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Telescopes

Reading: Chapter 4.2-4.5

Telescopes

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A telescope is a type of light collecting device

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Telescopes

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Telescopes come in various sizes and locations…

VLA, New Mexico

Arecibo, Puerto Rico

Telescopes

Technology helps make better telescopes…

Galileo’s telescope

Newton’s telescope

Hubble Space Telescope

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Reflection and Refraction

Interfaces and light propagation

• Light rays can be reflected (bounced off) or refracted (bent) at

an interface between two materials:

What is a lens?

Thin Lenses

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What is a lens? A Lens has usually two refracting

surfaces.

If the surfaces are close, we have a thin

lens.

Properties: Parallel rays converge at the focus, F2 (or

F1)

Thin Lenses

22Convex lenses

The eye

• Refraction can cause parallel light rays to converge to afocus

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• The focal plane is where light from different directions comesinto focus.

• The image behind a single (convex) lens is actually upside-down! Your brain flips the image.

The eye

• If you have perfect vision, then the focal plane is on your retina.

• If you have weak eyesight, lenses in your spectacles furtherrefract light to adjust the focal plane to lie on the retina.

The eye

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Corrective lenses are required since eye changes with age:

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(farsighted)

Diopter =1

Focal Length (in m)

The eye

• A camera focuses light like an eye and captures the imagewith a detector (film or CCDs)

• The CCD detectors in digital cameras are similar to thoseused in modern telescopes

Digitalcamerasdetect lightwith charge-coupleddevices(CCDs)

Camera

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High quality camera lens array:

Camera

17Pictures courtesy of LEICA

Multiple lenses partially correct for various aberrations

Camera

• What are the advantages of a camera over the eye?

– Image can be reliably stored for later analysis.

– Image has more details

– Exposure time (amount of light hitting detector) can becontrolled. Faint objects can be observed with longexposure times).

• What are the advantages of CCDs over film ?

– More sensitive to light.

– Broader dynamic range: bright and faint objects can berecorded at the same time.

– Image stored as digital data that can be processed on acomputer.

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Basic designs of telescopes

• Refracting telescope:

Focuses light with lenses

• Reflecting telescope:

Focuses light with mirrors

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Refracting Telescope

Angular magnification M is large when f1 is much

greater than f2

f1f2

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Refracting Telescope

• Refracting telescopes need to be very long, to

maximize the distance between the lenses

• They have large, heavy lenses for good light collection

Reflecting Telescope

• Reflecting telescopes can have much greaterdiameters

• Modern telescopes (built after 1900) arereflectors

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Designs for Reflecting Telescopes

Small telescopes can use other focal

arrangements that would be inconvenient

in larger telescopes

Example: McMath-Pierce Solar Telescope

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Reflecting Telescope

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Example: McMath-Pierce Solar Telescope

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Reflecting Telescope

• What are the advantages of a reflecting telescope over a

refracting telescope ?

– Only the reflecting surface of mirrors in a reflecting

telescope have to be perfectly shaped. In a lens the

entire shape of the lens and both surfaces are

important.

– Objective lenses are heavy and difficult to stabilize at

the top of the telescope. Heavy mirrors at the bottom

of the telescope are less problematic

– Lenses have chromatic aberrations that must be

corrected.

Reflecting Telescope

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What are the two most important

properties of a telescope?

1. Light-collecting area: Telescopes with

a larger collecting area can gather a

greater amount of light in a shorter time.

2. Angular resolution: Telescopes that are

larger are capable of taking images with

greater detail.

Light Collecting Area

• A telescope’s diameter tells us its light-collecting area: Area = !(diameter/2)2

• The largest telescopes currently in use havea diameter of about 10 meters

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Thought QuestionHow does the collecting area of a 10-meter

telescope compare with that of a 2-meter

telescope?

a) It’s 5 times greater.

b) It’s 10 times greater.

c) It’s 25 times greater.

Thought QuestionHow does the collecting area of a 10-meter

telescope compare with that of a 2-meter

telescope?

a) It’s 5 times greater.

b) It’s 10 times greater.

c) It’s 25 times greater.

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Angular Resolution

• The minimum angularseparation that thetelescope candistinguish.

• Also called resolvingpower

Recall that Angular separation= actual separation x 360 degrees/(2! x distance)

Angular Resolution

• Ultimate limit toresolution comes frominterference of lightwaves within atelescope.

• there is blurring called

a diffraction fringe

around every point of

light in the image.

• You cannot see any

detail smaller than the

fringe.

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Angular Resolution

• This limit onangular resolutionis known as thediffraction limit

• Larger telescopesare capable ofgreater resolutionbecause there’sless interference

Angular Resolution

• Diffraction limit depends on the wavelengthof light and diameter of the telescope

Diffraction limit (degrees) =25

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!"#

$%&

2

'wavelength

diameter

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Imaging

• Astronomicaldetectorsgenerallyrecord onlyone colour oflight at a time

• Several imagesmust becombined tomake full-colour pictures

Imaging

• The furthest objects in thispicture are 11 billion light-years from Earth

• This picture is a truecolour image made fromseparate exposures takenin blue, green, and far-redlight. It required 48 orbitsaround the Earth (morethan one day of exposuretime) to make theobservation.

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Imaging

HST photograph of Mars during closest approach in 2005

Imaging

• Astronomical detectors can record forms of light our eyes

can’t see

• False-colour or colour coded images use colour to represent

- different energies of nonvisible light

- different atoms in the object

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Imaging

• Columns of cool interstellar

hydrogen gas and dust that are

also incubators for new stars.

• The color image is constructed

from three separate images: Red

shows emission from sulfur ions,

green shows emission from

hydrogen, blue shows emission

from oxygen ions.

Eagle nebula: Pillars of creation

Imaging

Hubble Space Telescope:

Greatest hits

Orion Nebula: Composite

from Spitzer and Hubble

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Imaging

Websites of the different observatories have many more

images. Check out for example

- www.spitzer.caltech.edu/Media/mediaimages/index.shtml

- www.cfht.hawaii.edu/HawaiianStarlight/

- http://hubblesite.org/gallery/

Horsehead Nebula Whirlpool Galaxy

Spectroscopy

• A spectrograph separatesthe different wavelengthsof light before they hit thedetector

• Since the light is separatedout, more total light(longer exposure times) isrequired for the sametelescope to make aspectrum than to make animage.

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Spectroscopy

• Spectroscopy givesinformation about

- composition of stars andnebulae

- temperature of stars

- motion of stars andgalaxies (Doppler shift)

How does Earth’s atmosphere

affect ground-based observations?

• The best ground-based sites for

astronomical observing are

– Calm (not too windy)

– High (less atmosphere to see through)

– Dark (far from city lights: light pollution)

– Dry (few cloudy nights)

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Light Pollution

• Scattering of human-made lightin the atmosphere is a growingproblem for astronomy

Twinkling and Turbulence

Turbulent air flow in Earth’s atmosphere distortsour view, causing stars to appear to twinkle

Star viewed with ground-

based telescope

Same star viewed with

Hubble Space

Telescope

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Calm, High, Dark, Dry

• The bestobservingsites are atopremotemountains

Summit of Mauna Kea, Hawaii

Transmission in Atmosphere

• Only radio and visible light pass easily through Earth’satmosphere

• We need telescopes in space to observe other forms

• Space telescopes also avoid the problems of lightpollution and atmospheric turbulence

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How can we observe nonvisible

light?

• A standard satellitedish is essentially atelescope forobserving radiowaves

Primary mirror Secondary mirror

Radio Telescopes

• A radio telescopeis like a giantmirror that reflectsradio waves to afocus

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IR & UV Telescopes

• Infrared and ultraviolet-light telescopes operatelike visible-light telescopes but need to be aboveatmosphere to see all IR and UV wavelengths

SOFIA Spitzer

X-Ray Telescopes• X-ray telescopes

also need to beabove theatmosphere

• Focusing of X-raysrequires specialmirrors

• Mirrors are arrangedto focus X-rayphotons throughgrazing bounces offthe surfaceChandra

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X-Ray Telescopes

• Chandra images reveal that the rings of Saturn sparkle in X-

rays (blue dots in this X-ray/optical composite).

• The likely source for this radiation is the fluorescence

caused by solar X-rays striking oxygen atoms in the water

molecules that comprise most of the icy rings.

Gamma Ray Telescopes

• Gamma raytelescopes also needto be in space

• Focusing gammarays is extremelydifficult

Compton Observatory

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How can multiple telescopes

work together?

Interferometry

• Interferometery is atechnique for linkingtwo or more telescopesso that they have theangular resolution of asingle large one

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Interferometry

• First long distanceradio interferometry(3 km) wasdemonstrated inCanada in 1967

• Current radiotelescope networkspans globe

• Now becomingpossible withinfrared and visible-light telescopes

Very Large Array (VLA), New Mexico

Interferometry

• VLA, and Arecibo werefeatured in theHollywood movieContact based on theSETI (Search forExtraterrestrialIntelligence) program

• First major Hollywoodmovie with a femalephysicist as the maincharacter !

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Summary• How does light propagate through different materials ?

– Light rays travel in straight lines that can be reflectedor refracted (bent) at an interface between twomaterials

• How does your eye form an image?

– It uses refraction to bend parallel light rays so that theyform an image.

– The image is in focus if the focal plane is at the retina.

• How do we record images?

– Cameras focus light like your eye and record the imagewith a detector.

– The detectors (CCDs) in digital cameras are like thoseused on modern telescopes

Summary

• How does a telescope work?

– Refracting telescopes focus light with lenses

– Reflecting telescopes focus light with mirrors

– The vast majority of professional telescopesare reflectors

– Collecting area determines how much light atelescope can gather

– Angular resolution is the minimum angularseparation a telescope can distinguish

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Summary

• What are telescopes used for?

– Imaging

– Spectroscopy

Summary

• How does Earth’s atmosphere affect ground-based observations?

– Telescope sites are chosen to minimize theproblems of light pollution, atmosphericturbulence, and bad weather.

• Why do we put telescopes into space?

– Forms of light other than radio and visible donot pass through Earth’s atmosphere.

– Also, much sharper images are possiblebecause there is no turbulence.