Chapter 6:Chapter 6:Telescopes Telescopes – Portals of – Portals of

DiscoveryDiscovery

Visible light is only one type of electromagnetic

radiation emitted by stars

Each type of EM radiation travels at exactly the same speed - the speed of light!

Not all EM radiation can penetrate Earth’s atmosphere.

Different types of EM radiation Different types of EM radiation require different types of telescopesrequire different types of telescopes

• A refracting telescope uses a lens to A refracting telescope uses a lens to concentrate incoming lightconcentrate incoming light

• A reflecting telescope uses mirrors to A reflecting telescope uses mirrors to concentrate incoming starlightconcentrate incoming starlight

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Refracting TelescopeRefracting Telescope

• Refracting Refracting telescopes need telescopes need to be very long, to be very long, with large, heavy with large, heavy lenses.lenses.

A refracting telescope uses a

lens to concentrate

incoming light

Similar to a magnifying glass

A larger objective lens provides a

brighter (not bigger) image

lenses reverse images

Three main functions of a Three main functions of a telescopetelescope

• brightenbrighten(called (called light collecting arealight collecting area))

• see fine detailsee fine detail(called (called angular resolutionangular resolution))

and least important,and least important,

• magnifymagnifymagnification = (objective lens focal length magnification = (objective lens focal length // eyepiece lens eyepiece lens

focal length)focal length)

Refracting telescopes have drawbacksRefracting telescopes have drawbacks

• Spherical aberrationSpherical aberration

• Chromatic aberrationChromatic aberration

Special achromatic compound lenses and lens coatings can often fix this aberration

Refracting telescopes have Refracting telescopes have drawbacksdrawbacks

• Spherical Spherical aberrationaberration

• Chromatic Chromatic aberrationaberration

• Sagging due to Sagging due to gravity distorting the gravity distorting the lenslens

• Unwanted refractionsUnwanted refractions

• opaque to certain opaque to certain wavelengths of lightwavelengths of light

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Reflecting TelescopeReflecting Telescope

• Reflecting telescopes can have much greater Reflecting telescopes can have much greater diameters.diameters.

• Most modern telescopes are reflectors.Most modern telescopes are reflectors.

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Designs for Reflecting Designs for Reflecting TelescopesTelescopes

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Twin Keck telescopes on Mauna Kea in Hawaii

Segmented 10-meter mirror of a Keck telescope

Pick-up images.

Pick-up images.

Mirrors in Reflecting TelescopesMirrors in Reflecting Telescopes

AstronomerAstronomer’’s face two major s face two major obstacles in observing the starsobstacles in observing the stars

• Light Pollution from CitiesLight Pollution from Cities

• Effects of Twinkling from EarthEffects of Twinkling from Earth’’s s atmosphereatmosphere

Light PollutionLight Pollution

• Scattering of human-made light in the Scattering of human-made light in the atmosphere is a growing problem for atmosphere is a growing problem for astronomy.astronomy.

Tucson, Arizona in 1959 and 1980

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Bright star viewed with ground-based telescope

Same star viewed with Hubble Space Telescope

Twinkling and Turbulence

• Turbulent air flow in Earth's atmosphere distorts our view, causing stars to appear to twinkle.

Rapid changes in the density of Rapid changes in the density of EarthEarth’’s atmosphere cause passing s atmosphere cause passing starlight to quickly change direction, starlight to quickly change direction, making stars appear to twinkle.making stars appear to twinkle.

Advanced technology is spawning a new Advanced technology is spawning a new generation of equipment to view the generation of equipment to view the universeuniverse

• CCDs (charge-coupled devices)CCDs (charge-coupled devices)

• Large telescopes on remote mountain topsLarge telescopes on remote mountain tops– Mauna Kea in HawaiiMauna Kea in Hawaii– Cerro Pachon in ChileCerro Pachon in Chile

• Adaptive Optics to counteract the blurring of Adaptive Optics to counteract the blurring of EarthEarth’’s atmospheres atmosphere

• Orbiting space observatoriesOrbiting space observatories

A Charge-Coupled Device (CCD)A Charge-Coupled Device (CCD)

Ordinary Photographs vs. CCDs

Matching 10-m, multiple mirror Keck Telescopes in Hawaii with

adaptive optics

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Without adaptive optics With adaptive optics

Adaptive Optics

• Rapidly changing the shape of a telescope's mirror compensates for some of the effects of turbulence.

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Summit of Mauna Kea, Hawaii

Calm, High, Dark, Dry

• The best observing sites are atop remote mountains.

High above Earth’s atmosphere,

the Hubble Space Telescope provides stunning details about the

universe

Observations at wavelengths other than visible Observations at wavelengths other than visible light are revealing previously invisible sightslight are revealing previously invisible sights

Visible light image radio wavelength image

Radio wavelength observations are possible from Earth’s surface

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Radio Telescopes

• A radio telescope is like a giant mirror that reflects radio waves to a focus.

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SOFIA Spitzer

Infrared and Ultraviolet Telescopes

• Infrared and ultraviolet light telescopes operate like visible-light telescopes but need to be above atmosphere to see all wavelengths.

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X-Ray Telescopes

• Focusing of X-rays requires special mirrors.• Mirrors are arranged to focus X-ray photons through grazing

bounces off the surface.

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Fermi Gamma-Ray Observatory

Gamma-Ray Telescopes

• Gamma-ray telescopes also need to be in space.

• Focusing gamma rays is extremely difficult.

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Interferometry

• Interferometery is a technique for linking two or more telescopes so that they have the angular resolution of a single large one.

The Very Large Array (VLA) in New Mexico

Observations at other wavelengths are revealing previously invisible sights

UV

Ordinary visible

infrared

Map of Orion region

The Future of Telescopes?The Future of Telescopes?

• Telescopes at Lagrange PointsTelescopes at Lagrange Points

• Telescopes on other planets/moonsTelescopes on other planets/moons

• A large telescope to take over for Hubble A large telescope to take over for Hubble on the Moonon the Moon’’s polar regions polar region

• Better telescopes on Earth, larger, coupledBetter telescopes on Earth, larger, coupled

• Gravity wave detectorsGravity wave detectors

• Improved optics…..Improved optics…..