Observatories and TelescopesMauna 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 ?

Collecting Area

Lens

Retina

Eye could be a “refracting” telescope,but the size is very small

Collecting Area = Power• A = d2 ; 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 TelescopeMount 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

Objective Lens

Secondary Lens (Eyepiece)

Focus

Convex lens

Spheres of light from distant source

parallel rays at the observer

Different speeds in different media Bending or Refraction

Refraction of light beam

Light bends towards the perpendicular going into denser medium, and vice-versa

Normal(Perpendicular)

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 = ‘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

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

PrimaryMirrorPrime

Focus

Secondary Mirror

Cassegrain Focus

REFLECTING TELESCOPES

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 m = 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)