The Milky Way - Moore Public Schools · • Wavelengths of light are measured in units of...

© Cengage Learning 2016

Foundations of Astronomy | 13e

Seeds


Chapter 6

Light and Telescopes


Guidepost

• In this chapter, you will consider the

techniques astronomers use to study the

Universe

– What is light?

– How do telescopes work?

– What are the powers and limitations of

telescopes?


Guideposts (cont’d.)

– What kind of instruments do astronomers use

to record and analyze light gathered by

telescopes?

– Why are some telescopes located in space?


6-1 Radiation: Information from Space

• In astronomy, we cannot perform experiments

with our objects

– Stars, galaxies, etc.

• The only way to investigate them is by

analyzing the light (and other radiation) which

we observe from them


Common Misconception

• Misconception: we must be wary of the

word radiation

– Truth: Radiation is anything that radiates

away from a source and not all radiation

involves dangerous high-energy particles


Light as Waves

• Light waves are characterized by:

– Wavelength

– Frequency


Light as Waves (cont’d.)

• Wavelengths of light are measured in units

of nanometers (nm) or Ångström (Å)

• Visible light has wavelengths between

4000 Å and 7000 Å (= 400 – 700 nm)

1 nm = 10-9 m

1 Å = 10-10 m = 0.1 nm


Wavelengths and Colors

• Different colors of visible light correspond

to different wavelengths


Light as Particles

• Light can also appear as particles, called

photons (e.g., photoelectric effect)

• A photon has a specific energy E,

proportional to the frequency f

• The energy of a photon does not depend

on the intensity of the light


The Electromagnetic Spectrum



• Misconception: Radio waves are related to

sound

– Truth: Radio waves are a type of light that

your radio receiver transforms into sound


6-2 Telescopes

• Astronomers use telescopes to gather

more light from astronomical objects

– The larger the telescope, the more light it

gathers


• Refracting telescope:

lens focuses

light onto the focal

plane

• Reflecting telescope:

concave mirror

focuses light onto

the focal plane

Refracting and Reflecting Telescopes


Secondary Optics

• Secondary mirror: redirects the light path

towards the back or side of the incoming

light path

• Eyepiece: used to view and enlarge the

small image produced in the focal plane of

the primary optics.

Focal length


The Powers and Limitations of Telescopes

• Chromatic aberration: different

wavelengths are focused at different focal

lengths (prism effect)


The Powers and Limitations of Telescopes

(cont’d.)

• Light-gathering power: depends on the

surface area (A) of the primary lens or

mirror, proportional to diameter squared

A = p (D/2)2


Resolving Power

• Minimum angular distance amin between

two objects that can be separated:

• For optical wavelengths, this gives

amin = 1.22 (l/D)

amin = 11.6 arcsec / D[cm]


Seeing

• Weather conditions

and turbulence in

the atmosphere set

further limits to the

quality of

astronomical

images


Magnifying Power

• Ability of the telescope to make the image

appear bigger

• Depends on the ratio of focal lengths of

the primary mirror or lens (Fp) and the

eyepiece (Fe):

• A larger magnification does not improve

the resolving power of the telescope!

M = Fp/Fe



• Misconception: The purpose of an

astronomical telescope is to magnify

images

– Truth: Very high magnification does not

necessarily show more detail. Generally, the

amount of detail that a telescope can discern

is limited by its resolving power or the seeing

conditions


How Do We Know? 6-1

• Resolution and precision

– The precision of measurements is limited by

the resolution of the measurement technique

– The practical size of a pixel is set by the

resolution limit, and affected by:

• Atmospheric seeing

• Telescope optical quality and diffraction

– You can’t see details smaller than the pixel

size, so there is unavoidable uncertainty in all

scientific measurements


The Best Locations for a Telescope

• Far away from civilization – to avoid light

pollution


• On high mountain-tops – to avoid

atmospheric turbulence and other weather

effects

The Best Locations for a Telescope

(cont’d.)


Modern Optical Telescopes


Modern Optical Telescopes (cont’d.)

• The 4-m Mayall

Telescope at Kitt

Peak National

Observatory

(Arizona)


Advances in Modern Telescope Design

• Lighter mirrors

with lighter support

structures, to be

controlled

dynamically by

computers

– Floppy mirror

– Segmented mirror


Advances in Modern Telescope Design

(cont’d.)

• Simpler, stronger mountings (“Alt-azimuth

mountings”) to be controlled by computers


Examples of Modern Telescopic Design


Examples of Modern Telescopic Design (cont’d.)


Radio Astronomy

• Recall: radio waves of l ~ 1 cm – 1 m also

penetrate Earth’s atmosphere and can be

observed from the ground


Modern 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.


6-4 Airborne and Space Telescopes


Airborne Telescopes

• Infrared cameras need to be cooled to

very low temperatures, usually using liquid

nitrogen.


Space Telescopes

• The Hubble Space Telescope

• Launched in 1990

• Maintained and upgraded by several space

shuttle service missions throughout the 1990s

and early 2000s

• Avoids turbulence in Earth’s atmosphere

• Extends imaging and spectroscopy to infrared

and ultraviolet


The Hubble Space Telescope


Space Telescopes (cont’d.)

• HST successors

• James Webb Space Telescope

– Will be in solar orbit ~1 million miles from

Earth

• Herschel Space Observatory (2009)

– Carried a 3-m mirror and instruments cooled

almost to absolute zero


Space Telescopes (cont’d.)


High Energy Astronomy

• Telescopes observing gamma-rays, X-

rays, and ultraviolet sources must be

located high in Earth’s atmosphere or in

space

– General-purpose telescopes: e.g., Chandra

– Single-subject telescopes: e.g., Hindode


Chandra X-Ray Observatory


6-5 Astronomical Instruments and

Techniques

• Cameras and photometers

– Photographic plate (record image)

• Long exposure detect faint objects

• Brightness of objects not measured very precisely

– Photometers (measure intensity of the light)

• Sensitive light meter measures brightness of

objects very precisely

– Charge-coupled devices (CCDs)

• Records image and measures the brightness


CCD Imaging

• More sensitive than photographic plates

– Data can be read directly into computer

memory, allowing easy electronic

manipulations


Spectrographs

• Spectral lines in a spectrum tell us about

the chemical composition and other

properties of the observed object


Adaptive Optics

• Computer-controlled mirror support

adjusts the mirror surface (many times per

second) to compensate for distortions by

atmospheric turbulence


Interferometry

• Combine the

signals from

several smaller

telescopes to

simulate one

big mirror


Radio Maps and Interferometry

• Colors in a radio map can indicate

different intensities of the radio emission

from different locations on the sky

• Radio waves are much longer than visible

light – use interferometry to improve

resolution!


Radio Maps (cont’d.)


The Very Large Array (VLA)

• 27 dishes combined to simulate a large

dish of 36 km in diameter


6-6 Non-Electromagnetic Astronomy

• Radiation from space does not only come

in the form of electromagnetic radiation

– Particle astronomy

• Earth is constantly bombarded cosmic rays –

highly energetic subatomic particles traveling

through space at high velocities

– Gravity wave astronomy

• Gravity waves predicted to be produced by an

mass that accelerates, but would be extremely

weak and difficult to detect

• Inferred, but not yet detected


Discussion Questions

• Why would you not include sound waves

in the electromagnetic spectrum?

– Hint: See Figure 6-2. Do sound travel at the

speed of light? What about through a

vacuum?

• Why do optical astronomers often put their

telescopes at the tops of mountains,

whereas radio astronomers sometimes put

their telescopes in deep valleys?

– Hint: See Figure 6-3


Discussion Questions (cont’d.)

• Why does the wavelength response of the

human eye match the visual window of

Earth’s atmosphere so well? Why not the

radio window?

– Hint: The maximum energy of the Sun is ~500

nm (green)

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