Foundations of Astronomy 13e Seeds Chapter 6 Light and Telescopes
Guidepost In this chapter, you will consider the techniques astronomers use to study the Universe What is light? How do telescopes work? What are the powers and limitations of telescopes?
Guideposts (cont d.) What kind of instruments do astronomers use to record and analyze light gathered by telescopes? Why are some telescopes located in space?
6-1 Radiation: Information from Space In astronomy, we cannot perform experiments with our objects Stars, galaxies, etc. The only way to investigate them is by analyzing the light (and other radiation) which we observe from them
Common Misconception Misconception: we must be wary of the word radiation Truth: Radiation is anything that radiates away from a source and not all radiation involves dangerous high-energy particles
Light as Waves Light waves are characterized by: Wavelength Frequency
Light as Waves (cont d.) Wavelengths of light are measured in units of nanometers (nm) or Ångström (Å) 1 nm = 10-9 m 1 Å = 10-10 m = 0.1 nm Visible light has wavelengths between 4000 Å and 7000 Å (= 400 700 nm)
Wavelengths and Colors Different colors of visible light correspond to different wavelengths
Light as Particles Light can also appear as particles, called photons (e.g., photoelectric effect) A photon has a specific energy E, proportional to the frequency f The energy of a photon does not depend on the intensity of the light
The Electromagnetic Spectrum
Common Misconception Misconception: Radio waves are related to sound Truth: Radio waves are a type of light that your radio receiver transforms into sound
6-2 Telescopes Astronomers use telescopes to gather more light from astronomical objects The larger the telescope, the more light it gathers
Refracting and Reflecting Telescopes Refracting telescope: lens focuses light onto the focal plane Reflecting telescope: concave mirror focuses light onto the focal plane
Secondary Optics Secondary mirror: redirects the light path towards the back or side of the incoming light path Eyepiece: used to view and enlarge the small image produced in the focal plane of the primary optics. Focal length
The Powers and Limitations of Telescopes Chromatic aberration: different wavelengths are focused at different focal lengths (prism effect)
The Powers and Limitations of Telescopes (cont d.) Light-gathering power: depends on the surface area (A) of the primary lens or mirror, proportional to diameter squared A = p (D/2) 2
Resolving Power Minimum angular distance a min between two objects that can be separated: a min = 1.22 (l/d) For optical wavelengths, this gives a min = 11.6 arcsec / D[cm]
Seeing Weather conditions and turbulence in the atmosphere set further limits to the quality of astronomical images
Magnifying Power Ability of the telescope to make the image appear bigger Depends on the ratio of focal lengths of the primary mirror or lens (F p ) and the eyepiece (F e ): M = F p /F e A larger magnification does not improve the resolving power of the telescope!
Common Misconception Misconception: The purpose of an astronomical telescope is to magnify images Truth: Very high magnification does not necessarily show more detail. Generally, the amount of detail that a telescope can discern is limited by its resolving power or the seeing conditions
How Do We Know? 6-1 Resolution and precision The precision of measurements is limited by the resolution of the measurement technique The practical size of a pixel is set by the resolution limit, and affected by: Atmospheric seeing Telescope optical quality and diffraction You can t see details smaller than the pixel size, so there is unavoidable uncertainty in all scientific measurements
The Best Locations for a Telescope Far away from civilization to avoid light pollution
The Best Locations for a Telescope (cont d.) On high mountain-tops to avoid atmospheric turbulence and other weather effects
Modern Optical Telescopes
Modern Optical Telescopes (cont d.) The 4-m Mayall Telescope at Kitt Peak National Observatory (Arizona)
Advances in Modern Telescope Design Lighter mirrors with lighter support structures, to be controlled dynamically by computers Floppy mirror Segmented mirror
Advances in Modern Telescope Design (cont d.) Simpler, stronger mountings ( Alt-azimuth mountings ) to be controlled by computers
Examples of Modern Telescopic Design
Examples of Modern Telescopic Design (cont d.)
Radio Astronomy Recall: radio waves of l ~ 1 cm 1 m also penetrate Earth s atmosphere and can be observed from the ground
Modern Radio Telescopes Large dish focuses the energy of radio waves onto a small receiver (antenna) Amplified signals are stored in computers and converted into images, spectra, etc.
6-4 Airborne and Space Telescopes
Airborne Telescopes Infrared cameras need to be cooled to very low temperatures, usually using liquid nitrogen.
Space Telescopes The Hubble Space Telescope Launched in 1990 Maintained and upgraded by several space shuttle service missions throughout the 1990s and early 2000s Avoids turbulence in Earth s atmosphere Extends imaging and spectroscopy to infrared and ultraviolet
The Hubble Space Telescope
Space Telescopes (cont d.) HST successors James Webb Space Telescope Will be in solar orbit ~1 million miles from Earth Herschel Space Observatory (2009) Carried a 3-m mirror and instruments cooled almost to absolute zero
Space Telescopes (cont d.)
High Energy Astronomy Telescopes observing gamma-rays, X- rays, and ultraviolet sources must be located high in Earth s atmosphere or in space General-purpose telescopes: e.g., Chandra Single-subject telescopes: e.g., Hindode
Chandra X-Ray Observatory
6-5 Astronomical Instruments and Techniques Cameras and photometers Photographic plate (record image) Long exposure detect faint objects Brightness of objects not measured very precisely Photometers (measure intensity of the light) Sensitive light meter measures brightness of objects very precisely Charge-coupled devices (CCDs) Records image and measures the brightness
CCD Imaging More sensitive than photographic plates Data can be read directly into computer memory, allowing easy electronic manipulations
Spectrographs Spectral lines in a spectrum tell us about the chemical composition and other properties of the observed object
Adaptive Optics Computer-controlled mirror support adjusts the mirror surface (many times per second) to compensate for distortions by atmospheric turbulence
Interferometry Combine the signals from several smaller telescopes to simulate one big mirror
Radio Maps and Interferometry Colors in a radio map can indicate different intensities of the radio emission from different locations on the sky Radio waves are much longer than visible light use interferometry to improve resolution!
Radio Maps (cont d.)
The Very Large Array (VLA) 27 dishes combined to simulate a large dish of 36 km in diameter
6-6 Non-Electromagnetic Astronomy Radiation from space does not only come in the form of electromagnetic radiation Particle astronomy Earth is constantly bombarded cosmic rays highly energetic subatomic particles traveling through space at high velocities Gravity wave astronomy Gravity waves predicted to be produced by an mass that accelerates, but would be extremely weak and difficult to detect Inferred, but not yet detected
Discussion Questions Why would you not include sound waves in the electromagnetic spectrum? Hint: See Figure 6-2. Do sound travel at the speed of light? What about through a vacuum? Why do optical astronomers often put their telescopes at the tops of mountains, whereas radio astronomers sometimes put their telescopes in deep valleys? Hint: See Figure 6-3
Discussion Questions (cont d.) Why does the wavelength response of the human eye match the visual window of Earth s atmosphere so well? Why not the radio window? Hint: The maximum energy of the Sun is ~500 nm (green)