Presentation on theme: "Light and Telescopes Please pick up your assigned transmitter"— Presentation transcript:
1 Light and Telescopes Please pick up your assigned transmitter Please pick up your assigned transmitterand swipe your student ID for attendance tracking.
2 Light as a Wave (I) f = c/l l lUnit of frequency:1 Hz (“Hertz”) = 1/s.c = 300,000 km/s = 3*108 m/sLight waves are characterized by a wavelength l and a frequency f.f and l are related throughf = c/l
3 Light as a Wave (II)Wavelengths of light are measured in units of nanometers (nm) or Ångström (Å):1 nm = 10-9 m1 Å = m = 0.1 nmVisible light has wavelengths between 4000 Å and 7000 Å (= 400 – 700 nm).
4 What is the frequency of typical optical light with a wavelength of l = 5000 Å (= 5*10-7 m)? 1.5*104 Hz1.5*106 Hz6*1012 Hz6*1013 Hz6*1014 Hz:091234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859606162636465666768697071727374757677787980818283848586878889909192939495969798991001011021031041051061071081091101111121131141151160 of 116
5 Wavelengths and Colors 7000 Å4000 ÅDifferent colors of visible light correspond to different wavelengths.
6 The Electromagnetic Spectrum WavelengthFrequencyHigh flying air planes or satellitesNeed satellites to observe
7 Frequency and Temperature Every object emits electromagnetic radiation, according to its temperature:The hotter an object is, the higher the frequency at which it emits radiation.109 oK oK oK oKTemperature
8 The Sky in Different Wavelength Bands Radio WavesVisible lightg-raysInfraredX-rays
9 Which of the following forms of radiation indicates the highest temperatures? :08Visible lightInfrared radiationX-raysUltraviolet lightRadio waves1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859606162636465666768697071727374757677787980818283848586878889909192939495969798991001011021031041051061071081091101111121131141151160 of 116
10 The larger the telescope, the more light it gathers. Optical TelescopesAstronomers use telescopes to gather more light from astronomical objects.The larger the telescope, the more light it gathers.
11 Refracting / Reflecting Telescopes Refracting Telescope: Lens focuses light onto the focal planeFocal lengthReflecting Telescope: Concave Mirror focuses light onto the focal planeFocal lengthAlmost all modern telescopes are reflecting (mirror) telescopes.
12 Disadvantages of refracting telescopes Chromatic aberration: Different wavelengths are focused at different focal lengths (prism effect).Can be corrected, but not eliminated by second lens out of different material.Difficult and expensive to produce: All surfaces must be perfectly shaped; glass must be flawless; lens can only be supported at the edges
13 Secondary OpticsIn reflecting telescops: Secondary mirror, to re-direct 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.
14 In order to gather as much light as possible from the object you observe, you want to build the telescope mirror …:08As thick as possible.With as large a focal length as possible.With as small a focal length as possible.As large in diameter as possible.As small in diameter as possible.1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859606162636465666768697071727374757677787980818283848586878889909192939495969798991001011021031041051061071081091101111121131141151160 of 116
15 The Powers of a Telescope: Size does matter! Light-gathering power:Depends on the surface area A of the primary lens / mirror, proportional to diameter, D, squared.D
16 The Powers of a Telescope (II) 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)aminFor optical wavelengths, this givesamin = 11.6 arcsec / D[cm]
17 amin [arc seconds] = 11.6/D[cm] Ohio University owns a share of the 2.4-m MDM telescope (i.e., D = 2.4 m) on Kitt Peak, AZ. Can this telescope (in principle) resolve two stars which are 1 arc second apart?YesNo:05amin [arc seconds] = 11.6/D[cm]= 11.6/240 ≈ 0.05123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116The telescope could in principle (under ideal conditions) resolve objects that are 0.05 arc seconds apart!0 of 116
18 SeeingWeather conditions and turbulence in the atmosphere set further limits to the quality of astronomical imagesBad seeingGood seeing
19 The Hubble Space Telescope Launched in 1990; maintained and upgraded by several space shuttle service missions throughout the 1990s and early 2000’sAvoids turbulence in the Earth’s atmosphereExtends imaging and spectroscopy to (invisible) infrared and ultraviolet
20 The Advantage of HSTHST imageGround based image
21 The Powers of a Telescope (III) 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/FeA larger magnification does not improve the resolving power of the telescope!
22 Why are the Mountains of the Arizona Desert are one of the preferred locations for telescopes in the U.S.?Astronomers like mountain hiking.The climate is very dry.There are only few cities which light up the sky with artificial lights.It’s warmer than in most of the rest of the country.1. – 3.2. – 4.:081234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859606162636465666768697071727374757677787980818283848586878889909192939495969798991001011021031041051061071081091101111121131141151160 of 116
23 The Best Location for a Telescope Far away from civilization – to avoid light pollution
24 The Best Location for a Telescope (II) Paranal Observatory (ESO), ChileOn high mountain-tops – to avoid atmospheric turbulence (→ seeing) and other weather effects
25 Examples of Modern Telescope Design (I) Design of the Large Binocular Telescope (LBT)The Keck I telescope mirror
26 Examples of Modern Telescope Design (II) The Very Large Telescope (VLT)8.1-m mirror of the Gemini Telescopes
27 Could you use a telescope in your back-yard to observe infrared radiation from space? Yes, but you will need a special infrared detector.Yes, but you will need a special infrared filter in order not to burn your eyes (or your detector).No, because infrared radiation can not be detected at all.No, because infrared radiation is absorbed very high in the Earth’s atmosphere.No, because infrared radiation is absorbed in the lower atmosphere.:091234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859606162636465666768697071727374757677787980818283848586878889909192939495969798991001011021031041051061071081091101111121131141151160 of 116
28 NASA infrared telescope on Mauna Kea, Hawaii Infrared AstronomyMost infrared radiation is absorbed in the lower atmosphere.However, from high mountain tops or high-flying air planes, some infrared radiation can still be observed.NASA infrared telescope on Mauna Kea, Hawaii