Chapter 06

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Light and TelescopesChapter 6

Previous chapters have described the sky as it appears to our unaided eyes, but modern astronomers turn powerful telescopes on the sky. Chapter 6 introduces us to the modern astronomical telescope and its delicate instruments.

The study of the universe is so challenging, astronomers cannot ignore any source of information; that is why they use the entire spectrum, from gamma rays to radio waves. This chapter shows how critical it is for astronomers to understand the nature of light.

In each of the chapters that follow, we will study the universe using information gathered by the telescopes and instruments described in this chapter.

Guidepost

I. Radiation: Information from SpaceA. Light as a Wave and a ParticleB. The Electromagnetic Spectrum

II. Optical TelescopesA. Two Kinds of TelescopesB. The Powers of a TelescopeC. Buying a TelescopeD. New-Generation TelescopesE. Interferometry

III. Special InstrumentsA. Imaging SystemsB. The Spectrograph

Outline

IV. Radio TelescopesA. Operation of a Radio TelescopeB. Limitations of the Radio TelescopeC. Advantages of Radio Telescopes

V. Space AstronomyA. Infrared AstronomyB. Ultraviolet AstronomyC. X-Ray AstronomyD. Gamma-Ray TelescopesE. Cosmic RaysF. The Hubble Space Telescope

Outline (continued)

Light and Other Forms of Radiation

• The Electromagnetic Spectrum

In astronomy, we cannot perform experiments with our objects (stars, galaxies, …).

The only way to investigate them, is by analyzing the light (and other radiation) which we observe from them.

Light as a Wave (1)

• Light waves are characterized by a wavelength and a frequency f.

f = c/

c = 300,000 km/s = 3*108 m/s

• f and are related through

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 can also appear as particles, called photons (explains, e.g., photoelectric effect).

• A photon has a specific energy E, proportional to the frequency f:

E = h*f

h = 6.626x10-34 J*s is the Planck constant.

The energy of a photon does not depend on the intensity of the light!!!

The Electromagnetic Spectrum

Need satellites to observe

Wavelength

Frequency

High flying air planes or satellites

Optical TelescopesAstronomers use

telescopes to gather more light from

astronomical objects.

The larger the telescope, the more

light it gathers.

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 light path towards back or

side of incoming light

path.

Eyepiece: To view and

enlarge the small image produced in

the focal plane of the

primary optics.

Refractors and Reflectors

(SLIDESHOW MODE ONLY)

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

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 = (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.

min = 1.22 (/D)

Resolving power = minimum angular distance min between two objects that can be separated.

For optical wavelengths, this gives

min = 11.6 arcsec / D[cm]

min

Resolution and Telescopes

(SLIDESHOW MODE ONLY)

SeeingWeather conditions and turbulence in the atmosphere set further limits to the quality of astronomical images.

Bad seeing Good seeing

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!

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

Traditional Telescopes (1)

Traditional primary mirror: sturdy, heavy to avoid distortions.

Secondary mirror

Traditional Telescopes (2)

The 4-m Mayall

Telescope at Kitt Peak

National Observatory

(Arizona)

Advances in Modern Telescope Design

2. Simpler, stronger mountings (“Alt-azimuth mountings”) to be controlled by computers

1. Lighter mirrors with lighter support structures, to be controlled dynamically by computers

Floppy mirror Segmented mirror

Modern computer technology has made possible significant advances in telescope design:

Adaptive OpticsComputer-controlled mirror support adjusts the mirror surface (many times per second) to compensate for distortions by atmospheric turbulence

Examples of Modern Telescope Design (1)

Design of the Large Binocular

Telescope (LBT)

The Keck I telescope mirror

Examples of Modern Telescope Design (2)

8.1-m mirror of the Gemini Telescopes

The Very Large Telescope (VLT)

InterferometryRecall: Resolving power of a telescope depends on diameter D:

min = 1.22 /D.

This holds true even if not the entire surface is filled out.

• Combine the signals from several smaller telescopes to simulate one big mirror Interferometry

CCD ImagingCCD = Charge-coupled device

• More sensitive than photographic plates• Data can be read directly into computer memory, allowing easy electronic manipulations

Negative image to enhance contrasts

False-color image to visualize brightness contours

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

Radio AstronomyRecall: Radio waves of ~ 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 min = 1.22 /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 Astronomy

Radio 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.

Infrared Astronomy

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.

Space Astronomy

NASA’s Space Infrared Telescope Facility (SIRTF)

Ultraviolet Astronomy• Ultraviolet radiation with < 290 nm is

completely absorbed in the ozone layer of the atmosphere.

• Ultraviolet astronomy has to be done from satellites.

• Several successful ultraviolet astronomy satellites: IRAS, IUE, EUVE, FUSE

• Ultraviolet radiation traces hot (tens of thousands of degrees), moderately ionized gas in the Universe.

X-Ray Astronomy• X-rays are completely absorbed in the atmosphere.

• X-ray astronomy has to be done from satellites.

NASA’s Chandra X-ray Observatory

X-rays trace hot (million degrees), highly ionized gas in the Universe.

Gamma-Ray AstronomyGamma-rays: most energetic electromagnetic radiation;

traces the most violent processes in the Universe

The Compton Gamma-Ray Observatory

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

electromagnetic radiationwavelengthfrequencyNanometer (nm)Angstrom (Å)photoninfrared radiationultraviolet radiationatmospheric windowfocal lengthrefracting telescopereflecting telescopeprimary lens, mirrorobjective lens, mirroreyepiecechromatic aberrationachromatic lens

light-gathering powerresolving powerdiffraction fringeseeingmagnifying powerlight pollutionprime focussecondary mirrorCassegrain focusNewtonian focusSchmidt-Cassegrain focussidereal driveequatorial mountingpolar axisalt-azimuth mountingactive opticsadaptive optics

New Terms

interferometrycharge-coupled device (CCD)false-color imagespectrographgratingcomparison spectrumradio interferometercosmic ray

New Terms (continued)

1. Why does the wavelength response of the human eye match so well the visual window of Earth’s atmosphere?

2. Most people like beautiful sunsets with brightly glowing clouds, bright moonlit nights, and twinkling stars. Most astronomers don’t. Why?

Discussion Questions

Quiz Questions

1. The visible part of the electromagnetic spectrum can be divided into seven color bands of Red, Orange, Yellow, Green, Blue, Indigo, and Violet (from long to short wavelength). A single photon of which of these colors has the greatest amount of energy?

a. Redb. Orangec. Greend. Bluee. Violet

2. The entire electromagnetic spectrum can be divided into the seven bands of Radio, Microwave, Infrared, Visible, Ultraviolet, X-ray, and Gamma-ray (from longest to shortest wavelength). To which of these two bands is Earth's atmosphere the most transparent?

a. X-ray & Gamma-rayb. Ultraviolet & Infraredc. Visible & Ultravioletd. Microwave & Radioe. Visible & Radio

Quiz Questions

3. Why do the pupils of a cat's eyes open wider at night?

a. To reduce the buildup of cat eye wax.b. Cats are the only animals besides humans to observe the stars.c. The cat sleeps all day and is wide awake at night.d. To increase light gathering power.e. To attract a mate.

Quiz Questions

4. Astronomers are both hindered and assisted by chromatic aberration. In which device is chromatic aberration a big problem for astronomers?

a. The primary mirrors of reflecting telescopes.b. The primary lenses of refracting telescopes.c. The prism.d. Both a and b above.e. All of the above.

Quiz Questions

5. Why have no large refracting telescopes been built in the years since 1900?

a. Refracting telescopes suffer from chromatic aberration.b. Making large glass lenses without interior defects is difficult.c. Refracting telescopes have several surfaces to shape and polish. d. Large glass lenses are more difficult to support than large mirrors.e. All of the above.

Quiz Questions

6. What do large-diameter gently curved convex (thicker in the middle) lenses and large-diameter gently curved concave (thinner in the middle) mirrors have in common?

a. They both have short focal lengths.b. They both have long focal lengths.c. They can be used as primary light collectors for a telescope.d. Both a and c above.e. Both b and c above.

Quiz Questions

7. Which power of a telescope might be expressed as "0.5 seconds of arc"?

a. Light gathering power.b. Resolving power.c. Magnifying power.d. Both a and b above.e. Both a and c above.

Quiz Questions

8. Which power of a telescope is the least important?

a. Light gathering power.b. Resolving power.c. Magnifying power.d. Both a and b above.e. Both a and c above.

Quiz Questions

9. Which power of an optical telescope is determined by the diameter of the primary mirror or lens?

a. Light gathering power.b. Resolving power.c. Magnifying power.d. Both a and b above.e. Both a and c above.

Quiz Questions

10. What advantage do the builders of large telescopes today have over the previous generation of telescope builders?

a. Large mirrors can now be made thinner and lighter than before.b. Tracking celestial objects today is computer controlled and can take advantage of simpler, stronger mounts.c. High-speed computing today can be used to reduce the effect of Earth's atmosphere.d. Both b and c above.e. All of the above.

Quiz Questions

11. In which device do astronomers take advantage of chromatic aberration?

a. The primary mirrors of reflecting telescopes.b. The primary lenses of refracting telescopes.c. The prism.d. Both a and b above.e. All of the above.

Quiz Questions

12. Which power of a large ground-based optical telescope is severely limited by Earth's atmosphere on a cloudless night?

a. Light gathering power.b. Resolving power.c. Magnifying power.d. Both a and b above.e. Both a and c above.

Quiz Questions

13. The primary mirror of telescope A has a diameter of 20 cm, and the one in telescope B has a diameter of 100 cm. How do the light gathering powers of these two telescopes compare?

a. Telescope A has 5 times the light gathering power of telescope B.b. Telescope B has 5 times the light gathering power of telescope A.c. Telescope A has 25 times the light gathering power of telescope B.d. Telescope B has 25 times the light gathering power of telescope A.e. The light gathering power depends on the focal length of the eyepiece also.

Quiz Questions

14. What do the newer light-sensitive electronic CCD chips do better than the older photographic plates coated with light-sensitive chemicals?

a. They have a greater sensitivity to light.b. They can detect both bright and dim objects in a single exposure.c. Photometry can be done with the CCD images.d. The CCD images are easier to manipulate.e. All of the above.

Quiz Questions

15. What can radio telescopes do that optical telescopes cannot?

a. Find the location of cool hydrogen gas.b. See through dust clouds.c. Detect high temperature objects.d. Both a and b above.e. All of the above.

Quiz Questions

16. What is a disadvantage of radio telescopes compared to optical telescopes?

a. Radio photons have lower energy, thus radio waves have low intensity.b. Interference from nearby sources of radio waves.c. Poor resolving power.d. Both a and b above.e. All of the above.

Quiz Questions

17. Radio telescopes are often connected together to do interferometry. What is the primary problem overcome by radio interferometry?

a. Poor light gathering power.b. Poor resolving power.c. Poor magnifying power.d. Interference from nearby sources of radio waves.e. The low energy of radio photons.

Quiz Questions

18. Why are near-infrared telescopes located on mountaintops and ultraviolet telescopes in Earth orbit?

a. The primary infrared blocker, water vapor, is mostly in the lower atmosphere.b. The primary ultraviolet blocker, ozone, is located high in the atmosphere, far above mountaintops.c. Ultraviolet telescopes require the low temperature of space to operate.d. Both a and b above.e. Both a and c above.

Quiz Questions

19. Why must far-infrared telescopes be cooled to a low temperature?

a. To reduce interfering heat radiation emitted by the telescope.b. To protect the sensitive electronic amplifiers from overheating by sunlight.c. To improve their poor resolving power.d. To improve their poor magnifying power.e. To make use of the vast supplies of helium stockpiled by the United States.

Quiz Questions

20. Why are the sources of cosmic rays difficult to locate?

a. Cosmic rays are high-energy photons that penetrate the surfaces of telescope mirrors rather than reflecting to a focal point.b. Cosmic rays are charged particles, thus their paths are curved by magnetic fields, which masks the location of their source.c. Cosmic rays are neutral particles that weakly interact with matter and are difficult to detect.d. Cosmic rays are positively and negatively charged particles, which masks the location of their source.e. Cosmic rays are theoretical and have never been detected.

Quiz Questions

Answers

1. e2. e3. d4. b5. e6. e7. b8. c9. d10. e

11. c12. b13. d14. e15. d16. e17. b18. d19. a20. b