Transcript Document
Observatories and Telescopes Mauna Kea, Hawaii (14,000 ft) Why do telescopes need to be located at high altitude and dry climate ? Telescopes – Mirrors and Lenses • Telescopes are basically a large mirror (reflecting) or a lens (refracting) • Consider the human eye as ‘telescope’ • What determines the “power” of a telescope ? Retina Collecting Area Eye could be a “refracting” telescope, but the size is very small Lens Collecting Area = Power • A = p ( d / 2)2 ; d – diameter • Diameter of the telescope indicates its power • Largest optical telescope: Keck 1 and 2, each with a mirror of 10 m diameter • Large Binocular Telescope (LBT): Ohio State, Arizona, Germany, Italy Two 8.4 m mirrors in a binocular shaped mount • HST – Only 2.4 m, but with a huge advantage • How much more powerful than HST is the Keck (neglecting that advantage) ? • New 30m telescope on the drawing board Large Binocular Telescope Mount Graham, Arizona Objective and eyepiece Telescope Objective and Eyepiece • The main function of a telescope is to collect as much light as possible from the source, NOT to magnify an image • Need bigger and bigger telescopes! • The main mirror or lens of a telescope is called the OBJECTIVE • The Eye-Piece (small lens or mirror) is to magnify the image after it is formed from the light collected by the objective Properties of Light and Telescopes • Reflection Mirrors • Refraction Lenses Simple Refracting Telescope Convex lens Focus Objective Lens Secondary Lens (Eyepiece) Spheres of light from distant source parallel rays at the observer Different speeds in different media Bending or Refraction Refraction of light beam Normal (Perpendicular) Light bends towards the perpendicular going into denser medium, and vice-versa Refraction by prism and lens Refractive Index • Speed of light slows down in a medium ! • The ratio of the speed of light in vacuum to the speed in a medium c/v=m ‘mu’ is called the Refractive Index • Material R.I. Water 1.33 Glass 2.6 • Bending of light (diffraction) depends on R.I of the medium and wavelength of light l Chromatic Aberration: Different colors at different focus Chromatic aberration affects refracting telescopes; therefore use reflecting telescopes in modern observatories Law of Reflection: Angle i = Angle r REFLECTING TELESCOPES Prime Focus Secondary Mirror Primary Mirror Cassegrain Focus Reflecting Concave Mirror Telescopic Configurations Spherical and Parabolic Mirrors Wavelength range of observatories and telescopes • Ground based telescopes can measure - Visible (4000-7000 A), - Near-IR (0.7-2 microns), 1 mm = 10000 A - Radio ( ~ 1 mm or greater) All other wavelengths blocked out by the atmosphere • Space based observatories for Gamma ray, X-ray, UV, and Far-IR astronomy Visible (Optical) and Radio “Windows” in the Atmosphere Radar and Radio Astronomy • Radio telescopes (like huge satellite dish) collect radio waves from astronomical objects • Radar telescope = Transmitter + Radio Telescope • Doppler Radar Transmits radio waves towards an object and collects reflected radio waves; spread in signal shows distance and velocity • Largest radio telescope is the Arecibo, 1000 ft diameter, in Puerto Rico Radio Telescope Aracebo Radio Telescope(Puerto Rico) Rotational Speed and Doppler Shift (Line profile broadens on both the blue and red side) Doppler Radar Hubble Space Telescope Ground and HST images Unresolved Resolved Visible and IR images of Saturn Gamma-Ray View of the Sky Each wavelength band presents a different and mutually complementary view Telescope and Instruments Intensities of Lines in Absorption Spectra: Atoms absorb energy Emission spectra are a set of bright lines: atoms emit energy Observing Planets and Moons Spectra of Titan (Moon of Saturn): Methane (CH4)