Transcript Light and Telescopes - University of Redlands

Light and Telescopes
Astronomy: The Science of Seeing
Copyright – Anglo Australian Observatory
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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 lightyears away.
4. Tell what our Galaxy look like from the outside.
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Goals
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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 (l) and frequency (n) are related:
c = ln
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What’s the Wavelength Kenneth?
• Arrow 93.1 FM
• 93.1 MHz (Mega Hertz) = 93.1 x 106 cycles/sec
c = ln
3 x 1010 cm/sec = l x 93.1 x 106 cycles/sec
l = (3 x 1010 cm/sec)/(93.1 x 106 cycles/sec)
l = 322 cm =3.22 m
How big is your radio antenna?
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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 = ln
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Radio
g-ray
UV
Optical and infrared
X-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 times D2
If D increases, then S increases by D2
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If your telescope is 3 times bigger than mine, then your telescope can
see 32 = 9 times fainter objects than mine.
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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 Limit
If D increases then  decreases by the same amount.
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If your telescope is 3 times bigger than mine then you can see 3
times smaller angles (3 times smaller objects or detail).
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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
Space - HST
– 1.0m
Ground
- KPNO 4.0m – Copyright NOAO/AURA/NSF
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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.
NPOI - optical
VLA - radio
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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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Resolution and Seeing
HST
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.
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100-inch
40-inch
200-inch
400-inch
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Homework #3
• For Wednesday 9/18:
• Read Ty8, B6.4 – 6.5
• Do B6: Review Question 15, Problems 3, 4, 5
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