PHYS 1411 Introduction to Astronomy Light and Telescope Chapter 6 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? Outline I. Radiation: Information from Space A. Wave Characteristics B. Light as a Wave C. Light as a Particle D. The Electromagnetic Spectrum II. Optical Telescopes A. Properties of Light B. Two Kinds of Telescopes C. The Powers of a Telescope D. Problems with Lenses and Mirrors E. Light Pollution and Dark Skies Outline (continued) III. Special Instruments A. Imaging Systems B. The Spectrograph IV. Radio Telescopes A. Operation of a Radio Telescope B. Limitations of the Radio Telescope C. Advantages of Radio Telescopes V. Space Astronomy A. Astronomy in other wavelengths B. Hubble Space Telescope C. New Generation of Space Telescopes 1
Radiation: Information from Space What is a Wave? 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 Vibration are wiggle in time Wave are wiggle in space and time What do Waves do? Waves transport Energy Light is a type of wave Transverse and Longitudinal Waves Two common types of waves that differ because of the direction in which the medium vibrates compared with the direction of travel: longitudinal wave transverse wave Wave Description Vibration and wave characteristics Amplitude (A) distance from the midpoint to the crest or to the trough. SI unit depend on physical quantity Wavelength (λ) distance from the top of one crest to the top of the next crest, or distance between successive identical parts of the wave. SI units, meters Transfer of Energy: Light as Waves Light waves are characterized by: Wavelength Frequency Light as a Wave 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). 2
Light as a Particle Light and Colors Light can also appear as particles, called photons (explains, e.g., photoelectric effect). A photon has a specific energy E, proportional to the frequency f: E = h*f h = 6.626x10-34 J*s is the Planck constant. The energy of a photon does not depend on the intensity of the light!!! Different colors of visible light correspond to different wavelengths. The Electromagnetic Spectrum The Electromagnetic Spectrum and Atmosphere Wavelength Wavelength Frequency Frequency Need satellites to observe High flying air planes or satellites 3
What is a Telescopes? A Telescope is an instrument that collects light of a certain wavelength. Optical Telescope work in visible waves (light). Radio Telescope work in radio waves Many Space Telescope work in wavelengths that never reach us on Earth. Optical Telescopes The larger the telescope, the more light it gathers. Astronomers use telescopes to gather more light from astronomical objects. 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 LGP ~ A = (D/2) 2 Refracting/Reflecting Telescopes Focal length Refracting Telescope: Lens focuses light onto the focal plane Focal length Reflecting Telescope: Concave Mirror focuses light onto the focal plane Almost all modern telescopes are reflecting telescopes. 4
Secondary Optics In reflecting telescopes: Secondary mirror, to redirect light path towards back or side of incoming light path. Eyepiece: To view and enlarge the small image produced in the focal plane of the primary optics. Field of View in a Telescope Skyandtelescope.com Total amount of sky you can see with an eyepiece Reducing the aperture of eyepiece increases magnification but also decreases the field of view Magnifying Power Resolving 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 Minimum angular distance min between two objects that can be separated: min = 1.22 ( /D) For optical wavelengths, this gives min = 11.6 arcsec / D[cm] A larger magnification does not improve the resolving power of the telescope! 5
Summary of Equations and Worksheet http://astro.unl.edu/classaction/animations/ telescopes/telescope10.html LGP ~ A = (D/2) 2 M = F p /F e min = 11.6 arcsec / D[cm] Acknowledgment The slides in this lecture is for Tarleton: PHYS1411/PHYS1403 class use only Images and text material have been borrowed from various sources with appropriate citations in the slides, including PowerPoint slides from Seeds/Backman text that has been adopted for class. 6