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Astronomy 1 > Observational Methods > Flashcards

Observational Methods Flashcards

1
Q

Cosmic rays

A

High energy particles from space (supernovae, sun etc) detected at high altitude

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2
Q

Most atrononomical information is deduced from

A

Observations of EM radiation

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3
Q

EM waves

A

Oscillating electric and magnetic fields
Travel at c
C=v lambda
Or considered particles, E=hf

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4
Q

Atmospheric windows

A

Only certain parts of the EM spectrum can be viewed from the ground

Some parts completely opaque

Radio window completely transparent

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5
Q

Atmospheric windows - Main bands

A

Visible ~300-1100nm
Microwave/radio 10^-2 - 10m
Parts of infrared also observed from Earth

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6
Q

Effect of the Atmosphere

A

Gamma and x-rays absorbed by atoms and nuclei
UV mostly absorbed by O2 and O3
IR absorbed in different bands by H2O and CO2
Radio>10m blocked by charged particles in ionosphere

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7
Q

Luminosity

A

Power radiated by an object

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8
Q

Bolometric luminosity

A

Refers to power radiated at all wavelengths

Lbol= integral between 0 and infinity of Lmono d labda

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9
Q

Monochromatic luminosity

A

Measured over a small wavelength window

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10
Q

Power

A

Shaded area of curve monochromatic L x d lambda

Total power = integral (area under whole graph)

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11
Q

Flux

A

Power radiated per unit area received a distance d from an object (units Wm^-2)

F=L/4pid^2 (=L/ area)

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12
Q

Monochromatic flux

A

Exactly the same but using monochromatic luminosity in equation instead

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13
Q

Magnitude

A

Describes an object’s brightness

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14
Q

Apparent magnitude, m

A

How bright an object appears

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15
Q

Absolute magnitude, M

A

Intrinsic brightness of an object

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16
Q

Sirius

A

Brightest star in sky has m=-1.46

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17
Q

Apparent magnitude zero point

A

Apparently brighter star has smaller apparent magnitude than fainter star

m=0 point chosen to be vega

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18
Q

Apparent magnitude of sun

A

-27

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19
Q

Parallax

A

Difference in apparent position when an object is viewed along different lines of sight

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20
Q

Annual parallax

A

Difference in apparent position when a star is viewed from Earth and from the sun

Earth’s orbit gives different viewing positions
Calculate annual parallax from positions 6 months apart

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21
Q

Equation for annual parallax

A

Draw triangle to get tanP=SE/d = 1au/d

Since p very small, tanP~sinP~P (in rad)

So P~ 1au/d

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22
Q

Parsec

A

The distance at which an object has a parallax of one second of arc

Need P in radians

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23
Q

Absolute magnitude

A

Compares luminosities accounting for distance
M is the apparent magnitude a star would have at a distance of 10 parsecs

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24
Q

Two functions of telescopes

A

Make objects appear brighter and bigger

Collect a lot of light to enable faint objects to be seen and magnify objects to allow detail to be resolved

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25
Q

As astronomical objects very far away, light arrives at Earth

A

In essentially parallel rays

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26
Q

Convex lens

A

Brings parallel rays to focus at focal distance
Light ray passing through the centre is undeviated

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27
Q

Basic refracting telescope

A

Consists of primary/objective lens and an eyepiece lens

Lens separated by the sum of their focal lens

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28
Q

Exit pupil, d

A

Diameter occupied by outgoing light rays

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29
Q

Light collecting function

A

Light from large area, diameter D, collected into smaller area, diameter d

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30
Q

Magnification

A

Increase of the apparent angular size of an object

Ratio of of angular size of image when viewed through eyepiece to the actual angular size of the object
Or ratio of focal lengths of the objective and eyepiece

magnification=alpha e/ alpha o

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31
Q

Angular size

A

When object close, it subtends a large angle at the telescope

Far away object subtends a smaller angle

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32
Q

Tan alpha o

A

h/fo

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33
Q

Tan alpha e

A

h/fe

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34
Q

Large primary lens or small eyepiece gives

A

High magnification

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35
Q

For visual use, exit pupil should be

A

< or = diameter of dilated pupil of eye (~8mm)

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36
Q

Telescope size

A

Refers to diameter of objective/primary optic

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37
Q

Telescope speed

A

F0/D

F/10 or f-ten means F0/D=10

Fast lens gatherers more light in a shorter time and can have a shorter shutter speed

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38
Q

Is F/10 or F/22 faster

A

F/10

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39
Q

Light collection

A

Larger diameter collects more light
Larger area collects more power

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40
Q

Collected power

A

W=FA

for telescope of circular aperture of diameter D, A=pi(D/2)^2

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41
Q

Number of collected photons per second

A

nph=W/Ephoton

Ephoton=hf

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42
Q

Limiting magnitude

A

Apparent magnitude of the faintest object which can be observed through the telescope

Calculated via Pogsen’s equation mlim=6+5log10D/deye

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43
Q

Naked eye can detected stars of approximately

A

m=6 and below

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44
Q

Diffraction

A

Bending of wave passing through an aperture or around an object

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45
Q

Cross section through diffraction pattern from circular aperture

A

Central maximum contains 84% of the light - airy disk

First minima occurs at angular distance of 1.22lambda/D from centre

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46
Q

Two adjacent stars produce

A

Overlapping diffraction patterns

Stars far apart can be easily resolved
Close together start to overlap
At some point can just be resolved, any closer cannot separate

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47
Q

Can typically resolve stars if

A

Maximum of one overlaps the first minimum of the other
a~1.22 lambda/D

a=theoretical angular resolution=angular resolving power=Rayleigh’s criterion

Useful guide but depends on the eyesight of the observer

48
Q

Angular resolution of a means

A

Object with angular separation > or = a can be resolved

49
Q

Why not all light entering telescope reaches detector

A

Absorption of light in glass
Absorption/scattering by mirror/lens surface (dirt, bubbles, surface roughness)

50
Q

Telescope transmission T

A

Fraction of incident light which reaches detector

For multiple lenses or mirrors Ttot=T1xT2x…

51
Q

Typical values for telescope transmission

A

T=0.75 for mirrors
T=0.9 for lenses

52
Q

Single s

A

Energy arriving at detector for an observation time delta t

S=FATdelta t

But detectors not 100% efficient so quantum efficiency, n, added to equation

53
Q

For photons of equal energy, no of photons detected

A

N=S/hf = FATn delta t/ hf

54
Q

Noise (statistical uncertainty)

A

Present whenever we measure a signal

Can arise from source, telescope and detector, concentrate on noise from source being observed

55
Q

Poisson noise

A

Associated with random processes such as radioactive decay and emission of photons from source

Mean emission rate N
Fluctuations around mean, for large number of random, independent events typical variation is root N

So signal to noise ratio is SNR=N/ root N = root N

56
Q

Relative error can be greatly reduced when

A

Large number of photons are measured

57
Q

Why build telescopes on mountain tops

A

To avoid atmospheric distortion of light

58
Q

Atmosphere refractive index (density)

A

Changes in space and time
Atmospheric flow, turbulence

Refraction of rays in atmosphere gives random deviations

59
Q

Scintillation

A

Variations of flux entering telescope, brightness variations

60
Q

Seeing

A

Variations in apparent direction of origin of light

Seeing value refers to average size of image

0.5” is very good

61
Q

Two types of telescope

A

Refractor, primary optic is a lens
Reflector, primary optic is a mirror

62
Q

Chromatic aberration

A

Different wavelengths being focussed at different positions

63
Q

Spherical aberration

A

Paraxial rays which hit lens at different distances from optical axis are focused at different distances

Corrected by using aspherical lenses

64
Q

Disadvantages of refractors

A

Chromatic aberration
Spherical aberration for spherical lenses
Lenses are heavy
Require very uniform glass

Largest practical size for refractor is ~1m diameter

65
Q

Reflectors

A

Use a mirror as primary optic
Ideal shape - concave paraboloid

Focus all wavelengths to single point

66
Q

Parabolic mirror

A

Preferable to spherical but highly susceptible to astigmatism, leads to spread in image position if rays not exactly paraxial

67
Q

Newtonian reflector

A

Concave primary to focus light, flat secondary folding mirror to divert light to focal point outside telescope

68
Q

Cassegrain reflector

A

Concave primary, convex secondary, reflecting light through a gap in the primary

Allows long focal length, high magnification telescope

69
Q

Schmidt-Cassegrain reflector

A

Primary mirror is spherical
Refracting Schmidt correct plate used to compensate for spherical aberration

70
Q

Gregorian reflector

A

Primary mirror is concave paraboloid
Real image is formed before light reaches secondary mirror
At secondary mirror, rays diverging so it is concave to further focus them

71
Q

Hubble Space Telescope

A

Benefit of being above atmosphere seeing effects so get very clear images

72
Q

James Webb Space Telescope

A

Primary mirror with 18 hexagonal mirror segments
Near and mid infrared imaging and spectroscopy

73
Q

Disadvantages of secondary mirrors

A

Loss of light (some of aperture blocked)
Diffraction around secondary mirror and holder leading to image distortion

74
Q

Active optics

A

Primary mirror can change shape due to mechanical stress and thermal expansion/contraction
Slow changes of order 1s or longer
Large mirrors only possible with active optics
Monitor mirror shape/image quality
Apply corrections via mechanical actuators which change mirror shape

75
Q

Largest mirror practical to avoid sagging under own weight

A

About 8m

Segmentation critical to allow very large mirrors

76
Q

Adaptive optics

A

Corrects for rapid (millisecond timescale) effects of seeing

77
Q

Process for adaptive optics

A

Beam splitter directs some light to a wavefront sensor
Wavefront sensor analyses wavefront
Wavefront analysis used to calculate correction
Corrections applied to a deformable ‘adaptive mirror’
Corrections aim to make wavefronts more planar, improving image quality

78
Q

Shack Hartmann wavefront sensor

A

Light focussed by an array of small lenses onto a position sensitive detector

Actual image positions are compared to the plane wave case to measure wavefront distortion

79
Q

Guide star

A

Often the target if observation is too faint to properly measure wavefront distortion

Nearby guide star monitored instead- light has followed similar path through atmosphere

80
Q

Artificial guide star

A

Where bright, nearby star not available
Lasers used to produce light source in atmosphere

Eg: Rayleigh guide star, near UV, use backscattering from high in atmosphere to measure wavefront distortion

Sodium: 587nm laser used to excite sodium atoms high in atmosphere, creating glow

81
Q

Chemical photography

A

Photographic plates used in the past
Still provides best spatial detail and largest formats
Entirely superseded by electronic detectors which rely on photo electric effect

82
Q

Photo electric effect

A

Photon striking alkali metal or semiconductor can eject photon if Ep>wf

Eelectron=Ep-wf

83
Q

Quantum efficiency of material

A

n=no of ejected electrons/no of incident photons

84
Q

CCDs

A

Charge coupled devices are sensors which are spatially sensitive to photons

Semiconductor chips utilise photoelectric effect
Array of pixels to provide position sensitivity

85
Q

How CCDs work

A

Incident photon hits CCD pixel, photon ejects electron from its place in semiconductor lattice
Bias voltage draws electron into a ‘well’, held until it is read out
More photons on pixel will result in more electrons stored in well
After exposure, collected electrons read out for analysis

86
Q

What does it mean by read out

A

Voltage applied
Electrons move pixel to pixel
All e move right, third row to transfer register
Move step down, first reading
Another step down, next reading

87
Q

Pixels in CCDs

A

Usually 5-10 micro m squares
Size of CCD is around 4cm side length (found by square rooting total no of pixels and multiply by size of each pixel)

88
Q

CCDs are

A

Sensitive in optical band
Stable
Quick to read out
Linear
Must be cooled to reduce dark current (e emitted by random thermal fluctuations)
Limited in size

89
Q

Photomultiplier

A

Photons incident on photocathode, liberating electron
E accelerated to another electrode held at higher potential
Collision liberates several e, accelerated to next electrode etc
Generates pulses of charge at anode

90
Q

Cold hydrogen

A

Emits at wavelength 21cm due to transition of the neutral ground state
Passes easily through atmosphere
21cm radio observations used to map structures

91
Q

Angular resolution of single radio telescope is

A

Rayleigh’s criterion
For radio wavelengths >0.01m angular resolution poor

92
Q

Antennae

A

Used to transmit or receive radio waves
Simplest is metal rod
Electric field of radio wave makes e oscillate producing voltage which can be detected

93
Q

Angular resolution of antennae described by

A

Beam
The angle of the cone within which a source is detected

94
Q

1 Jansky (Jy)

A

10^-26 W/m^2/Hz

95
Q

Waveguides

A

Simple antenna has poor angular resolution and low sensitivity

Improve sensitivity by waveguides which re-radiate any energy falling onto them towards the dipole and a reflector behind antenna

96
Q

How to see high resolution images from radio telescopes

A

Arrays of antennae and dishes used together in an interferometer configuration to improve angular resolution

97
Q

Interferometers

A

Parallel rays arrive at two antennae separated by D
Signals combined at receiver
pd=l=Dsin theta

Constructive interference when integer multiple n lambda
Small angle approx when source close to zenith

98
Q

As source moves across sky it’s position is tracked and interferometric signal

A

oscillates as an interference pattern is mapped out

Angular resolution is precision with which position can be measured delta theta = delta n lambda/D

99
Q

X rays emitted by

A

Very hot gases

100
Q

Thermal Bremsstrahlung radiation

A

Galactic clusters forming from merging of galaxies/groups of galaxies

In falling matter collides with existing gas and is heated, leading to x ray emission

101
Q

Crab Nebula

A

Remnant of core collapse supernova
Pulsar at centre

102
Q

X ray interaction with matter

A

High photon energy tend to penetrate matter
Refractive index of many materials is close to 1 in X-ray band

Not deviated much and eventually absorbed without much change in direction
Can scatter easily rather than reflect at normal incidence

103
Q

Grazing incidence

A

X-rays can be reflected at grazing incidence
Large angle of incidence

104
Q

X-ray mirrors

A

Nearly parallel to incoming rays
X-rays can be focused by first reflecting from parabolic mirror then from hyperbolic mirror

105
Q

Problem with X-ray telescopes and mirrors

A

Not much collecting area compared to an optical telescope, particularly serious as X-ray flux tends to be low

106
Q

Solution for X-ray telescopes and mirrors

A

Many cylindrical nested mirrors

107
Q

Problem: Until recently could only use grazing incidence optics, lambda >0.1nm

A

Shorter wavelength, imperfections/roughness of mirror’s surfaces is a problem, defects similar size to wavelength

108
Q

New multilayer optical coatings

A

Allow higher energy x-rays to be imaged

Alternating coatings layers of tungsten/silicon or platinum/silicon carbide used

109
Q

Collimating optics/ Fourier grid

A

Alternative to focussing optics for x-ray imaging
Simplest technique - a collimator restricts viewing angle to locate origin of x-rays

110
Q

Modulation collimator

A

Two arrays of wire grids allow only x-rays from narrow strips of sky to be detected

111
Q

Coded aperture

A

A mask/collimator which transmits some X-rays in a pattern is placed in front of the detector

The position/ shape of the pattern allows reconstruction of the source direction

112
Q

CCDs for X-rays

A

Due to high energy of X-ray photons, one X-ray can liberate many electrons
No of e proportional to photon energy
Read out identifies location of X-ray on grid and energy of X-ray
X-rays of particular energy emitted from particular atoms can be identified - very useful

113
Q

Micro channel plate

A

Plates with small channels- micron scale glass tubes coated to emit electrons when struck by x-rays
Electrons accelerated by voltage to hit second array of tubes
Collisions with walls of tubes produce more e, causes large pulse of e at anode
Wire grid allows position sensitivity

114
Q

CHANDRA - high resolution camera

A

Micro channel plates coated to emit electrons when struck by x-rays
Electrons accelerated down tube by voltage, emitting more electrons from collisions with tube walls
Crossed wire grid detects signal and allows position of x-ray to be determined

115
Q

CHANDRA - advanced CCD imaging spectrometer

A

Array of CCDs
Can measure energy of every detected photon
Map x-rays produced by different elements

Astronomy 1 (6 decks)

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