Astronomy a beginners guide to the universe 8th CHaisson mcmillan chapter 03

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Chapter 3 Telescopes

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3.1 Optical Telescopes

• Refracting lens

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• Image formation

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• Reflecting and refracting telescopes

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• Modern telescopes are all reflectors:

– Light traveling through lens is refracted differently

depending on wavelength.

– Some light traveling through lens is absorbed.

– A large lens can be very heavy and can only be

supported at the edge.

– A lens needs two optically acceptable surfaces, but a mirror needs only one.

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• Types of reflecting telescopes

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• Details of the Keck telescope

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• Image acquisition: Charge-coupled devices

(CCDs) are electronic devices that can be quickly read out and reset

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Discovery 3.1: The Hubble Space Telescope

• The Hubble Space Telescope has several

instruments

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Discovery 3.1: The Hubble Space Telescope

• Resolution achievable by the Hubble Space

Telescope

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• The figure, part (b) was

taken with a telescope

twice the size of (a)

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3.2 Telescope Size

• Multiple telescopes: Mauna Kea

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• The VLT (Very Large Telescope), Atacama, Chile

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• Resolving power: When better, can distinguish

objects that are closer together

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3.3 High-Resolution Astronomy

• Atmospheric blurring due to air movements

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3.3 High-Resolution Astronomy

• Solutions:

– Put telescopes on mountaintops, especially in

deserts.

– Put telescopes in space.

– Use active optics—control mirrors by bending them slightly to correct for atmospheric distortion.

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3.4 Radio Astronomy

• Largest radio telescope: 300-m dish at Arecibo

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• 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.

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• Interferometry:

– Combines information from several widely separated radio

telescopes as if it came from a single dish.

– Resolution will be that

of a dish whose diameter = largest separation between dishes.

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

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• These telescopes can get radio images whose

resolution is close to optical

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• Interferometry can also be done with visible light, but

it is much harder due to shorter wavelengths

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3.5 Space-Based Astronomies

• Infrared radiation can image where visible radiation

is blocked by interstellar matter or atmospheric

particles

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• X-ray image of supernova remnant Cassiopeia A

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• Gamma rays are the most high-energy radiation we can detect This supernova remnant would be nearly invisible without the Fermi satellite and its gamma-ray detector

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• Much can be learned from

observing the same

astronomical object at

many wavelengths Here,

the Milky Way

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Summary of Chapter 3

• Refracting telescopes make images with a lens

• Reflecting telescopes make images with a mirror

• Modern research telescopes are all reflectors

• CCDs are used for data collection

• Data can be formed into images, analyzed

spectroscopically, or used to measure intensity

• Large telescopes gather much more light, allowing study of very faint sources

• Large telescopes also have better resolution

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Summary of Chapter 3, cont.

• Resolution of ground-based optical telescopes is limited by atmospheric effects

• Resolution of radio or space-based telescopes is limited by diffraction

• Active and adaptive optics can minimize

atmospheric effects

• Radio telescopes need large collection area;

diffraction is limited

• Interferometry can greatly improve resolution

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Summary of Chapter 3, cont

• Infrared and ultraviolet telescopes are similar to optical

• Ultraviolet telescopes must be above the

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