Observatories and Telescopes Mauna Kea Hawaii 14 000

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 ?

$Retina Collecting Area Eye could be a “refracting” telescope, but the size is very$

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 30 m telescope on the drawing board

Large Binocular Telescope Mount Graham, Arizona

Large Telescopes Thirty-meter class telescopes (TMT) under construction

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 (simply change the eyepiece to magnify image)

$Properties of Light and Telescopes • Reflection Mirrors • Refraction Lenses$

Properties of Light and Telescopes • Reflection Mirrors • Refraction Lenses

$Simple Refracting Telescope Convex lens Focus Objective Lens Secondary Lens (Eyepiece)$

Simple Refracting Telescope Convex lens Focus Objective Lens Secondary Lens (Eyepiece)

Spheres of light from distant source parallel rays at the observer

$Different speeds in different media Bending or Refraction$

Different speeds in different media Bending or Refraction

$Refraction of light beam Normal (Perpendicular) Light bends towards the perpendicular going into denser$

Refraction of light beam Normal (Perpendicular) Light bends towards the perpendicular going into denser medium, and vice-versa

$Refraction by prism and lens$

Refraction by prism and lens

$Refractive Index • Speed of light slows down in a medium ! • The$

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

$Chromatic Aberration: Different colors at different focus Chromatic aberration affects refracting telescopes; therefore use$

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 • No longer! 1600 ft diameter in China