elements of thespectrometer; transmission and reflection systems; spectral resolvingpower.

Multi-band astronomy - radio antennas and interferometry; X-raytelescopes; neutrino detectors; -ray astronomy; gravitational wavedetectors.

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Learning Objectives

1) Describe the electromagnetic spectrum, associated units, andthe properties of the atmosphere in different spectral regions.

2) Explain the use of telescopes as flux collectors, includingquantitative assessment of angular resolving power, magnifyingpower, telescope transmission and limiting magnitude.

3) Describe the main types of modern astronomical detectors(CCDs and photomultipliers) and spectrometers, and explain theprinciples underlying their use.

4) Apply knowledge of the above directly to experimental

measurements, and to solving quantitative problems involving theoperation of multi-element telescope, instrument and detectorsystems.

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Scale


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EM Stuff


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ATLAS detector

Size: 46 m long, 25 m high and 25 m wide.The ATLAS detector is the largest volume

particle detector ever constructed.Weight: 7000 tonnes

Design: barrel plus end capsLocation: Meyrin, Switzerland.

More than 3000 scientists from 174 institutes in 38 countries work on ATLAS.

ATLAS

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ATLAS SCT

ATLAS SCT

61 m2 of silicon detectors6 million readout channels

Operated at -20CDesign: barrel plus end capsBuilt across 4 continents

Nov 28th 2011 Bristol - Val OShea 8

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Nearly all information from space arrives at Earth aselectromagnetic (EM) radiation e.g. light.

Other sources of information include: cosmic rays, neutrinos,meteorites, gravitational waves, and materials collected byspacecraft.

EM radiation can directly give information on an objects skyposition, apparent brightness, spectral energy distribution,Doppler shift, temporal variations, etc...With the application of some physical principles, this data can

be used to give information on the sources distance,luminosity, temperature, chemical composition, size, rotation,etc.

Electromagnetic radiation travels as a transverse wave ofoscillating electric and magnetic fields.

EM Radiation

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The wavelength and frequency of anEM wave obey:

c =

where c is the speed of light (invacuum this is 2.997925 108 m s1)

is the wavelength (often given innanometres = 109 m, or ngstrm= 1010 m)

and is the frequency (Hz)

long wavelength low frequencyshort wavelength high frequency

EM Radiation

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Electromagnetic radiation can also be thought of in a quantuminterpretation - as discrete packets of energy known asphotons.

The energy of a photon is related to its frequency viaE = h

where h is Plancks constant = 6.626 1034 J s.

high frequency high energy

Energy is often expressed in terms of electron volts (eV),e (q) the electron charge = 1.6 1019 C => 1eV = 1.61019 J

long wavelength low frequency lower energy

short wavelength high frequency higher energy

EM Radiation

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Useful'Prefixes'

Prefix' Factor'

E'2'Exa' 1018'

P'2'Peta' 1015'

T'2'Tera' 101'

G'2'Giga' 109'

M'2'Mega' 106'

K'2'Kilo' 103'

c'2'centi' 102'

m'2'milli' 1023'

m'2'micro' 1026'

n'2'nano' 1029'

p'2'pico' 1021'

f'2'femto' 10215'

a'2'ato' 10218'

!

Often in physics andastronomy, you will workw ith numbers manyorders of magnitudegreater or smaller than

you see in everyday life.

To assist with this, wehave table with the mostused prefixes and theirrelevant factors.

Orders of Magnitude

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For most of the EM spectrum the Earth is a very bad viewing platform. The atmosphere isopaque at most wavelengths, but there are two main windows:visible window 3001100 nmradio window 102102 m

10 MHz10 GHz Outside of these windows radiation is mostly absorbed

In the visible window we still have dust, pollution and water vapour that can dim and scatterstarlight. Due to this observatories tend to be at high, clear, dry sites e.g. Chile, Hawaii. Forthe rest of the spectrum we generally have to put our detectors/telescopes in space.

Atmospheric windowshttp://en.wikipedia.org/wiki/Image:Atmospheric_electromagnetic_transmittance_or_opacity.jpg.

EM Radiation and our Atmosphere

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EM Radiation and our Atmosphere

There are also many other means ofobserving that are neither on theground nor in space.

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The apparent brightness of a star is referred to byits magnitude. The magnitude system has its origins

with Hipparchus (120BC). He described the brighteststars as of the first magnitude.Later, Ptolemy (180AD) assigned a scale to thissystem by saying that the faintest stars visible to thenaked eye were five orders of magnitude fainter thanthe brightest ones i.e. the brightest stars aremagnitude m = 1, and the faintest are magnitude m = 6.

In 1856, Pogson gave a quantitative basis for themagnitude scale, by noticing that the eyes responseto changes in brightness was not linear butlogarithmic, so a magnitude difference corresponds toa brightness ratio. He defined a magnitude differenceof 5 to correspond to a brightness ratio of 100.

Apparent Magnitude

Magnitude 6

1

1 10 100 log Flux

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This magnitude system was developed withonly the human eye as a detector, andthere- fore only refers to the brightness

of the stars around 550 nm.However, two stars of equal brightness inthe visible band (V band) will notnecessarily be of the same brightness atother wavelengths.

This means that different, well specified,standard magnitude systems are needed.

The most common system is the Johnson,or UBV system - notionally Ultraviolet (360 nm), Blue ( 440 nm), Visible ( 550nm), Red, Infrared . . .

Apparent Magnitude

ccentral

wavelength

U 360nm ultrviolet

B 440nm blue

V 550nm visible

R 700nm red

I 0.9 m infrared

J 1.2mH 1.6m

K 2.2m

L 3.4mfurtherinfra

red

M 4.9m

N 10.2m

Q 20m What is the magnitude difference between two starswith a flux ratio of 500?m = |m1 m2| = | 2.5log10(500)|, = 6.75.

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The magnitude system only allows you to describe the apparent brightness of objects relativeto other objects, but often you need a more physical way of describing it. Two such ways arethe objects luminosity and the energy flux you observe.

Magnitude & Flux

An objects luminosity is the power it radiates (energy emitted per second - often given, inastronomy, in units of erg s1, where 1 erg= 107 J). The luminosity can be defined over a smallwavelength range (i.e. when measuring a particular part of the EM spectrum) or can besummed up over the whole spectrum, which is often called the bolometric luminosity.

Measure the amount of light energy per unit area per unit time.

! = !"#$%& !"#$%&!"#$

= !4!!!

!!!!!!!!

! = !4!!!!

!"#!!!!!!!!!!!!

1pc = 3.26ltyrs = 3.09 1016 m).

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Why are Distances Important?

Distances are necessary for estimating:

Total energy emitted by an object (Luminosity)Masses of objects from their orbital motionsTrue motions through space of starsPhysical sizes of objects

The problem is that distances are very hard to measure...

Parallax

Where:

p = parallax angle in arcseconds

d = distance in "Parsecs"

A "natural" unit for distances in astronomy: the Parallax-Second or Parsec.

d= 1p

206,265 AU3.26 Light Years3.086x1016 m

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!! =!

4!!

! !!! = !4!10

!!

! ! = 2.5!"#!"

!4!!!

!4!10!

!

!! =

2

.

5!"#!"10

!

!

!

= 5log1010 + 5log10d

mM = 5log10d5

Where d is in parsecs. So for a star at d=100pc with m=6

mM =525=5M =1

Absolute Magnitude

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