Two Properties of Thermal (Blackbody) Radiation
Relative intensity
per unit area
1. Hotter objects emit more light at all wavelengths per unit area.
2. Hotter objects emit photons with a higher average energy (bluer).
Quantitative Review of Thermal (Blackbody) Radiation
• Stefan-Boltzmann Law
• Gives the power of emitted blackbody radiation
emitted by an object at temperature T:
P = ε Aσ T4
P = radiated power in Watts.
ε = emissivity (a unitless number ranging from 0 to 1 where ε=0
would be a highly reflective surface and ε=1 would be a perfect
absorber/emitter.)
A = area of emitter surface (in square meters).
σ = 5.67x10-8 W/m2K4 (this is the Stefan-Boltzmann constant - a
fundamental constant).
T = Temperature in degrees Kelvin (K).
• Hotter objects emit A LOT more light (emitted
power proportional to T4 !).
Quantitative Review of Thermal (Blackbody) Radiation
• Wien’s Displacement Law
• Gives the wavelength where the peak of emitted
blackbody radiation occurs:
λmax = 2.9x10-3 / T
λmax = wavelength in meters
T = Temperature in degrees Kelvin (K).
Units of constant (2.9 x 10-3) is meters-Kelvin.
• Hotter objects emit the peak of their radiation at
smaller wavelengths (i.e. higher frequencies or
higher energies).
Telescopes
Collecting Light with Telescopes
• Why use telescopes?
• Why put telescopes into space?
• How is technology revolutionizing astronomy?
Why use telescopes?
• Telescopes collect (A LOT) more light than our eyes
⇒ light-collecting area
• Telescopes can see more detail than our eyes
⇒ angular resolution
• Telescopes/instruments can detect light that is invisible to
our eyes (e.g., infrared, ultraviolet)
⇒ multi-wavelength sensitivity
Bigger is better (for telescopes)
1. Larger light-collecting area:
a bigger telescope sees fainter objects.
More distant objects are fainter, so bigger telescope
also sees out to larger distances.
2. Better angular resolution:
a bigger mirror can make sharper images.
Theoretically best possible angular resolution is
inversely proportional to mirror size:
2x larger telescope can make 2x sharper images.
Bigger telescopes have better
angular resolution ⇒ see finer details
• Diffraction Limit
θd = λ / D
θd = diffraction limit angle (in radians)
λ = wavelength
D = telescope diameter
• Diffraction limit angle is inversely proportional to the
telescope diameter (larger D means smaller angle means
higher resolution).
• Diffraction limit angle is proportional to the wavelength
(shorter wavelength means smaller angle means higher
resolution).
Two Types of Telescope Designs
• Refracting telescope: uses lenses (older)
Yerkes 1-m refractor
Two Types of Telescope Designs
• Reflecting telescope: uses mirrors (newer)
Gemini North 8-m
Observatories
• Properties of and ideal ground-based, astronomical
observatory
• Far from background lights
• Far from sources of electrical interference
• Large fraction of nights clear (free from clouds)
• High altitude
• Minimal atmospheric turbulance (good seeing).
Mauna Kea, Hawaii
Keck I & II Telescopes:
Largest optical/IR telescopes in the world
Keck Telescope: Top view
Keck primary mirror is segmented
The Electromagnetic Spectrum
Access to the entire spectrum
provides information about many
different astrophysical processes
Observing different wavelengths of light
requires different telescope designs
Radio telescope (Arecibo, Puerto Rico)
Radio Telescopes
VLA (Very Large Array)
New Mexico
Observing different wavelengths of light
requires different telescope designs
Xray telescopes use “grazing incidence” optics
Chandra (X-ray)
Why put telescopes in space?
• NOT because they
are closer to the
stars!
• Because Earth’s
atmosphere causes
3 problems for
telescopes.
Hubble Space Telescope
Problem #1: Light pollution
Man-made lights brighten the night sky,
making it harder to observe faint objects
Problem #2: Atmospheric “seeing”
Turbulence causes twinkling,
which blurs astronomical images.
Star viewed with
ground-based telescope
View from Hubble
Space Telescope
Problem #3: Limited transmission
Atmosphere absorbs most wavelengths of light,
including all UV & X-rayand most infrared photons.
Space-based telescopes solve all 3 problems
• Location/technology
can help overcome
light pollution and
turbulence.
• But nothing short of
going to space can
solve problem of
atmospheric
absorption of light
Chandra X-ray Observatory
Hubble Space Telescope
Why not put all the telescopes in space?
2m diameter, $4 billion
8m diameter, $100 million
• Cost & safety
• Ease of upgrade.
• New technologies (e.g. adaptive optics) can
make ground-based telescopes very powerful.
JWST (James Webb Space Telescope)
“Next Generation” Space Telescope
• Replacement for Hubble.
• Much larger diameter – 6.5m
vs 2.4m
• Emphasis on infrared
wavelengths (to study the
universe at high redshift)
• Will not orbit the Earth –
rather placed at L2 Lagrange
point beyond the moon.
• Launches in 2013.
JWST will reside at L2 beyond orbit of moon
JWST deployment
The Moon would be a great spot for an observatory
(but at what price?)
How is technology
revolutionizing astronomy?
• It makes possible more powerful and
more capable TELESCOPES and
INSTRUMENTS
– Light detector technology
– Adaptive optics
– Interferometry
Telescopes use instruments to
collect and measure light
Telescopes use instruments to
collect and measure light
Most common types:
• Imaging camera
• Spectrograph
IR spectrometer (spectrograph) on the
NASA 3-meter IRTF (IR Telescope Facility)
Electronic light detectors are at the
heart of all modern optical/IR instruments
(bigger = more pixels = better)
12288 pixels
8192 pixels
200 Megabytes per image
CFH12K wide field
images with CFHT
MegaPrime on the
3.6-meter CFHT (Canada-France-Hawaii Telescope)
5700 kg, 2 meter tall
1 degree field of view
CFHT/Megacam: 377 million pixels
750 Megabytes per image
Adaptive optics (AO) imaging
Compensate for atmospheric turbulence using a small mirror
which rapidly changes shape in real-time: sharper images.
AO off
AO on
Schematic of adaptive optics system
Adaptive Optics
Correcting for the turbulent atmosphere
High Resolution Optical View of the Galactic Center
Adaptive Optics
Neptune at 1.65 microns
With Keck adaptive optics
2.3 arc sec
Without adaptive optics
May 24, 1999
June 27, 1999
Adaptive Optics Systems require that a
bright star be visible in the field of view to
allow measurement of the atmospheric
wavefront distortions in order to correct for
them.
• This limits AO observations to objects with nearby
bright stars…
… unless…
• You make your own artificial star using a laser
(called a Laser Guide Star).
Next-Generation Large Ground-Based Optical and IR Telescopes
TMT
Mirror Support System for TMT
• Each hexagonal Segment has an active support system
TMT Animation
Giant Magellan Telescope
ESO ELT (or OWL)
Recap: Telescopes
• How do telescopes
help us learn about
the universe?
– We can see fainter
objects and more detail
than we can see by eye.
– Specialized telescopes
allow us to study other
wavelengths besides
visible light.
• Why do we put
telescopes in
space?
– They are above
Earth’s atmosphere
and therefore not
subject to light
pollution, atmospheric
distortion, or
atmospheric
absorption of light.