5 Telescopes How big is yours?

5 Goals Telescopes Angular Sizes “Seeing” Magnitudes

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5 Radio Optical and infrared X-ray UV  -ray

5 What is the Purpose of a Telescope? 1.Increase the amount of light we see. Sensitivity is proportional to Collecting Area. S = constant times  D 2 If D increases, then S increases by D 2 If your telescope is 3 times bigger than mine, then your telescope can see 3 2 = 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?

5 Sensitivity

5 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. 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?

5 Resolution

5 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 milliarcsecond = 1 arcsecond –Polaris is 46 milliarcseconds in diameter –An astronaut on the Moon is 2 milliarcseconds tall!

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5 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.

5 Types of Telescopes

5 Refractor

5 Yerkes 40-inch refractor optical

5 Reflector

5 Hooker 100-inch reflector optical

5 Keck twin 10- meter reflector optical/IR

5 radio Arecibo 300-foot reflector

5 Interferometry Combine the light from two or more telescopes to simulate the RESOLUTION of one giant telescope. VLA - radio NPOI - optical

5 Atmospheric Seeing

5 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

5 Ground - KPNO 4.0m – Copyright NOAO/AURA/NSF Space - HST – 1.0m Hubble Space Telescope

5 HST Resolution and Seeing Neptune with the Palomar 200-inch reflector and HST

5 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.

5 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

5 Magnitude vs. Brightness Mag. Difference Factors of 2.5Brightness Diff = = 2.5 X = 2.5 X 2.5 X = 2.5 X 2.5 X 2.5 X = 2.5 X 2.5 X 2.5 X 2.5 X = 2.5 X 2.5 X 2.5 X 2.5 X 2.5 X

5 But what does it look like from the back?