Roger Freedman Robert Geller William Kaufmann III Universe Tenth Edition Chapter 6 Optics and Telescopes By reading this chapter, you will learn 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 Refraction 1
Refraction 2
Refraction and Lenses Light Rays from Distant Objects are (nearly) Parallel 3
A Lens Creates an Extended Image of an Extended Object Qualities that make a good telescope: Light gathering ability (aperture): the bigger the lens, mirror, or dish, the more light it can gather Resolution: the ability to distinguish to distant objects that are close to each other Tracking: the ability to follow an object in the sky for long exposures Adaptive Optics Magnification Location, Location, Location PDF 6 1: A refracting telescope uses a lens to concentrate incoming light at a focus A Refracting Telescope 4
But not a bigger image! Light Gathering Power Early Telescopes Galileo didn t invent the telescope, but he improved on the Flemish design Circa 1600 A refractor Largest Refractor: Yerkes 40 circa 1870 5
A Large Refracting Telescope A Light Refracting Telescope Reflection 6
A Newtonian Telescope Designs for Reflecting Telescope The Secondary Mirror Does Not Cause a Hole in the Image 7
Friedrich Wilhelm Herschel Late 18 th C. 20 ft A reflector 8
Mt. Wilson 100 Hooker reflector Mt. Palomar 200 Hale telescope Angular Resolution 9
Mauna Kea Keck (ex) interferometer Large Binocular Telescope, Mt. Graham Don t forget the mount! What you place the telescope on is vital; any stray vibrations distort the image Also, the sky moves (as seen from Earth) You need a mount that is both stable and tracks the sky Enter Joseph Fraunhoffer 10
How the sky moves German Equatorial Mount Alt/Azimuth Mounts Altitude (up and down angle) Azimuth (around motion) If you take the A105L lab you ll use Dobsonians like this Good and cheap, but you do all the work slewing the scope 11
6 3: Telescope images are degraded by the blurring effects of the atmosphere and by light pollution The term Astronomers use to indicate how good or bad the view is of the sky is Seeing More of a problem for visible light observers Particulates Scattered light Atmospheric Absorption Twinkle The Telescopes of Mauna Kea Adaptive Optics Guide star laser bounces off Na in the upper atmosphere Beam reflected back to chip which then deforms to sharpen image See next slide 12
Creating an Artificial Star 6 4: A charge coupled device is commonly used to record the image at a telescope s focus Charge Coupled Devices (CCDs) and Imaging 13
6 5: Spectrographs record spectra of astronomical objects A Grating Spectrograph Remember this? 14
6 6: A radio telescope uses a large concave dish to reflect radio waves to a focus A Radio Telescope 15
The First Radio Telescope Karl Jansky, in 1932, discovered radio waves impinging the Earth from outer space Modern Telescopes: Owens Valley Modern Telescopes: Arecibo 16
Modern Telescopes: VLA Optical and Radio Views of Saturn 6 7: Telescopes in orbit around Earth detect radiation that does not penetrate the atmosphere The Transparency of Earth s Atmosphere 17
Different molecules absorb different wavelengths Telescopes in Orbit Herschel Space Observatory 18
Orion Seen at Ultraviolet, Infrared and Visible Wavelengths The Hubble Space Telescope James Webb Space Telescope 2018 launch 19
The Fermi Gamma Ray Telescope The Entire Sky at Five Wavelength Ranges 20
Key Ideas Refracting Telescopes: Refracting telescopes, or refractors, produce images by bending light rays as they pass through glass lenses. Glass impurities, chromatic aberration, weight, 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. 21
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. 22