33Copyright – Anglo Australian Observatory
Light and TelescopesLight and Telescopes
Astronomy: The Science of Seeing
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How do you do Astronomy?
• How do Chemists do Chemistry?– Make solutions, mix chemicals …
• How do Biologists do Biology?– Breed fruit flies, (and whatever else
biologists do).
• They devise and conduct experiments in their labs.
• But how do you do that for astronomy?
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Light
• Astronomy is a “passive” science.• We can’t (yet) go to the stars or
other galaxies.
•The Universe must come to us.•We rely on light exclusively!
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What you see is all you get!
• So you need to squeeze EVERY last drop of information out of the light we get.
• This semester we’ll see how we can use light to:
1. Weigh a planet.
2. Take a star’s temperature.
3. Tell what’s in the center of a star a thousand light-years away.
4. Tell what our Galaxy look like from the outside.
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GoalsGoals
• What is light?• What are the different types of light?• What is the purpose of a telescope?• What can we see with a telescope?
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The “Visible” Spectrum
• When you think of “light”, what do you think of?
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What is Light?
• Light is a wave of energy.• Moves through a vacuum.• Travels at the speed of light (a
CONSTANT): c = 3 x 1010 cm/s
• The wavelength () and frequency () are related:
c =
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What’s the Wavelength Kenneth?
• Arrow 93.1 FM• 93.1 MHz (Mega Hertz) = 93.1 x 106
cycles/secc = 3 x 1010 cm/sec = x 93.1 x 106 cycles/sec
= (3 x 1010 cm/sec)/(93.1 x 106 cycles/sec) = 322 cm =3.22 m
How big is your radio antenna?
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To Sum Up…To Sum Up…
• Radio waves, microwaves, rainbows, UV waves, x-rays, etc are ALL forms of light (electromagnetic waves).
• They ALL travel through space at the speed of light. c
• The higher the frequency, the shorter the wavelength. c =
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Radio Optical and infrared
X-rayUV-ray
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Types of Telescopes
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What is the Purpose of a Telescope?
1. Increase the amount of light we see.Sensitivity is proportional to Collecting Area (area of the
circular opening of the telescope).S = constant timesD2
If D increases, then S increases by D2
• If your telescope is 3 times bigger than mine, then your telescope can see 32 = 9 times fainter objects than mine.
• Can you read a book at night? What’s the faintest star you can see with your naked eye?
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Sensitivity
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What is the Purpose of a Telescope?
2. Increase the detail (resolution) we see.Resolution is inversely proportional to Telescope
Diameter. constant times 1/D Diffraction LimitIf D increases then decreases by the same amount.
• If your telescope is 3 times bigger than mine then you can see 3 times smaller angles (3 times smaller objects or detail).
• Can you read a street sign a block away? Can you see the binary star in the Big Dipper with your naked eye?
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Resolution
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Angles
• The sky is 360 arc degrees around.• 60 arcminutes = 1 arc degree
– The Full Moon is about half an arc degree = 30 arcminutes.
• 60 arcseconds = 1 arcminute– Mars is about 2 arcminutes now.
• 1000 milliarcsecond = 1 arcsecond– Polaris is 3 milliarcseconds in diameter– An astronaut on the Moon is 2 milliarcseconds
tall!
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Angular Size
• Angular size: How big does something look as viewed from the Earth?– During a solar eclipse, the Moon looks big
enough to cover the Sun.
• The Sun is a million times larger than the Earth.
• The Moon is a fourth the size of the Earth.• The distance from the Earth determines
their ANGULAR SIZE.
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Atmospheric Seeing
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Beat the Seeing
• Seeing degrades resolution
• Can put a telescope in space (Hubble Space Telescope)
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Hubble Space Telescope
Ground - KPNO 4.0m – Copyright NOAO/AURA/NSFSpace - HST – 1.0m
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Beat the Seeing
• Seeing degrades resolution
• Can put a telescope in space (Hubble Space Telescope)– Expensive!
• Interferometers (NPOI, VLA)
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Interferometry• Combine the light from two or more
telescopes to simulate the RESOLUTION of one giant telescope.
VLA - radio
NPOI - optical
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Beat the Seeing
• Seeing degrades resolution• Can put a telescope in space
(Hubble Space Telescope)– Expensive!
• Interferometers (NPOI, VLA)– Complicated!
• Adaptive optics can recover resolution
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HST
Resolution and Seeing
Neptune with the Palomar 200-inch reflector and HST
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Seeing and Magnification• Larger than a few inches, a telescope’s
resolution stops getting better due to seeing.
• Don’t be fooled by advertisements claiming huge magnification increases!– “Amazing 500X magnification!”
• But sensitivity ALWAYS increases with bigger telescopes.
3340-inch
100-inch
200-inch 400-inch
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Homework #3Homework #3
• For Wednesday 9/18:• Read Ty8, B6.4 – 6.5• Do B6: Review Question 15, Problems
3, 4, 5