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Light and Telescopes Please pick up your assigned transmitter and swipe your student ID for attendance tracking.
Light as a Wave (I) Light waves are characterized by a wavelength and a frequency f. f = c/ c = 300,000 km/s = 3*10 8 m/s f and are related through Unit of frequency: 1 Hz (“Hertz”) = 1/s.
Light as a Wave (II) Wavelengths of light are measured in units of nanometers (nm) or Ångström (Å): 1 nm = m 1 Å = m = 0.1 nm Visible light has wavelengths between 4000 Å and 7000 Å (= 400 – 700 nm).
What is the frequency of typical optical light with a wavelength of = 5000 Å (= 5*10 -7 m)? 1.1.5*10 4 Hz 2.1.5*10 6 Hz 3.6*10 12 Hz 4.6*10 13 Hz 5.6*10 14 Hz
Wavelengths and Colors Different colors of visible light correspond to different wavelengths Å 7000 Å
The Electromagnetic Spectrum Need satellites to observe Wavelength Frequency High flying air planes or satellites
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. Temperature 10 9 o K 10 6 o K 10 3 o K 1 o K
The Sky in Different Wavelength Bands Radio Waves Infrared Visible light X-rays -rays
Which of the following forms of radiation indicates the highest temperatures? 1.Visible light 2.Infrared radiation 3.X-rays 4.Ultraviolet light 5.Radio waves
Optical Telescopes Astronomers use telescopes to gather more light from astronomical objects. The larger the telescope, the more light it gathers.
Refracting / Reflecting Telescopes Refracting Telescope: Lens focuses light onto the focal plane Reflecting Telescope: Concave Mirror focuses light onto the focal plane Focal length Almost all modern telescopes are reflecting (mirror) telescopes.
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
Secondary Optics In 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.
In order to gather as much light as possible from the object you observe, you want to build the telescope mirror … 1.As thick as possible. 2.With as large a focal length as possible. 3.With as small a focal length as possible. 4.As large in diameter as possible. 5.As small in diameter as possible.
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, D, squared. D
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. min = 1.22 ( /D) Resolving power = minimum angular distance min between two objects that can be separated. For optical wavelengths, this gives min = 11.6 arcsec / D[cm] min
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? 1.Yes 2.No min [arc seconds] = 11.6/D[cm] = 11.6/240 ≈ 0.05 The telescope could in principle (under ideal conditions) resolve objects that are 0.05 arc seconds apart!
Seeing Weather conditions and turbulence in the atmosphere set further limits to the quality of astronomical images Bad seeingGood seeing
The Hubble Space Telescope Avoids turbulence in the Earth’s atmosphere Extends imaging and spectroscopy to (invisible) infrared and ultraviolet Launched in 1990; maintained and upgraded by several space shuttle service missions throughout the 1990s and early 2000’s
The Advantage of HST HST image Ground based image
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 (F o ) and the eyepiece (F e ): M = F o /F e A larger magnification does not improve the resolving power of the telescope!
Why are the Mountains of the Arizona Desert are one of the preferred locations for telescopes in the U.S.? 1.Astronomers like mountain hiking. 2.The climate is very dry. 3.There are only few cities which light up the sky with artificial lights. 4.It’s warmer than in most of the rest of the country – – 4.
The Best Location for a Telescope Far away from civilization – to avoid light pollution
The Best Location for a Telescope (II) On high mountain-tops – to avoid atmospheric turbulence (→ seeing) and other weather effects Paranal Observatory (ESO), Chile
Examples of Modern Telescope Design (I) Design of the Large Binocular Telescope (LBT) The Keck I telescope mirror
Examples of Modern Telescope Design (II) 8.1-m mirror of the Gemini Telescopes The Very Large Telescope (VLT)
Could you use a telescope in your back-yard to observe infrared radiation from space? 1.Yes, but you will need a special infrared detector. 2.Yes, but you will need a special infrared filter in order not to burn your eyes (or your detector). 3.No, because infrared radiation can not be detected at all. 4.No, because infrared radiation is absorbed very high in the Earth’s atmosphere. 5.No, because infrared radiation is absorbed in the lower atmosphere.
Infrared Astronomy Most 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