Light and Telescope 3/4/2018. PHYS 1403 Introduction to Astronomy. Guideposts (cont d.) Guidepost. Outline (continued) Outline.

Transcription

1 PHYS 1403 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

2 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 = m = 0.1 nm Visible light has wavelengths between 4000 Å and 7000 Å (= nm). 2

3 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

4 n of air =1.33 N, and n of water=1.33 n of plastic = 1.49 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

5 Secondary Optics In reflecting telescopes: Secondary mirror, to redirect light path towards back or side of incoming light path. 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 Eyepiece: To view and enlarge the A larger magnification does small image produced in not improve the resolving the focal power of the telescope! plane of the primary optics. RocketMime Field of View in a Telescope Resolving Power Minimum angular distance min between two objects that can be separated. D is diameter of mirror or lens min = 1.22 ( /D) For optical wavelengths, this gives min = 11.6 arcsec / D[cm] Skyandtelescope.com Total amount of sky you can see with an eyepiece Reducing the aperture (diameter) of eyepiece increases magnification but also decreases the field of view 5

6 Hobby Telescopes Eyepiece Refractor - Lens Jtwastronomy.com Reflector - Mirror Astroshop.eu Modern Optical Telescopes Modern Optical Telescopes (cont d.) Cassegrain Focus Schnmidt - Cassegrain Focus Prime Focus The 4-m Mayall Telescope at Kitt Peak National Observatory (Arizona) Advances in Modern Telescope Design Examples of Modern Telescopic Design Lighter mirrors with lighter support structures, to be controlled dynamically by computers Floppy mirror Segmented mirror 6

7 Examples of Modern Telescopic Design (cont d.) * Observatory Telescopes Mounts Funding Federal Government (DOE FIPSE) Tarleton State University Telescope require mounts that allow rotation Manufacturer Astronomical Consultant and Equipment Inc.(ACE) 32-inch (0.8m) Reflector f/ratio= f/7 Remotely Control ACE Software CCD Camera Finger Lakes - FOV = 17 x 17 Resolution /pixel Imaging and Photometry UBVRI, Hα, OIII and Neutral Density counterweight Popular Equatorial Mount No counterweight needed GeekWrapped.com Starizona.com 7

8 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. 8