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