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Page 19 Mount Palomar near San Diego: Mount Palomar near San Diego: Prime focus cage and an inhabitant Prime focus cage and an inhabitant • • NOTE: NOTE: Smoking and Smoking and drinking are not drinking are not permitted in the prime permitted in the prime focus cage On web focus cage On web page of Anglo page of Anglo Australian Telescope Australian Telescope • • Until the 1970 Until the 1970 ’ ’ s, women s, women weren weren ’ ’ t permitted t permitted either either Page 20 Today Today ’ ’ s reflecting telescopes s reflecting telescopes • • Cassegrain Cassegrain focus: focus: – – Light enters from top Light enters from top – – Bounces off primary Bounces off primary mirror mirror – – Bounces off Bounces off secondary mirror secondary mirror – – Goes through hole in Goes through hole in primary mirror to primary mirror to focus focus Page 21 Examples of real telescopes Examples of real telescopes • • Backyard telescope: Backyard telescope: – – 3.8 3.8 ” ” diameter refracting lens diameter refracting lens – – Costs ~ 300 at Amazon.com Costs ~ 300 at Amazon.com – – Completely computerized: it will Completely computerized: it will find the planets and galaxies for find the planets and galaxies for you you Page 22 Largest optical telescopes in world Largest optical telescopes in world • • Twin Keck Telescopes on top of Mauna Kea Twin Keck Telescopes on top of Mauna Kea volcano in Hawaii volcano in Hawaii Page 23 36 hexagonal segments make up 36 hexagonal segments make up the full Keck mirror the full Keck mirror Page 24 Keck Keck ’ ’ s 10-meter diameter mirror is s 10-meter diameter mirror is made of 36 segments made of 36 segments Page 25 One Keck segment in storage One Keck segment in storage Page 26 Future plans are even more Future plans are even more ambitious ambitious Thirty Meter Telescope Keck Telescope Page 27 Future plans are even more Future plans are even more ambitious ambitious People Page 28 Concept of angular resolution Concept of angular resolution Car Lights Car Lights Angular resolution Angular resolution • • The ability to separate two objects. The ability to separate two objects. • • The angle between two objects decreases as your The angle between two objects decreases as your distance to them increases. distance to them increases. • • The smallest angle at which you can distinguish two The smallest angle at which you can distinguish two objects is your objects is your angular resolution angular resolution . . Page 29 How big is one arc second of How big is one arc second of angular separation? angular separation? • • A full circle on the sky contains 360 degrees A full circle on the sky contains 360 degrees or 2 or 2 π π radians radians – – Each degree is 60 arc minutes Each degree is 60 arc minutes – – Each arc minute is 60 arc seconds Each arc minute is 60 arc seconds 1 arc sec 1 arc min 60 arc sec 1 degree 60 arc min 2 radians 360 degrees ❂ ❂ 2 60 60 360 radians = 4.8 10 -6 radian = 4.8 µrad 5 µrad or 1 µrad 0.2 arc sec Page 30 What does it mean for an object to What does it mean for an object to “ “ subtend an angle subtend an angle θ ” ” ? ? θ θ is the apparent angular size of the object is the apparent angular size of the object Your eye A distant object angle θ Page 31 “ “ Small angle formula Small angle formula ” ” • • sin sin θ θ ~ ~ θ θ if if θ θ is is 1 1 radian radian • • s = d sin s = d sin θ θ ~ d ~ d θ θ • • Example: how many ar Example: how many ar c sec does a nickel subtend if it c sec does a nickel subtend if it is located 2 km away? is located 2 km away? d s θ A dime is about 1 cm across, so s d 1 cm 2 km 1 km 1000 m 1 m 100 cm = 1 2 10 5 radians 1 ➭ rad 10 -6 rad = 5 ➭ rad = 1 arc sec Page 32 Concept Question Concept Question From Earth, planet A subtends an angle of 5 arc sec, and From Earth, planet A subtends an angle of 5 arc sec, and planet B subtends an angle of 10 arc sec. If the radius planet B subtends an angle of 10 arc sec. If the radius of planet A equals the radius of planet B, then of planet A equals the radius of planet B, then a planet A is twice as big as planet B. a planet A is twice as big as planet B. b planet A is twice as far as planet B. b planet A is twice as far as planet B. c planet A is half as far as planet B. c planet A is half as far as planet B. d planet A and planet B are the same distance. d planet A and planet B are the same distance. e planet A is five times as far as planet B. e planet A is five times as far as planet B. Page 33 What do astronomers do with What do astronomers do with telescopes? telescopes? • • Imaging: Imaging: Taking digital pictures of the sky Taking digital pictures of the sky • • Spectroscopy: Spectroscopy: Breaking light into spectra Breaking light into spectra • • Timing: Timing: Measuring how light output varies with Measuring how light output varies with time time Page 34 Imaging Imaging • • Filters Filters are placed in are placed in front of a camera to front of a camera to allow only certain allow only certain colors to be imaged colors to be imaged • • Single color images Single color images are then are then superimposed to superimposed to form true color form true color images. images. Page 35 How can we see images of How can we see images of nonvisible nonvisible light? light? • • Electronic detectors such as Electronic detectors such as CCDs CCDs can can record light our eyes cant see record light our eyes cant see • • We can then represent the recorded light We can then represent the recorded light with some kind of color coding, to reveal with some kind of color coding, to reveal details that would otherwise be invisible to details that would otherwise be invisible to our eyes our eyes Page 36 Crab Nebula - supernova remnant Crab Nebula - supernova remnant where a star blew up 1000 yrs ago where a star blew up 1000 yrs ago Infra-red light Infra-red light Visible light Visible light X-rays X-rays From above the atmosphere Page 37 In principle, larger telescopes In principle, larger telescopes should give should give sharper sharper images images • • Concept of Concept of “ “ diffraction limit diffraction limit ” ” – – Smallest angle on sky that a telescope can resolve Smallest angle on sky that a telescope can resolve – – Numerically: Numerically: d = D radians where = wavelength of light, D = telescope diameter in the same units as diffraction limit = 2.5 10 5 wavelength of light diam of telescope arc seconds In same units In same units Page 38 Image of a point source seen Image of a point source seen through a circular telescope mirror through a circular telescope mirror • • Size of central spot ~ Size of central spot ~ λ λ D D Diffraction limit animation Diffraction limit animation Page 39 Example of diffraction limit Example of diffraction limit • • Keck Telescope, visible light Keck Telescope, visible light • • BUT BUT : Turbulence in the Earth : Turbulence in the Earth ’ ’ s atmosphere blurs s atmosphere blurs images, so even the largest telescopes can images, so even the largest telescopes can ’ ’ t t “ “ see see ” ” better than about 1 arc second better than about 1 arc second – – A decrease of a factor of 1 0.0125 = 80 in resolution A decrease of a factor of 1 0.0125 = 80 in resolution telescope diameter D = 10 meters wavelength of light = 5000 Angstroms = 5 10 -7 meter diffraction limit = 2.5 10 5 ✭ ✮ 5 10 -7 10 arc seconds = 0.0125 arc second Page 40 Images of a bright star, Images of a bright star, Arcturus Arcturus Lick Observatory, 1 m telescope Long exposure image Short exposure image Diffraction limit of telescope Page 41 Snapshots of turbulence, Snapshots of turbulence, Lick Observatory Lick Observatory These are all images of a star, taken with very short exposure times 100 milliseconds Page 42 How to correct for atmospheric blurring How to correct for atmospheric blurring Measure details Measure details of blurring from of blurring from “ “ guide star guide star ” ” near the object near the object you want to you want to observe observe Calculate on a Calculate on a computer the computer the shape to apply shape to apply to deformable to deformable mirror to correct mirror to correct blurring blurring Light from both guide Light from both guide star and astronomical star and astronomical object is reflected object is reflected from deformable from deformable mirror; distortions mirror; distortions are removed are removed Page 43 Infra-red images of a star, from Lick Infra-red images of a star, from Lick Observatory adaptive optics system Observatory adaptive optics system With adaptive optics No adaptive optics Page 44 Adaptive optics increases peak intensity Adaptive optics increases peak intensity of a point source of a point source Lick Observatory, Near infrared images of a star No AO With AO No AO With AO Intensity Page 45 Deformable mirror is small mirror Deformable mirror is small mirror behind main mirror of telescope behind main mirror of telescope Page 46 Mirror changes its shape because Mirror changes its shape because actuators push and pull on it actuators push and pull on it • • Actuators are glued to back of thin Actuators are glued to back of thin glass mirror glass mirror • • When you apply a voltage to an When you apply a voltage to an actuator, it expands or contracts in actuator, it expands or contracts in length, pushing or pulling on the length, pushing or pulling on the mirror mirror Page 47 Neptune in infra-red light, Neptune in infra-red light, Keck Telescope adaptive optics Keck Telescope adaptive optics Without adaptive optics With adaptive optics

2.3 arc sec