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TelescopesTelescopes

How big is yours?

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GoalsGoals

• Telescopes• Angular Sizes• “Seeing”• Magnitudes

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Radio Optical and infrared

X-rayUV-ray

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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.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 46 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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Types of Telescopes

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Refractor

55Yerkes 40-inch refractor

optical

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Reflector

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Hooker 100-inch reflector

optical

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Keck twin 10-meter reflector

optical/IR

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radio

Arecibo 300-foot reflector

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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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Atmospheric Seeing

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Beat the Seeing

• Seeing degrades resolution• Can put a telescope in

space (Hubble Space Telescope)– Expensive!

• Can make interferometers (NPOI)– Complicated!

• Adaptive optics can recover resolution

55Ground - KPNO 4.0m – Copyright NOAO/AURA/NSFSpace - HST – 1.0m

Hubble Space Telescope

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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.

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Magnitude Scale

• The SMALLER the number the BRIGHTER the star!

• Every difference of 5 magnitudes is a 100X difference in BRIGHTNESS.

• Every difference of 1 magnitude = 2.5X brightness

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Magnitude vs. Brightness

Mag. Difference

Factors of 2.5 Brightness Diff.

1 2.51 = 2.5 2.5

2 2.52 = 2.5 X 2.5 6.3

3 2.53 = 2.5 X 2.5 X 2.5 16

4 2.54 = 2.5 X 2.5 X 2.5 X 2.5 40

5 2.55 = 2.5 X 2.5 X 2.5 X 2.5 X 2.5

100

6 2.56 = 2.5 X 2.5 X 2.5 X 2.5 X 2.5 X 2.5

250

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But what does itlook like from theback?