1 Week 7: 10/5 & 10/7 • Capturing that radiation • Chapter 6 (Telescopes & Sensors) Optical to Radio Summary • What are we sensing? Matter! – Matter is made of atoms (nucleus w/ protons, neutrons & cloud of electrons – Emits photons • Phases of matter? – Changing temperatures or pressure changes phases → changes emissions → changes light! 5.4 Learning from Light • Recognizing that “light” is giving us detailed information about what is happening at an atomic, hence compositional, level… • Our goals for learning – What are the three basic types of spectra? – How does light tell us what things are made of? – How does light tell us the temperatures of planets and stars? – How do we interpret an actual spectrum? What are the three basic types of spectra? Continuous Spectrum Emission Line Spectrum Absorption Line Spectrum Spectra of astrophysical objects are usually combinations of these three basic types Three Types of Spectra
Transcript
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Week 7: 10/5 & 10/7
• Capturing that radiation
• Chapter 6 (Telescopes & Sensors)
Optical to Radio
Summary• What are we sensing? Matter!
– Matter is made of atoms (nucleus w/ protons, neutrons & cloud of electrons
– Emits photons
• Phases of matter? – Changing temperatures or pressure changes
phases → changes emissions → changes light!
5.4 Learning from Light
• Recognizing that “light” is giving us detailed information about what is happening at an atomic, hence compositional, level…
• Our goals for learning– What are the three basic types of spectra?
– How does light tell us what things are made of?
– How does light tell us the temperatures of planets and stars?
– How do we interpret an actual spectrum?
What are the three basic types of spectra?
Continuous Spectrum
Emission Line SpectrumAbsorption Line Spectrum
Spectra of astrophysical objects are usually combinations of these three basic types
Three Types of Spectra
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Continuous Spectrum
• The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption
Emission Line Spectrum
• A thin or low-density cloud of gas emits light only at specific wavelengths that depend on its composition and temperature, producing a spectrum with bright emission lines
Absorption Line Spectrum
• A cloud of gas between us and a light bulb can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum (also applies to stellar atmospheres)
How does light tell us what things are made of?
Spectrum of the Sun
Chemical Fingerprints
• Each type of atom has a unique set of energy levels
• Each transition corresponds to a unique photon energy, frequency, and wavelength
Energy levels of Hydrogen
Chemical Fingerprints
• Downward transitions produce a unique pattern of emission lines
• Observing the fingerprints in a spectrum tells us which kinds of atoms are present
Selective absorption of Selective absorption of radiation in the Atmosphereradiation in the Atmosphere
OO22 and Oand O33 absorb almost 100% of the UV absorb almost 100% of the UV radiation at a radiation at a < 0.3 < 0.3 m.m.
0.1 0.3 0.5 0.7 1 5 10 15 20
HH22O and COO and CO22 are strong absorbers of IR radiation are strong absorbers of IR radiation and poor absorbers of visible radiation.and poor absorbers of visible radiation.
0 1 0 3 0 5 0 7 1 5 10 15 20
0.1 0.3 0.5 0.7 1 5 10 15 20
0.1 0.3 0.5 0.7 1 5 10 15 20
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CHCH44 and Nand N22O are also O are also strong absorbers of IR radiationstrong absorbers of IR radiation
Common in outer
solar system
Quick Test:
Which letter(s) labels absorption lines?
A B C D E
Quick Test:Which letter(s) labels the peak
(greatest intensity) of infrared light?
A B C D E
Thought Question
Which letter(s) labels emission lines?
A B C D E
How do we interpret an actual spectrum?
• By carefully studying the features in a spectrum, we can learn a great deal about the object that created it.
What is this object?
Reflected Sunlight: Continuous spectrum of visible light is like the Sun’s except that some of the blue light has been absorbed - object must look red
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What is this object?
Thermal Radiation: Infrared spectrum peaks at a wavelength corresponding to a temperature of 225 KMust be pretty cold!
What is this object?
Carbon Dioxide: Absorption lines are the fingerprint of CO2 in the atmosphere
What is this object?
Ultraviolet Emission Lines: Indicate a hot upper atmosphere
What is this object?
Mars!
Recap• What are the three basic type of spectra?
– Continuous spectrum, emission line spectrum, absorption line spectrum
• How does light tell us what things are• How does light tell us what things are made of?– Each atom has a unique fingerprint.– We can determine which atoms something is
made of by looking for their fingerprints in the spectrum.
And…• How does light tell us the temperatures of
planets and stars?– All stars emit a continuous spectrum that depends on
temperaturetemperature.
– The spectrum of that thermal radiation tells us the object’s temperature.
• How do we interpret an actual spectrum?– By carefully studying the features in a spectrum, we
can learn a great deal about the object that created it.
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5.5 The Doppler Effect
• Our goals for learning– How does light tell us the speed of a distant
jobject?
– How does light tell us the rotation rate of an object?
How does light tell us the speed of a distant object?
The Doppler Effect
The Doppler Effect
Same for Light
Measuring the Shift
Stationary
Moving Away
Moving Away Faster
Measuring the Doppler Effect from shifts in the wavelengths of emission lines
Moving Toward
Moving Toward Faster
The amount of blue or red shift tells us an object’s speed toward or away
ffrom us:
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Doppler Shift tells us ONLY about the part of an object’s motion toward or away from us:
Quick Test:
I measure a line in the lab at 500.7 nm.The same line in a star has wavelength 502.8 nm.
450nm=blue 700nm=red
a) It is moving away from me.
b) It is moving toward me.
c) It has unusually long spectral lines.
Redshifted to hereLab Spectra
How does light tell us the rotation rate of an object?
• Different Doppler shifts from different id f t tisides of a rotating
object spread out its spectral lines
Spectrum of a Rotating Object
• Spectral lines are wider when an object rotates faster
Recap• “Light can tell us:
– What something is made of
– What its temperature is
– If it is a solid or gas
• How does light tell us the speed of a distant object?How does light tell us the speed of a distant object?– The Doppler effect tells us how fast an object is moving toward or away
from us.
• Blueshift: objects moving toward us
• Redshift: objects moving away from us
• How does light tell us the rotation rate of an object?– The width of an object’s spectral lines can tell us how fast it is rotating
End of Chapter 5 slides
Chapter 6: Telescopes
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Telescope• Instrument for gathering and focusing
radiation from distant objects
Typically X-ray, Visible, IR, Radio
Functional Design• Gather radiation
• Focus to a point (magnify)
• Create an image for analysis
Electromagnetic Spectrum Electromagnetic Spectrum
The Mark I EyeballThe Mark I EyeballThe Mark I EyeballThe Mark I Eyeball• Biochemical Sensor
• Senses– 16 gray scales
2 000 l– 2,000 colors
• Resolution– Spatial = 0.1 mm*
– Spectral = 0.15 nm
• Dynamic Range– 300,000 steps
* depends on distance totarget. Roughly 200 metersfrom orbital altitudes
How does your eye form an image?
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Focusing Light
• Refraction can cause parallel light rays to converge to a focus
Image Formation
• The focal plane is where light from different directions comes into focus
• The image behind a single (convex) lens is actually upside-down!
How do we record images? Focusing Light
Digital cameras detect light with charge-coupled devices (CCDs)
• A camera focuses light like an eye and captures the image with a detector
• The CCD detectors in digital cameras are similar to those used in modern telescopes
• Our goals for learning– What are the two most important p
properties of a telescope?
– What are the two basic designs of telescopes?
– What do astronomers do with telescopes?
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M.A.M.1. Mounting – If you can’t see it, nothing else
matters2. Aperture – Diameter of the light-gathering
portion (mirror, lens, or antenna)3. Magnification – How much bigger is it?
• Everything gets magnified (see #1 above)• Handheld maximum typically 7-10X• Useful Magnification ~ 40X per inch of aperture
— 2.4 inch telescope → 100 power— 6 inch telescope → 240 power— Atmosphere limits maximum power to ~ 300X
The Magnification Myth
Magnification• 1X = 1 times the human eye, 10X = 10 times the
human eye, etc.• Magnification = How much closer, hence bigger
Important properties of a telescope?
1. Light-collecting area: Telescopes with a larger collecting area (aperture) can gatherlarger collecting area (aperture) can gather a greater amount of light in a shorter time.
2. Angular resolution: Telescopes with a larger aperture are capable of taking images with greater detail.
Light Collecting Area• Aperture• A telescope’s diameter tells us its light
collecting area: Area = πr2g• The largest telescopes
currently in use have a diameter of about 10 meters
• Galileo’s telescope had an apertureof about 1 inch (6X)
Bigger is better Angular Resolution
• The minimumangular separation g pthat the telescope can distinguish.
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The two basic telescope designs
• Refracting telescope: Focuses light with lenses
• Reflecting telescope: Focuses light with mirrors
Refracting Telescope
• Refracting telescopes usually very long, with large, heavy lenses
Reflecting Telescope
• Reflecting telescopes easier to build with much greater diameters
• Most modern telescopes are reflectors
Designs for Reflecting Telescopes
Mirrors in Reflecting Telescopes
Twin Keck telescopes on Mauna Kea in Hawaii
Segmented 10-meter mirror of a Keck telescope
Imaging
• Astronomical detectors generally record only one color of light at a time
• Several images must be combined to make full-color pictures
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Imaging• Astronomical
detectors can record forms of light our eyes can’t see
• Color is sometimes used to represent different energies of nonvisible light
Spectroscopy• A spectrograph
separates the different wavelengths of light before they
Diffractiongrating breaksLight from g y
hit the detectorg glight intospectrum
Detectorrecordsspectrum
gonly one starenters
Spectroscopy
• Graphing relative brightness of light at eachlight at each wavelength shows the details in a spectrum
How does Earth’s atmosphere affect ground-based observations?
• The best ground-based sites for astronomical observing are– Calm (not too windy)
– High (less atmosphere to see through)
– Dark (far from city lights)
– Dry (few cloudy nights)
Light Pollution
• Scattering of human-made light in the atmosphere is a growing problem for astronomy
Light Pollution
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Light Pollution
Darkest Country on Earth Remediation
A funny thing happened A funny thing happened on the groundon the ground
1990: The whole point of the Hubble was to get away1990: The whole point of the Hubble was to get awayfrom light pollution, dust, turbulence, and clouds.from light pollution, dust, turbulence, and clouds.
Twinkling and Turbulence
Turbulent air flow in Earth’s atmosphere distorts our view, causing stars to appear to twinkle
Star viewed with ground-based telescope
Same star viewed with Hubble Space Telescope
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Adaptive Optics
Rapidly changing the shape of a telescope’s mirror compensates for some of the effects of turbulence
Without adaptive optics With adaptive optics
Adaptive OpticsAdaptive Optics
Adaptive Optics
Increasing Signal to Noise ratioOver-sampling of hundreds (or thousands) of images
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Why do we put telescopes into space?
Transmission in Atmosphere
• Only radio and visible light pass easily through Earth’s atmosphere
• We need telescopes in space to observe other forms
Non-Optical ImagingSeeing things in a different light
GammaIR
Vis X-ray
X-Ray RGB composite
Magnetic Field Interactions
Earth
X-ray
Earthnot toscale
Magnetospheres
X-ray
Jupiter in X-ray and Visible light
Radio Telescopes
• Size is a function of wavelength, so they’re BIGthey re BIG
• Interferometry common
• Also on spacecraft
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Same Function
• Collect• Focus• Produce Image
Imaging Magnetic Fields
Interferometry
• Signals from many ll b t tsmall but separate
telescopes are electronically combined to equal one very large telescope
Ground-based Astronomy in Space
• Farside of the Moon observing site
N t h– No atmosphere– Very long
nights– No light
pollution– Minimal radio
noise
Spacecraft Payloads• Telescope are
critical payloads
Cassini Payload
• Typical manifest– Imaging Radar– UV/Visible instrument– Near IR instrument– Wide field (panoramic
