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Light and Telescopes Almost all astronomical information is obtained through the electromagnetic radiation, i.e. light, we receive from cosmic objects Assignment Chapter 6. All of it Goals 1) To investigate the nature of light 2) To become familiar with the electromagnetic spectrum 3) To introduce telescopes 4) To understand how we collect and study light using telescope 5) All of this is covered in Chapter 6 What is light? Light is electromagnetic radiation, i.e. coupled electric and magnetic fields that oscillate in strength and that propagate in space while carrying energy. Technically, light is the part of electromagnetic (e.m.) radiation that humans (and other animals) see Humans also sense (“see” with sense other than sight) other part of the e.m. spectrum, like heat (through skin) Although incorrectly, we usually call “light” all type of electromagnetic radiation, like X-ray light or UltraViolet light Light really is a small portion of the spectrum of e.m. radiation Types of e.m. radiation differ from each other by wavelengths • Blue light: short wavelength; red: long one • X-ray: very short wavelength; radio: very long one Identical situation with sound pitch • High pitch: short wavelength; bass: long one What is Electromagnetic Radiation? Made of propagating waves of electric and magnetic field It carries energy with it • Sometimes called “radiant energy” • Think – solar power, photosynthesis, photo-electric cells, the fireplace … It also carries information • the signal received by your car radio • the signals received by telescopes staring at stars • the signals received by your eyes right now! What is the electromagnetic wave? It is electricity and magnetism moving through space. So, when we say the speed of light is “c” what we really mean is that the speed of the electromagnetic wave is “c”, regardless of its frequency Light as a wave Waves you can see: e.g., ocean waves Waves you cannot see: • sound wave • electromagnetic waves Light is an electromagnetic wave Light as a Wave l c = 300,000 km/s = 3*108 m/s • Light waves are characterized by a wavelength l and a frequency f. • f and l are related through f = c/l Properties of Waves Wavelength – the distance between crests (or troughs) of a wave. For light in general: Frequency – the speed = c = s/t = λ number of crests λ=c (or troughs) that λ = c/ pass by each second. frequency Speed – the rate at wavelength which a crest (or speed of light = 3x105 km/s trough) moves. in vacuum Light as a Wave (2) • 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 comes in quanta of energy called photons – little bullets of energy. • Photons are massless, but they have momentum and and energy. • They also react to a gravitational field (because they follow the curved space-time). Wave-particle duality All types of electromagnetic radiation act as both waves and particles. The two views are connected by the relation E = h = h c / l h is the Planck's constant, c is the speed of light is the frequency, l is the wavelength The energy of a photon does not depend on the intensity of the light!!! Intensity A photon's energy depends on the wavelength (or frequency) only, NOT the intensity. But the energy you experience depends also on the intensity (total number of photons). It turns out that particles of matter, such as electrons, also behave as both wave and particle. The theory that describes these puzzles and their solution, and how light and atoms interact is quantum mechanics. In Summary: properties of Light All light travels through (vacuum) space with a velocity = 3x105 km/s The frequency (or wavelength) of photon determines how much energy the photon has: The number of photons (how many) determines the intensity Light can be described in terms of either energy, frequency, or wavelength. Visible Light Shorter Wavelength Longer Wavelength Remember: visible light isn’t the whole story. It’s just a small part of the entire electromagnetic spectrum Short Wavelength Long Wavelength (high frequency) (high energy) (low frequency) (low energy) Wavelengths and size of things Example of Electromagnetic Radiation Short wavelength Long wavelength If light is thermally generated, by a heated body, the dominant color reflects the temperature of the body Compared to visible light, radio waves have: higher energy and longer wavelength higher energy and shorter wavelength lower energy and longer wavelength lower energy and shorter wavelength all light has the same energy The Multi-wavelength Sun Radio infrared optical X-ray Optical Sky Near-infrared sky Boldt et al. Radio Sky Soft X-ray Sky Different wavelength carry different type of information • Visible light: the glow of stars (dust blocks light) • Infrared: the glow of dust Visible light (top) and infrared (bottom) image of the Sombrero Galaxy Matter interacts with light in four different ways: Absorption – the energy in the photon is absorbed by the matter and turned into thermal energy Reflection – no energy is transferred and the photon “bounces” off in a new (and predictable) direction E.g., Your hand feels warm in front of a fire. E.g., Your bathroom mirror Transmission – no energy is transferred and the photon passes through the matter unchanged. Emission – matter gives off light in two different ways. We’ll come back to this next lecture. Our eyes work via the process of: transmission reflection absorption emission none of the above A red ball is red because: it only emits frequencies corresponding to red it only reflects frequencies corresponding to red it only transmits frequencies corresponding to red it only absorbs frequencies corresponding to red Telescopes The largest optical telescopes in the world: The twin 10-m Keck telescopes (Hawaii) The Hubble Space Telescope (HST) An ultraviolet (1000-3500) Ang, Optical (3500-8500) Ang, and near-infrared (8500-16000) Ang telescope The Five College Radio Astronomy Observatory (now defunct) UMass LMT The 50-m Large Millimeter Telescope The largest millimeterwavelength telescope in the world U Mass and Mexico Refracting/Reflecting Telescopes Focal length Focal length Refracting Telescope: Lens focuses light onto the focal plane Reflecting Telescope: Concave Mirror focuses light onto the focal plane Almost all modern telescopes are reflecting telescopes. Secondary Optics In reflecting telescopes: Secondary mirror, to redirect the light path towards the back or side of the incoming light path. Eyepiece: To view and enlarge the small image produced in the focal plane of the primary optics. Disadvantages of Refracting Telescopes • Chromatic aberration: Different wavelengths are focused at different focal lengths (prism effect). • Difficult and expensive to produce: All surfaces must be perfectly shaped; glass must be flawless; lens can only be supported at the edges Can be corrected, but not eliminated by second lens out of different material What telescopes are for? Why do they need to be big? The main feature of a telescope is its capacity to collect as much light as possible • Like an antenna: the stronger the signal the clearest the transmission. • Well, guess what: an antenna *is* a telescope (a radio telescope, that is) The larger the light collector, I.e. the primary mirror or lens, the more powerful the telescope (Light Gather Power= LGP) • LGP ~ 4 p D2 • LGPA/LGPB = (DA/DB)2 • A telescope twice as large collects four times as much light The other primary feature is image sharpness, to faithfully reproduce details • Resolving power: a = 11.6/D The last, and least important, feature is magnification The Powers of a Telescope: Size Does Matter 1. Light-gathering power: Depends on the surface area A of the primary lens / mirror, proportional to diameter squared: A = p (D/2)2 D The Powers of a Telescope (2) 2. Resolving power: Wave nature of light => The telescope aperture produces fringe rings that set a limit to the resolution of the telescope. Resolving power = minimum angular distance amin between two objects that can be separated. amin = 1.22 (l/D) For optical wavelengths, this gives amin = 11.6 arcsec / D[cm] amin Seeing Weather conditions and turbulence in the atmosphere set further limits to the quality of astronomical images. Bad seeing Good seeing Seeing Seeing Deep Imaging of the sky: at the edge of the Universe To study galaxy formation both space-based sensitivity and angular resolution required!! Note how many more details and faint objects can be observed with the Hubble Space Telescope Ground Telescope Subaru + SUPREME Space: HST + ACS The Powers of a Telescope (3) 3. Magnifying Power = ability of the telescope to make the image appear bigger. The magnification depends on the ratio of focal lengths of the primary mirror/lens (Fo) and the eyepiece (Fe): M = Fo/Fe A larger magnification does not improve the resolving power of the telescope! Telescopes do not have to be only Optical Different wavelengths carry different type of information (optical: stars; X-ray: black hole; infrared: dust; radio: gas) To detect different wavelengths of light, eg. X-ray, UV, optical, infrared, radio, different technologies are required For example, special mirrors are necessary for X-ray telescopes or else the radiation would pass through them. Hence, it is necessary to specialize telescopes to the wavelength of light one wishes to study. We X-ray, UV, optical, infrared and radio telescopes Different locations for telescopes In addition, the Earth’s atmosphere affects light of different wavelengths differently: 1. 2. 3. 4. As a consequence some telescopes can operate on the ground: • • optical, near-infrared, radio Some can only work in space • • • It totally absorbs X-ray and UV light: X-ray and UV telescopes MUST be placed in space It blurs the optical light, I.e. it destroys sharpness. It also adds the glare of the night sky (yup! There is such thing) to optical and infrared light, which makes faint sources hard to see. It totally absorbs some (important) infrared light X-ray, UV, mid- and far-infrared For high-resolution (super-sharp) observations, or for observations of very faint sources (i.e. to avoid the glare of the Earth’s atmospherer) either space telescopes or very advanced technologies (adaptive optics) are required. The Best Location for a Telescope Far away from civilization – to avoid light pollution The Best Location for a Telescope (2) Paranal Observatory (ESO), Chile On high mountain-tops – to avoid atmospheric turbulence ( seeing) and other weather effects Most wavelengths cannot penetrate the Earth's atmosphere Observing Beyond the Ends of the Visible Spectrum Most infrared radiation is absorbed in the lower atmosphere. NASA infrared telescope on Mauna Kea, Hawaii Infrared cameras need to be cooled to very low temperatures, usually using liquid nitrogen. However, from high mountain tops or high- The Hubble Space Telescope • Launched in 1990; maintained and upgraded by several space shuttle service missions throughout the 1990s and early 2000’s • Avoids turbulence in the Earth’s atmosphere • Extends imaging and spectroscopy to (invisible) infrared and ultraviolet Infrared Astronomy from Orbit: NASA’s Spitzer Space Telescope Infrared light with wavelengths much longer than visible light (“Far Infrared”) can only be observed from space. Why different wavelengths are required Regardless of the technology, different wavelengths carries different information: • Shorter wavelengths carry information on very energetic phenomena (e.g. black holes, star formation) • Optical wavelengths carry information on the structures of galaxies and their motions (the assembly of the bodies of galaxies, their size) • Longer wavelengths carry information on the chemical composition, physical state (gas and dust, presence, chemical elements; temperature) Telescope Instruments Cameras: • To obtain images at desired wavelength or wavelengths (color images) • This yields the morphology, size of the sources Spectrographs: • To study the intensity of the various wavelengths (colors) • This yields the physical nature (star, galaxy, balck hole), chemical composition, physical properties (temperature, density), dynamics (motions, mass), distance of the sources Variability (change with time) There are three basic aspects of the light from an object that we can study from the Earth. Intensity (spatial distribution of the light) Spectra (composition of the object and the object’s velocity) The Spectrograph Using a prism (or a grating), light can be split up into different wavelengths (colors!) to produce a spectrum. Spectral lines in a spectrum tell us about the chemical composition and other properties of the observed object . Spectral Lines of Some Elements Argon Helium Mercury Sodium Neon Spectral lines are like a cosmic barcode system for elements. Traditional Telescopes Secondary mirror Traditional primary mirror: sturdy, heavy to avoid distortions Traditional Telescopes The 4-m Mayall Telescope at Kitt Peak National Observatory (Arizona) Advances in Modern Telescope Design (1) Modern computer technology has made significant advances in telescope design possible: Segmented mirror 1. Lighter mirrors with lighter support structures, to be controlled dynamically by computers Floppy mirror Adaptive Optics Computer-controlled mirror support adjusts the mirror surface (many times per second) to compensate for distortions by atmospheric turbulence A laser beam produces an artificial star, which is used for the computer-based seeing correction. Advances in Modern Telescope Design (2) 2. Simpler, stronger mountings (“Alt-azimuth mountings”) to be controlled by computers Examples of Modern Telescope Design Examples of Modern Telescope Design CCD Imaging (no photographic films any longer CCD = Charge-coupled device • Much more sensitive than photographic plates (90% vs. 1%) • Data can be read directly into computer memory, allowing easy electronic manipulations and analysis Visible light (top) and infrared (bottom) image of the Sombrero Galaxy Radio Astronomy Recall: Radio waves of l ~ 1 cm – 1 m also penetrate the Earth’s atmosphere and can be observed from the ground. 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. Radio Interferometry Just as for optical telescopes, the resolving power of a radio telescope is amin = 1.22 l/D. For radio telescopes, this is a big problem: Radio waves are much longer than visible light. Use interferometry to improve resolution! Radio Interferometry (2) The Very Large Array (VLA): 27 dishes are combined to simulate a large dish of 36 km in diameter. Even larger arrays consist of dishes spread out over the entire U.S. (VLBA = Very Long Baseline Array) or even the whole Earth (VLBI = Very Long Baseline Interferometry)! The Largest Radio Telescopes The 300-m telescope in Arecibo, Puerto Rico. The 100-m Green Bank Telescope in Green Bank, WVa. Science of Radio Astronomy Radio astronomy reveals several features, not visible at other wavelengths: • Neutral hydrogen clouds (which don’t emit any visible light), containing ~ 90 % of all the atoms in the Universe • Molecules (often located in dense clouds, where visible light is completely absorbed) • Radio waves penetrate gas and dust clouds, so we can observe regions from which visible light is heavily absorbed. Life at the telescope. I The telescope, before sunset The MMT 6.5-m telescope, Univ. of Arizona The trusty Night Assistant, who does all the work Life at the telescope. II The diligent Student, who makes sure the work is done right The hard-working Professor, who bosses everybody around