Universe Tenth Edition Chapter 6 Optics and Telescopes Roger Freedman Robert Geller William Kaufmann III

6-1 How a refracting telescope uses a lens to form an image 6-2 How a reflecting telescope uses a curved mirror to form an image 6-3 How a telescope ’ s size and the Earth ’ s atmosphere limit the sharpness of a telescopic image 6-4 How electronic light detectors have revolutionized astronomy 6-5 How telescopes are used to obtain spectra of astronomical objects 6-6 The advantages of using telescopes that detect radio waves from space 6-7 The advantages of placing telescopes in Earth orbit By reading this chapter, you will learn

Refraction

Refraction and Lenses

Light Rays from Distant Objects are Parallel

A Lens Creates and Extended Image of an Extended Object

A Refracting Telescope 6-1: A refracting telescope uses a lens to concentrate incoming light at a focus

Light Gathering Power

Chromatic Aberration

A Light Refracting Telescope

A Large Refracting Telescope

Reflection

6-2: A reflecting telescope uses a mirror to concentrate incoming light at a focus

A Newtonian Telescope

Designs for Reflecting Telescope

The Secondary Mirror Does Not Cause a Hole in the Image

Spherical Aberration

Angular Resolution 6-3: Telescope images are degraded by the blurring effects of the atmosphere and by light pollution

The Telescopes of Mauna Kea

Creating an Artificial “Star”

Using Adaptive Optics to “Unblur” Telescope Images

6-4: A charge-coupled device is commonly used to record the image at a telescope’s focus Charge-Coupled Devices (CCDs) and Imaging

6-5: Spectrographs record spectra of astronomical objects A Grating Spectrograph

6-6: A radio telescope uses a large concave dish to reflect radio waves to a focus A Radio Telescope

The Very Large Array (VLA)

Optical and Radio Views of Saturn

Optical View of Saturn

Radio View of Saturn

6-7: Telescopes in orbit around Earth detect radiation that does not penetrate the atmosphere The Transparency of Earth’s Atmosphere

Herschel Space Observatory

Orion Seen at Ultraviolet, Infrared and Visible Wavelengths

The Hubble Space Telescope

The James Webb Space Telescope (JWST)

The Fermi Gamma-Ray Telescope

The Entire Sky at Five Wavelength Ranges

Key Ideas Refracting Telescopes: Refracting telescopes, or refractors, produce images by bending light rays as they pass through glass lenses. Chromatic aberration is an optical defect whereby light of different wavelengths is bent in different amounts by a lens. Glass impurities, chromatic aberration, opacity to certain wavelengths, and structural difficulties make it inadvisable to build extremely large refractors.

Key Ideas Reflecting Telescopes: Reflecting telescopes, or reflectors, produce images by reflecting light rays to a focus point from curved mirrors. Reflectors are not subject to most of the problems that limit the useful size of refractors.

Key Ideas Angular Resolution: A telescope ’ s angular resolution, which indicates ability to see fine details, is limited by two key factors. Diffraction is an intrinsic property of light waves. Its effects can be minimized by using a larger objective lens or mirror. The blurring effects of atmospheric turbulence can be minimized by placing the telescope atop a tall mountain with very smooth air. They can be dramatically reduced by the use of adaptive optics and can be eliminated entirely by placing the telescope in orbit.

Key Ideas Charge-Coupled Devices: Sensitive light detectors called charge-coupled devices (CCDs) are often used at a telescope ’ s focus to record faint images. Spectrographs: A spectrograph uses a diffraction grating to form the spectrum of an astronomical object. Radio Telescopes: Radio telescopes use large reflecting dishes to focus radio waves onto a detector. Very large dishes provide reasonably sharp radio images. Higher resolution is achieved with interferometry techniques that link smaller dishes together.

Key Ideas Transparency of the Earth ’ s Atmosphere: The Earth ’ s atmosphere absorbs much of the radiation that arrives from space. The atmosphere is transparent chiefly in two wavelength ranges known as the optical window and the radio window. A few wavelengths in the near-infrared also reach the ground. Telescopes in Space: For observations at wavelengths to which the Earth ’ s atmosphere is opaque, astronomers depend on telescopes carried above the atmosphere by rockets or spacecraft. Satellite-based observatories provide new information about the universe and permit coordinated observation of the sky at all wavelengths.