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