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Chapter 3 Telescopes

Telescopes

Images can be formed through reflection or refraction.Reflecting mirror

Optical Telescopes

Collect a large amount of light

Refracting lens

Optical Telescopes

Reflecting and refracting telescopes

Optical Telescopes

Modern telescopes are all reflectors:• Light traveling through lens is refracted

differently depending on wavelength.• Some light traveling through lens is absorbed.• Large lens can be very heavy, and can only be

supported at edge.• Lens needs two optically acceptable surfaces,

mirror only needs one.

Optical Telescopes

Question 1

Modern telescopes use mirrors rather than lenses for all of these reasons EXCEPT

a) light passing through lenses can be absorbed or scattered.

b) large lenses can be very heavy.c) large lenses are more difficult to make.d) mirrors can be computer controlled to

improve resolution.e) reflecting telescopes aren’t affected by

the atmosphere as much.

Question 1

Modern telescopes use mirrors rather than lenses for all of these reasons EXCEPT

a) light passing through lenses can be absorbed or scattered.

b) large lenses can be very heavy.c) large lenses are more difficult to make.d) mirrors can be computer controlled to

improve resolution.e) reflecting telescopes aren’t affected by

the atmosphere as much.

Reflecting instruments like the KECK telescopes can be made larger, and more capable, than refractors.

Image acquisition: Charge-coupled devices (CCDs) are electronic devices, can be quickly read out and reset.

Optical Telescopes

CCD Camera Sensor

a) they don’t require chemical development.b) digital data is easily stored & transmitted.c) CCDs are more light sensitive than film.d) CCD images can be developed faster.e) All of the above are true.

An advantage of CCDs over photographic film is

Question 2

a) they don’t require chemical development.b) digital data is easily stored & transmitted.c) CCDs are more light sensitive than film.d) CCD images can be developed faster.e) All of the above are true.

An advantage of CCDs over photographic film is

Question 2

Image processing by computers can sharpen images.

Optical Telescopes

Ground Based Telescope

Early HST

Computer Enhanced HST

Later HST

The Hubble Space Telescope has several instruments.

The Hubble Space Telescope

Resolution achievable by the Hubble Space Telescope limited by optics and not atmosphere

The Hubble Space Telescope

Ground Based – M100

Light-gathering power:Improves detailBrightness proportional to square of radius of mirrorThe figure, part (b) was taken with a telescope twice the size of (a)

Telescope Size

Area = p R2

Multiple telescopes: Mauna Kea

Telescope Size

Telescope SizeThe VLT (Very Large Telescope), Atacama, Chile

a) bend around corners and edges.b) separate into its component colors.c) bend through a lens.d) disperse within a prism.e) reflect off a mirror.

Diffraction is the tendency of light to

Question 3

a) bend around corners and edges.b) separate into its component colors.c) bend through a lens.d) disperse within a prism.e) reflect off a mirror.

Diffraction is the tendency of light to

Question 3

Diffraction affects all telescopes and limits the sharpness of all images.

Resolving power: Ability to distinguish objects that are close together.Resolution is proportional to wavelength and inversely proportional to telescope size.

Telescope Size

Question 4

Resolution is improved by using

a) larger telescopes & longer wavelengths. b) infrared light.c) larger telescopes & shorter wavelengths. d) lower frequency light.e) visible light.

Question 4

Resolution is improved by using

a) larger telescopes & longer wavelengths. b) infrared light.c) larger telescopes & shorter wavelengths. d) lower frequency light.e) visible light.

Diffraction limits resolution; larger telescopes and shorter-wave light produces sharper images.

10’ 1”

Effect of improving resolution:(a) 10′; (b) 1′; (c) 5″; (d) 1″

Telescope Size

a) the quality of the telescope’s optics.b) the transparency of a telescope’s lens.c) the sharpness of vision of your eyes.d) the image quality due to air stability.e) the sky’s clarity & absence of clouds.

Seeing in astronomy is a measurement of

Question 5

Seeing in astronomy is a measurement of

Question 5

“Good Seeing” occurs when the atmosphere is clear and the air is still.

Turbulent air produces “poor seeing,”

and fuzzier images.

a) the quality of the telescope’s optics.b) the transparency of a telescope’s lens.c) the sharpness of vision of your eyes.d) the image quality due to air stability.e) the sky’s clarity & absence of clouds.

Point images of a star

Smeared overall image of star

Question 6

Adaptive optics refers to

a) making telescopes larger or smaller. b) reducing atmospheric blurring using

computer control.c) collecting different kinds of light

with one type of telescope. d) using multiple linked telescopes.

Question 6

Adaptive optics refers to

a) making telescopes larger or smaller. b) reducing atmospheric blurring using

computer control.c) collecting different kinds of light

with one type of telescope. d) using multiple linked telescopes.

Shaping a mirror in “real time” can dramatically improve resolution.Cluster R136

Atmospheric blurring due to air movements

High-Resolution Astronomy

Solutions:• Put telescopes on mountaintops, especially in

deserts.• Put telescopes in space.• Active (adaptive) optics – control mirrors based

on temperature and orientation.

Solutions:• Without active (adaptive) optics a 1m

telescope is limited to about 2’• With active (adaptive) optics the same 1m

telescope could approach 0.1”.

Radio telescopes:• Similar to optical reflecting telescopes• Prime focus• Less sensitive to imperfections (due to longer

wavelength); can be made very large

Radio Astronomy

105 m Green Bank, WV

Radio dishes are large because

Question 7

a) radio photons don’t carry much energy. b) they are painted white.c) they are cheap to make.d) they are can operate during the day.

Radio dishes are large in order to

Question 7

Resolution is worse with long-wave light, so radio telescopes must be large

to compensate.

a) radio photons don’t carry much energy. b) they are painted white.c) they are cheap to make.d) they are can operate during the day.

Largest radio telescope: 300-m dish at Arecibo

Radio Astronomy

Question 8

Radio telescopes are useful because

a) observations can be made day & night.b) we can see objects that don’t emit

visible light.c) radio waves are not blocked by

interstellar dust.d) they can be linked to form

interferometers.e) All of the above are true.

Question 8

Radio telescopes are useful because

a) observations can be made day & night.b) we can see objects that don’t emit

visible light.c) radio waves are not blocked by

interstellar dust.d) they can be linked to form

interferometers.e) All of the above are true.

The Very Large Array links separate radio telescopes to create

much better resolution.

Longer wavelength means poorer angular resolution.Advantages of radio astronomy:• Can observe 24 hours a day.• Clouds, rain,

and snow don’t interfere.

• Observations at an entirely different frequency; get totally different information.

Radio Astronomy

Centaurus A

Interferometry:• Combines information

from several widely spread radio telescopes as if it came from a single dish.• Resolution will be that of dish whose diameter = largest separation between dishes.

Radio Astronomy

Interferometry requires preserving the phase relationship between waves over the distance between individual telescopes.

Can get radio images whose resolution is close to optical.

Radio Astronomy

VLA 4 m Kitt Peak

Radio AstronomyInterferometry can also be done with visible light, but much harder due to shorter wavelengths.

CHARA 1 m at Mount Wilson, CA

Infrared radiation can image where visible radiation is blocked; generally can use optical telescope mirrors and lenses.

Other Astronomies

Orion Nebula

Question 9

Infrared telescopes are very useful for observing

a) pulsars & black holes.b) from locations on the ground.c) hot stars & intergalactic gas.d) neutron stars. e) cool stars & star-forming regions.

Question 9

Infrared telescopes are very useful for observing

a) pulsars & black holes.b) from locations on the ground.c) hot stars & intergalactic gas.d) neutron stars. e) cool stars & star-forming regions.

Infrared images of star-forming “nurseries” can reveal objects

still shrouded in cocoons of gas and dust.

Infrared telescopes can also be in space or flown on balloons.

Other Astronomies

Ultraviolet images(a)The Cygnus loop supernova remnant(b) M81

Other Astronomies

X rays and gamma rays will not reflect off mirrors as other wavelengths do; need new techniques.X rays will reflect at a very shallow angle, and can therefore be focused.

Other Astronomies

X-ray image of supernova remnant Cassiopeia A

Other Astronomies

Gamma rays cannot be focused at all; images are therefore coarse.

Other Astronomies

Galaxy 3C279

Much can be learned from observing the same astronomical object at many wavelengths. Here, the Milky Way.

Other Astronomies

INFRARED

VISIBLE

GAMMA-RAY

Question 10

The Hubble Space Telescope (HST) offers sharper images than ground telescopes primarily because

a) HST is closer to planets & stars.b) HST uses a larger primary mirror.c) it gathers X-ray light.d) HST orbits above the atmosphere.e) it stays on the nighttime side of

Earth.

Question 10

The Hubble Space Telescope (HST) offers sharper images than ground telescopes primarily because

a) HST is closer to planets & stars.b) HST uses a larger primary mirror.c) it gathers X-ray light.d) HST orbits above the atmosphere.e) it stays on the nighttime side of

Earth.

HST orbits less than 400 miles above Earth – not much closer

to stars & planets!

But it can gather UV, visible, and IR light, unaffected by

Earth’s atmosphere.

Example Exam Questions

One advantage of the Hubble Space telescope over ground based ones is that

A. it can better focus x-ray images. B. in orbit, it can operate close to its diffraction

limit at visible wavelengths. C. it is larger than any Earth-based scopes. D. its adaptive optics controls atmospheric blurring better. E. it can make better observations of the ozone

layer.

What problem does adaptive optics correct?

A. defects in the optics of the telescope, such as the original Hubble mirror

B. turbulence in the earth's atmosphere which creates twinkling C. the opacity of the earth's atmosphere to some

wavelengths of light D. chromatic aberration due to use of only a single

lens objective E. the light pollution of urban areas

An emission line results from an electron falling from a higher to lower energy orbital around its atomic nucleus.

A. True B. False

The angular resolution of an 8 inch diameter telescope is _______ greater than that of a 2 inch diameter telescope.

A. 2× B. 4× C. 8× D. 9× E. 16×

The larger the red shift, the faster the distant galaxy is rushing toward us.

A. True B. False

Changing the electric field will have no effect on the magnetic fields of a body.

A. True B. False

Doubling the temperature of a black body will double the total energy it radiates. A. True B. False

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