Light and Telescopes
Almost all astronomical information is obtained through the electromagnetic radiation, i.e. light, we receive from cosmic objects
Assignment
Chapter 6. All of it
Goals1)To investigate the nature of light
2)To become familiar with the electromagneticspectrum
3)To introduce telescopes
4)To understand how we collect and study light using telescope
5) All of this is covered in Chapter 6
What is light? Light is electromagnetic radiation, i.e. coupled electric and
magnetic fields that oscillate in strength and that propagate in space while carrying energy.
Technically, light is the part of electromagnetic (e.m.) radiation that humans (and other animals) see
Humans also sense (“see” with sense other than sight) other part of the e.m. spectrum, like heat (through skin)
Although incorrectly, we usually call “light” all type of electromagnetic radiation, like X-ray light or UltraViolet light
Light really is a small portion of the spectrum of e.m. radiation Types of e.m. radiation differ from each other by wavelengths
• Blue light: short wavelength; red: long one• X-ray: very short wavelength; radio: very long one
Identical situation with sound pitch• High pitch: short wavelength; bass: long one
What is Electromagnetic Radiation?
Made of propagating waves of electric and magnetic field
It carries energy with it• Sometimes called “radiant energy”• Think – solar power, photosynthesis,
photo-electric cells, the fireplace …
It also carries information • the signal received by your car radio• the signals received by telescopes staring at
stars• the signals received by your eyes right now!
What is the electromagnetic wave?
It is electricity and magnetism moving through space.
So, when we say the speed of light is “c” what we really mean is that the speed of the electromagnetic wave is “c”, regardless of its frequency
Light as a wave Waves you can see:
e.g., ocean waves Waves you cannot
see:• sound wave• electromagnetic
waves
Light is an electromagnetic wave
Light as a Wave
• Light waves are characterized by a wavelength l and a frequency f.
f = c/l
c = 300,000 km/s = 3*108 m/s
• f and l are related through
l
Properties of Waves Wavelength – the
distance between crests (or troughs) of a wave.
Frequency – the number of crests (or troughs) that pass by each second.
Speed – the rate at which a crest (or trough) moves.
For light in general:speed = c = s/t = λ
λ = cλ = c/
wavelength frequency
speed of light = 3x105 km/s in vacuum
Light as a Wave (2)• Wavelengths of light are measured in
units of nanometers (nm) or Ångström (Å):1 nm = 10-9 m
1 Å = 10-10 m = 0.1 nm
Visible light has wavelengths between 4000 Å and 7000 Å (= 400 – 700 nm).
Wavelengths and Colors
Different colors of visible light correspond to different
wavelengths.
Light as particles
• Light comes in quanta of energy called photons – little bullets of energy.
• Photons are massless, but they have momentum and and energy.
• They also react to a gravitational field (because they follow the curved space-time).
Wave-particle duality
All types of electromagnetic radiation act as both waves and particles.
The two views are connected by the relation
E = h = h c / l
h is the Planck's constant, c is the speed of light is the frequency, l is the wavelength
The energy of a photon does not depend on the intensity of the light!!!
Intensity
A photon's energy depends on the wavelength (or frequency) only, NOT the intensity.But the energy you experience depends also on the intensity (total number of photons).
It turns out that particles of matter, such as electrons, also behave as both wave and particle.
The theory that describes these puzzles and their solution, and how light and atoms interact is quantum mechanics.
In Summary: properties of Light All light travels through (vacuum) space
with a velocity = 3x105 km/s
The frequency (or wavelength) of photon determines how much energy the photon has:
The number of photons (how many) determines the intensity
Light can be described in terms of either energy, frequency, or wavelength.
Visible Light ShorterWavelength
LongerWavelength
Remember: visible light isn’t the whole story. It’s just a small part of the entire electromagnetic
spectrum
Long Wavelength(high frequency)(high energy)
Short Wavelength(low frequency)(low energy)
Wavelengths and size of things
Example of Electromagnetic Radiation
Short wavelength Long wavelength
If light is thermally generated, by a heated body, the dominant color reflects the temperature of the body
Compared to visible light, radio waves have:
higher energy and longer wavelength higher energy and shorter
wavelength lower energy and longer wavelength lower energy and shorter wavelength all light has the same energy
The Multi-wavelength Sun
Radio infrared
X-rayoptical
Optical Sky
Near-infrared sky
Boldt et al.
Radio Sky
Soft X-ray Sky
Different wavelength carry different type of information
• Visible light: the glow of stars (dust blocks light)
• Infrared: the glow of dust
Visible light (top) and infrared (bottom) image of the
Sombrero Galaxy
Matter interacts with light in four different ways:
Absorption – the energy in the photon is absorbed by the matter and turned into thermal energy
E.g., Your hand feels warm in front of a fire. Reflection – no energy is transferred and the
photon “bounces” off in a new (and predictable) direction
E.g., Your bathroom mirror Transmission – no energy is transferred and the
photon passes through the matter unchanged. Emission – matter gives off light in two different
ways. We’ll come back to this next lecture.
Our eyes work via the process of:
transmission reflection absorption emission none of the above
A red ball is red because:it only emits frequencies
corresponding to red
it only reflects frequencies corresponding to red
it only transmits frequencies corresponding to red
it only absorbs frequencies corresponding to red
Telescopes
The largest optical telescopes in the world:The twin 10-m Keck telescopes (Hawaii)
The HubbleSpace Telescope (HST)
An ultraviolet(1000-3500) Ang,
Optical(3500-8500) Ang,
and near-infrared (8500-16000) Ang
telescope
The Five College Radio Astronomy
Observatory(now defunct)
UMass LMT
The 50-m Large Millimeter Telescope
The largest millimeter-wavelength telescope in the world
U Mass and Mexico
Refracting/Reflecting Telescopes
Refracting Telescope:
Lens focuses light onto the focal plane
Reflecting Telescope:
Concave Mirror focuses light onto the focal
plane
Almost all modern telescopes are reflecting telescopes.
Focal length
Focal length
Secondary OpticsIn reflecting telescopes: Secondary
mirror, to re-direct the light path towards
the back or side of the incoming
light path.
Eyepiece: To view and
enlarge the small image produced in
the focal plane of the
primary optics.
Disadvantages of Refracting Telescopes• Chromatic aberration: Different
wavelengths are focused at different focal lengths (prism effect).
Can be corrected, but not eliminated by second lens out of different material
• Difficult and expensive to produce: All surfaces
must be perfectly shaped; glass must be flawless; lens can only
be supported at the edges
What telescopes are for?Why do they need to be big?
The main feature of a telescope is its capacity to collect as much light as possible• Like an antenna: the stronger the signal the clearest the transmission.• Well, guess what: an antenna *is* a telescope (a radio telescope, that
is) The larger the light collector, I.e. the primary mirror or lens, the
more powerful the telescope (Light Gather Power= LGP)• LGP ~ 4 p D2
• LGPA/LGPB = (DA/DB)2
• A telescope twice as large collects four times as much light
The other primary feature is image sharpness, to faithfully reproduce details• Resolving power: a = 11.6/D
The last, and least important, feature is magnification
The Powers of a Telescope:Size Does Matter
1. Light-gathering power: Depends on the surface area A of the primary lens / mirror, proportional to diameter squared:
A = p (D/2)2
D
The Powers of a Telescope (2)
2. Resolving power: Wave nature of light => The telescope aperture produces fringe rings that set a limit to the resolution of the telescope.
amin = 1.22 (l/D)
Resolving power = minimum angular distance amin between two objects that can be separated.
For optical wavelengths, this gives
amin = 11.6 arcsec / D[cm]
amin
SeeingWeather conditions and turbulence in the atmosphere set further limits to the quality of astronomical images.
Bad seeing Good seeing
Seeing
Seeing
Deep Imaging of the sky:at the edge of the Universe
Ground Telescope Subaru + SUPREME Space: HST + ACS
To study galaxy formation both space-based sensitivity and angular resolution required!!
Note how many more details and faint objects can be observed with the Hubble Space Telescope
The Powers of a Telescope (3)
3. Magnifying Power = ability of the telescope to make the image appear bigger.
The magnification depends on the ratio of focal lengths of the primary mirror/lens (Fo) and the eyepiece (Fe):
M = Fo/Fe
A larger magnification does not improve the resolving power of the telescope!
Telescopes do not have to be only Optical
Different wavelengths carry different type of information (optical: stars; X-ray: black hole; infrared: dust; radio: gas)
To detect different wavelengths of light, eg. X-ray, UV, optical, infrared, radio, different technologies are required
For example, special mirrors are necessary for X-ray telescopes or else the radiation would pass through them.
Hence, it is necessary to specialize telescopes to the wavelength of light one wishes to study.
We X-ray, UV, optical, infrared and radio telescopes
Different locations for telescopes In addition, the Earth’s atmosphere affects light of different
wavelengths differently:1. It totally absorbs X-ray and UV light: X-ray and UV telescopes MUST
be placed in space2. It blurs the optical light, I.e. it destroys sharpness. 3. It also adds the glare of the night sky (yup! There is such thing) to
optical and infrared light, which makes faint sources hard to see.4. It totally absorbs some (important) infrared light
• As a consequence some telescopes can operate on the ground: • optical, near-infrared, radio
• Some can only work in space• X-ray, UV, mid- and far-infrared
• For high-resolution (super-sharp) observations, or for observations of very faint sources (i.e. to avoid the glare of the Earth’s atmospherer) either space telescopes or very advanced technologies (adaptive optics) are required.
The Best Location for a Telescope
Far away from civilization – to avoid light pollution
The Best Location for a Telescope (2)
On high mountain-tops – to avoid atmospheric turbulence ( seeing) and other weather effects
Paranal Observatory (ESO), Chile
Most wavelengths cannot penetrate the Earth's atmosphere
Observing Beyond the Ends of the Visible Spectrum
However, from high mountain tops or high-flying air planes, some infrared radiation can still be observed.
NASA infrared telescope on Mauna Kea, Hawaii
Most infrared radiation is absorbed in the lower atmosphere.
Infrared cameras need to be cooled to very low temperatures, usually using liquid nitrogen.
The Hubble Space Telescope
• Avoids turbulence in the Earth’s atmosphere
• Extends imaging and spectroscopy to (invisible) infrared and ultraviolet
• Launched in 1990; maintained and upgraded by several space shuttle
service missions throughout the 1990s and early 2000’s
Infrared Astronomy from Orbit: NASA’s Spitzer Space Telescope
Infrared light with wavelengths much longer than visible light (“Far Infrared”) can only be
observed from space.
Why different wavelengths are required
Regardless of the technology, different wavelengths carries different information:• Shorter wavelengths carry information on
very energetic phenomena (e.g. black holes, star formation)
• Optical wavelengths carry information on the structures of galaxies and their motions (the assembly of the bodies of galaxies, their size)
• Longer wavelengths carry information on the chemical composition, physical state (gas and dust, presence, chemical elements; temperature)
Telescope Instruments
Cameras:• To obtain images at desired wavelength or
wavelengths (color images)• This yields the morphology, size of the sources
Spectrographs:• To study the intensity of the various
wavelengths (colors)• This yields the physical nature (star, galaxy,
balck hole), chemical composition, physical properties (temperature, density), dynamics (motions, mass), distance of the sources
Intensity(spatial distribution of the light) Spectra
(composition of the objectand the object’s velocity)
There are three basic aspects ofthe light from an object that we can study from the Earth.
Variability(change with time)
The SpectrographUsing a prism (or a grating), light can be split up into different wavelengths
(colors!) to produce a spectrum.
Spectral lines in a spectrum tell us about the chemical composition and other properties of the observed object .
Spectral Lines of Some Elements
Argon
Helium
Mercury
Sodium
Neon
Spectral lines are like a cosmic barcode system for elements.
Traditional Telescopes
Traditional primary mirror: sturdy, heavy to avoid distortions
Secondary mirror
Traditional Telescopes
The 4-m Mayall
Telescope at Kitt Peak National
Observatory (Arizona)
Advances in Modern Telescope Design (1)
1. Lighter mirrors with lighter support structures, to be controlled dynamically by computers
Floppy mirror
Segmented mirror
Modern computer technology has made significant advances in telescope design possible:
Adaptive OpticsComputer-controlled mirror support adjusts the mirror surface (many times per second) to compensate for
distortions by atmospheric turbulence
A laser beam produces an artificial star, which is used for the
computer-based seeing correction.
Advances in Modern Telescope Design (2)
2. Simpler, stronger mountings (“Alt-azimuth mountings”) to be controlled by computers
Examples of Modern Telescope Design
Examples of Modern Telescope Design
CCD Imaging(no photographic films any longer
CCD = Charge-coupled device
• Much more sensitive than photographic plates (90% vs. 1%)
• Data can be read directly into computer memory, allowing easy electronic manipulations and analysis
Visible light (top) and infrared (bottom) image of the
Sombrero Galaxy
Radio AstronomyRecall: Radio waves of l ~ 1 cm – 1 m also penetrate the Earth’s atmosphere and can be
observed from the ground.
Radio Telescopes
Large dish focuses the energy of radio waves onto a small receiver (antenna)
Amplified signals are stored in computers and converted into images, spectra, etc.
Radio InterferometryJust as for optical telescopes, the resolving power of a radio telescope is amin = 1.22 l/D.
For radio telescopes, this is a big problem: Radio waves are much longer than visible light.
Use interferometry to improve resolution!
Radio Interferometry (2)The Very Large Array (VLA): 27 dishes are combined
to simulate a large dish of 36 km in diameter.
Even larger arrays consist of dishes spread out over the entire U.S. (VLBA = Very Long Baseline Array) or even the whole Earth (VLBI = Very Long Baseline Interferometry)!
The Largest Radio Telescopes
The 100-m Green Bank Telescope in Green Bank, WVa.
The 300-m telescope in Arecibo, Puerto Rico.
Science of Radio AstronomyRadio astronomy reveals several features, not visible at other wavelengths:
• Neutral hydrogen clouds (which don’t emit any visible light), containing ~ 90 % of all the atoms in the Universe
• Molecules (often located in dense clouds, where visible light is completely absorbed)
• Radio waves penetrate gas and dust clouds, so we can observe regions from which visible light is heavily absorbed.
Life at the telescope. I
The trusty Night Assistant, who does all the work
The telescope, before sunsetThe MMT 6.5-m telescope, Univ. of Arizona
Life at the telescope. II
The diligent Student,who makes sure the work is done right
The hard-working Professor, who bosses everybody around
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