PHYS 1403 Introduction to Astronomy Light and Telescope Chapter 6 Reminder/Announcement 1. Extension for Term Project 1: Now Due on Monday November 12 th 2. You will be required to bring your cross staff with you for verification and use during the lab activity on November 12 th. 3. No class and Lab on Monday November 19 th. You can use that time for Term Project 2 or Sunspots lab that will be assigned November 12 th. 4. Term Project 2 will be assigned after November 5 th. Chapter Outline 1. Radiation: Information from Space a) Wave Characteristics b) The Electromagnetic Spectrum c) Dual Nature of Light 2. Optical Telescopes a) Two Kinds of Telescopes b) What Telescope should I buy? c) Telescope Mounts d) Tarleton Telescope Chapter Outline (continued) 1. What to do with a Telescope? a) Viewing with Eye b) Taking pictures (CCD camera) c) Braking white light in colors 2. Radio Telescopes a) Operation of a Radio Telescope b) Limitations of the Radio Telescope c) Advantages of Radio Telescopes 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 3. Space Astronomy a) Astronomy in other wavelengths b) Hubble Space Telescope c) New Generation of Space Telescopes 1
What is a 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 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 How do we describe a wave? Vibration and wave characteristics Amplitude (A) distance from the midpoint to the crest or to the trough. SI unit depend on physical quantity, it could be meters if it is wave in the ocean, intensity if it is a light wave. 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 How do we describe a wave? Vibration and wave characteristics Frequency (f or ν) How many wavelengths pass a reference point in 1 second. SI units are cycles/seconds or Hertz (Hz). Velocity (v) How fast waves are moving. SI units, meters/seconds No matter what type of wave we have, the product of wavelength and frequency gives you the speed of the wave. v For light waves c Where c is the speed of light Energy of a Wave For waves the energy is obtained by multiplying the frequency by a specific constant. E Where h is a constant known as Planck constant Max Planck was the one who proposed this relationship and later Einstein also used the idea to explain photoelectric effect (converting light into electricity. Noticed that at any one time the wave can only have one frequency, which means one energy. If the frequency changes, energy changes. We can sort waves according to their increasing frequency (hence energy), and decreasing wavelength (since f ~ 1/λ) and call it a spectrum. Electromagnetic Wave Oscillating charges produces transverse waves. These are called electromagnetic waves because there is electric and magnetic waves and energy associated with oscillations. Business Insider UK 2
Electromagnetic Spectrum Assemblage of electromagnetic waves according to increasing frequency. Waves visible to the eye is called visible waves. It is generally called light, but any electromagnetic wave can be called light. Just some waves are invisible to the eye. What is Light? It s a form of energy that our eyes can detect. Wavelengths of light are measured in units of nanometers (nm) or Angström (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). Google Sites https://sites.google.com/site/chempendix/em-spectrum What is Light? Light has dual nature It sometimes behaves as waves and at other times as a particle. When it behaves like a particle. The particle is called a photon. Each photon has energy, wavelength and frequency but no mass. The energy of a photon is given by E The particle nature was explained by Einstein when he was studying photoelectric effect. In the particle model, light is considered to be a beam made up of particles hitting the target. White Visible Light and Colors MPC - WordPress.com Different colors of visible light correspond to different wavelengths. The Electromagnetic Spectrum and Atmosphere What is a Telescopes? Wavelength Need satellites to observe High flying air planes or satellites Frequency A Telescope is an instrument that collects electromagnetic wave 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. 3
Optical Telescopes 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 The larger the telescope, the more light it gathers. Refracting/Reflecting Telescopes Focal length Refracting Telescope: Lens focuses light onto the focal plane Reflecting Telescope: Concave Mirror focuses light onto the focal Focal length plane Almost all modern telescopes are reflecting telescopes. 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. 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 ): Field of View in a Telescope M = F p /F e RocketMime A larger magnification does not improve the resolving power of the telescope! 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 4
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] Hobby Telescopes Eyepiece Refractor - Lens Jtwastronomy.com Reflector - Mirror Astroshop.eu Modern Optical Telescopes Schmidt - Cassegrain Focus Telescopes Mounts Telescope require mounts that allow rotation Cassegrain Focus Prime Focus counterweight 5
Popular Equatorial Mount No counterweight needed * Observatory GeekWrapped.com Starizona.com Funding Federal Government (DOE FIPSE) Tarleton State University 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 - 0.5 /pixel Imaging and Photometry UBVRI, Hα, OIII and Neutral Density 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