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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
3
!"#
$%&
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.