astronomy chp 6

8/13/2019 astronomy chp 6

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Light and Telescopes

Chapter 6


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Previous chapters have described the sky as it appears

to our unaided eyes, but modern astronomers turnpowerful telescopes on the sky. Chapter 6 introduces usto the modern astronomical telescope and its delicateinstruments.

The study of the universe is so challenging, astronomerscannot ignore any source of information; that is why theyuse the entire spectrum, from gamma rays to radiowaves. This chapter shows how critical it is forastronomers to understand the nature of light.

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

Guidepost


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I. Radiation: Information from Space

A. Light as a Wave and a ParticleB. The Electromagnetic Spectrum

II. Optical Telescopes

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

III. Special InstrumentsA. Imaging SystemsB. The Spectrograph

Outline


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IV. Radio Telescopes

A. 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)


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Light and Other Forms ofRadiation

The Electromagnetic Spectrum

In astronomy, we cannot perform experimentswith our objects (stars, galaxies, ).

The only way to investigate them, is by

analyzing the light (and other radiation) whichwe observe from them.


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Light as a Wave (1)

Light waves are characterized by awavelength l and a frequency f.

f = c/l

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

f and lare related through

l


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Light as a Wave (2)

Wavelengths of light are measured in unitsof nanometers (nm) or ngstrm ():

1 nm = 10-9m

1 = 10-10m = 0.1 nm

Visible lighthas wavelengths between4000 and 7000 (= 400700 nm).


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Wavelengths and Colors

Differentcolors of visible light

correspond to different wavelengths.


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Light as Particles

Light can also appear as particles, calledphotons (explains, e.g., photoelectric effect).

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

E = h*f

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

The energy of a photon does notdepend on the intensityof the light!!!


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The Electromagnetic Spectrum

Need satellitesto observe

Wavelength

Frequency

Highflying airplanes orsatellites


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Optical TelescopesAstronomers use

telescopes to gathermore light from

astronomical objects.

The larger thetelescope, the more

light it gathers.


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Refracting/Reflecting Telescopes

RefractingTelescope:Lensfocuseslight onto thefocal plane

ReflectingTelescope:

Concave Mirrorfocuses lightonto the focal

plane

Almost all modern telescopes are reflecting telescopes.

Focal length

Focal length


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Secondary OpticsIn reflectingtelescopes:Secondary

mirror, to re-direct light pathtowards back or

side of

incoming lightpath.

Eyepiece:Toview and

enlarge thesmall imageproduced in

the focalplane of the

primary

optics.


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Refractors and Reflectors

(SLIDESHOW MODE ONLY)

Di d f R f i
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Disadvantages of RefractingTelescopes

Chromatic aberration:Differentwavelengths are focused at differentfocal lengths (prism effect).

Can becorrected, butnot eliminatedby second lensout of differentmaterial.

Difficult and expensive to

produce: All surfaces must be

perfectly shaped; glass must

be flawless; lens can only be

supported at the edges


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The Powers of a Telescope:Size Does Matter

1. Light-gatheringpower:Dependson the surfacearea A of the

primary lens /mirror,proportional todiameter

squared:

A = p(D/2)2

D


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The Powers of a Telescope (2)

2. Resolving power: Wave nature oflight => The telescope apertureproduces fringe rings that set alimit to the resolution of thetelescope.

amin= 1.22 (l/D)

Resolving power = minimumangular distance aminbetweentwo objects that can beseparated.

For optical wavelengths, this gives

amin= 11.6 arcsec / D[cm]

amin


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Resolution and Telescopes

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SeeingWeatherconditionsandturbulence intheatmosphere

set furtherlimits to thequality ofastronomical

images.

Bad seeing Good seeing


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The Powers of a Telescope (3)

3. Magnifying Power= ability of thetelescope to make the image appearbigger.

The magnification depends on the ratio of focal

lengths of the primary mirror/lens (Fo) and theeyepiece (Fe):

M = Fo/Fe

A larger magnification does not improve theresolving power of the telescope!

Th B t L ti f


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The Best Location for aTelescope

Far away from civilizationto avoid light pollution

Th B t L ti f


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The Best Location for aTelescope (2)

On high mountain-topsto avoid atmosphericturbulence (seeing) and other weather effects

Paranal Observatory (ESO), Chile


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Traditional Telescopes (1)

Traditional primary mirror: sturdy,

heavy to avoid distortions.

Secondary mirror


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Traditional Telescopes (2)

The 4-mMayall

Telescope atKitt PeakNational

Observatory

(Arizona)


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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 telescopedesign:


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Adaptive OpticsComputer-controlled mirror support adjusts the mirror

surface (many times per second) to compensate fordistortions by atmospheric turbulence

E l f M d T l


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Examples of Modern TelescopeDesign (1)

Design of the LargeBinocular Telescope

(LBT)

The Keck I telescope mirror

E l f M d T l


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Examples of Modern TelescopeDesign (2)

8.1-m mirror of the Gemini Telescopes

The Very Large Telescope (VLT)


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CCD ImagingCCD= Charge-coupled device

More sensitive thanphotographic plates

Data can be readdirectly into computermemory, allowing easyelectronic manipulations

Negative image toenhance contrasts

False-color image to visualizebrightness contours


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The SpectrographUsing aprism(or a grating), light canbe split up into different wavelengths

(colors!) to produce a spectrum.

Spectral linesin a spectrum

tell us about the chemicalcomposition and otherproperties of the observedobject


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Radio AstronomyRecall: Radio wavesof l~ 1 cm1 m alsopenetrate the Earths atmosphere and can be

observed from the ground.


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Radio Telescopes

Large dish focusesthe energy of radiowaves onto a smallreceiver (antenna)

Amplified signals arestored in computersand converted into

images, spectra, etc.

f


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Radio InterferometryJust as for optical telescopes, the resolving power ofa radio telescope is a

min= 1.22 l/D.

For radio telescopes, this is a big problem: Radiowaves are much longer than visible light

Use interferometry to improve resolution!

R di I f (2)


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Radio Interferometry (2)The VeryLarge Array(VLA): 27dishes arecombined tosimulate a

large dish of36 km indiameter.

Even larger arrays consist of dishes spread out over theentire U.S. (VLBA = Very Long Baseline Array) or even the

whole Earth (VLBI = Very Long Baseline Interferometry)!

Th L R di T l


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The Largest Radio Telescopes

The 100-m Green Bank Telescope in

Green Bank, WVa.

The 300-m telescope in

Arecibo, Puerto Rico

S i f R di A t


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Science of Radio Astronomy

Radio astronomy reveals several features,

not visible at other wavelengths:

Neutral hydrogen clouds (which dont emit anyvisible 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, sowe can observe regions from which visible lightis heavily absorbed.

I f d A t


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Infrared Astronomy

However,from highmountaintops or high-

flying airplanes,someinfraredradiationcan still beobserved.

NASA infrared telescope on Mauna Kea, Hawaii

Most infrared radiation is absorbed in the loweratmosphere.

S A t


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Space Astronomy

NASAs Space Infrared Telescope


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NASA s Space Infrared Telescope

Facility (SIRTF)

Ult i l t A t


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Ultraviolet Astronomy

Ultraviolet radiation with l< 290 nm is

completely absorbed in the ozone layer ofthe atmosphere.

Ultraviolet astronomy has to be done from

satellites.

Several successful ultraviolet astronomysatellites: IRAS, IUE, EUVE, FUSE

Ultraviolet radiation traces hot (tens ofthousands of degrees), moderately ionizedgas in the Universe.

X R A t


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

X-ray astronomy has to be done from satellites.

NASAs

Chandra X-ray

Observatory

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

G R A t


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

traces the most violent processes in the Universe

The Compton

Gamma-Ray

Observatory

Th H bbl S T l


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The Hubble Space Telescope

Avoidsturbulence inthe Earths

atmosphere

Extendsimaging andspectroscopyto (invisible)infrared andultraviolet

Launched in 1990;maintained andupgraded by severalspace shuttle servicemissions throughout

the 1990s and early2000s

N T


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electromagnetic radiationwavelengthfrequencyNanometer (nm)

Angstrom ()photoninfrared radiation

ultraviolet radiationatmospheric windowfocal lengthrefracting telescopereflecting telescope

primary lens, mirrorobjective lens, mirroreyepiecechromatic aberrationachromatic lens

light-gathering powerresolving powerdiffraction fringeseeingmagnifying powerlight pollutionprime focus

secondary mirrorCassegrain focusNewtonian focusSchmidt-Cassegrain focussidereal drive

equatorial mountingpolar axisalt-azimuth mountingactive opticsadaptive optics

New Terms


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Disc ssion Q estions


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1. Why does the wavelength response of the humaneye match so well the visual window of Earths

atmosphere?

2. Most people like beautiful sunsets with brightly

glowing clouds, bright moonlit nights, and twinklingstars. Most astronomers dont. Why?

Discussion Questions


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Quiz Questions

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

a. Redb. Orangec. Greend. Blue

e. Violet


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2. The entire electromagnetic spectrum can be divided into theseven 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 mosttransparent?

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

e. Visible & Radio

Quiz Questions


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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 thestars.

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


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4. Astronomers are both hindered and assisted by chromaticaberration. In which device is chromatic aberration a bigproblem 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


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5. Why have no large refracting telescopes been built in theyears 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 andpolish.d. Large glass lenses are more difficult to support than largemirrors.e. All of the above.

Quiz Questions


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6. What do large-diameter gently curved convex (thicker in themiddle) 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


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


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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


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



Quiz Questions


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10. What advantage do the builders of large telescopes todayhave 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 ofEarth's atmosphere.d. Both b and c above.e. All of the above.

Quiz Questions


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11. In which device do astronomers take advantage ofchromatic 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


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



Quiz Questions


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13. The primary mirror of telescope A has a diameter of 20 cm, andthe one in telescope B has a diameter of 100 cm. How do the lightgathering 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 theeyepiece also.

Quiz Questions

Q Q


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14. What do the newer light-sensitive electronic CCD chips dobetter 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 singleexposure.c. Photometry can be done with the CCD images.d. The CCD images are easier to manipulate.e. All of the above.

Quiz Questions

Q i Q i


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15. What can radio telescopes do that optical telescopescannot?

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

Q i Q ti


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16. What is a disadvantage of radio telescopes compared tooptical telescopes?

a. Radio photons have lower energy, thus radio waves havelow 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

Q i Q ti


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17. Radio telescopes are often connected together to dointerferometry. What is the primary problem overcome by radiointerferometry?

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

Q i Q ti


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

a. The primary infrared blocker, water vapor, is mostly in thelower atmosphere.

b. The primary ultraviolet blocker, ozone, is located high in theatmosphere, far above mountaintops.c. Ultraviolet telescopes require the low temperature of space tooperate.d. Both a and b above.e. Both a and c above.

Quiz Questions

Q i Q ti


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

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 theUnited States.

Quiz Questions


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A


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Answers

1. e2. e3. d4. b5. e

6. e7. b8. c9. d10. e

11. c12. b13. d14. e15. d

16. e17. b18. d19. a20. b

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