Transcript Document
Telescopes How big is yours? 5 Goals • • • • Telescopes Angular Sizes “Seeing” Magnitudes 5 5 Radio g-ray UV Optical and infrared X-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 D2 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? 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. • 1000 milliarcsecond = 1 arcsecond – Polaris is 46 milliarcseconds in diameter – An astronaut on the Moon is 2 milliarcseconds tall! 5 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 optical Yerkes 40-inch refractor 5 Reflector 5 Hooker 100-inch reflector optical 5 Keck twin 10meter reflector optical/IR 5 Arecibo 300-foot reflector radio 5 Interferometry • Combine the light from two or more telescopes to simulate the RESOLUTION of one giant telescope. NPOI - optical VLA - radio 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 Hubble Space Telescope Space – 1.0m Ground- HST - KPNO 4.0m – Copyright NOAO/AURA/NSF 5 Resolution and Seeing HST 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 1 Factors of 2.5 2.51 = 2.5 Brightness Diff. 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 5 But what does it look like from the back? 5