ASTR 1120 ASTR 1120 General Astronomy: General Astronomy: Stars & Galaxies Stars & Galaxies � AST CLASS Learning from light: temperature (from continuum spectrum) chemical composition (from spectral lines) velocity (from Doppler shift) � ODA � Detecting light: TELESCOPES
On to Telescopes On to Telescopes Mauna Kea Observatories, Big Island, HI Mauna Kea Observatories, Big Island, HI
Imaging with our Eyes Imaging with our Eyes • pupil pupil – – allows light to enter the eye allows light to enter the eye • • lens lens – – focuses light to create an image focuses light to create an image • • retina retina – – detects the light and generates signals detects the light and generates signals • sent to brain sent to brain
Telescopes and Telescopes and cameras work much cameras work much like our eyes like our eyes
Optical Telescopes of Two Types of Two Types Optical Telescopes REFRACTOR (LENSES) REFRACTOR (LENSES) REFLECTOR (MIRRORS) REFLECTOR (MIRRORS)
World’ ’s Largest Refractor s Largest Refractor World Yerkes (1 m) (1 m) Yerkes
World’ ’s Largest Optical Reflector s Largest Optical Reflector World Twin Keck Twin Keck Telescopes (10 m) Telescopes (10 m)
Reflecting vs vs Refracting Refracting Reflecting 1. A mirror only needs a high-quality surface coating, the rest of the glass doesn’t matter • Surface can be recoated as necessary 2. Big lenses are heavy! – Big mirrors are heavy too but they can be supported from the back • Newest telescopes use multiple smaller mirrors 3. Lenses focus different colors of light at different places
Clicker Question Clicker Question Why is the largest reflector ten times Why is the largest reflector ten times larger than the largest refractor? larger than the largest refractor? A. Metal for the long tube of the refractors is too expensive. B. Reflecting telescopes are easier to clean since their mirrors are exposed. C. Lenses will crack if taken to high altitudes. D. Large mirrors are easier to make accurately than large lenses. E. Reflecting telescopes work at more wavelengths.
Clicker Question Clicker Question Why is the largest reflector ten times Why is the largest reflector ten times larger than the largest refractor? larger than the largest refractor? A. Metal for the long tube of the refractors is too expensive. B. Reflecting telescopes are easier to clean since their mirrors are exposed. C. Lenses will crack if taken to high altitudes. D. Large mirrors are easier to make accurately than large lenses. E. Reflecting telescopes work at more wavelengths.
Size DOES Matter! Size DOES Matter! 1. Light-Collecting Area Light-Collecting Area 1. Telescopes with a larger collecting area can Telescopes with a larger collecting area can • • gather a greater amount of light in a shorter gather a greater amount of light in a shorter time. time. 2. Angular Angular Resolution 2. Resolution • Telescopes that are larger are capable of taking images with greater detail.
Light Collecting Power Light Collecting Power • Think of telescope as a Think of telescope as a “ “photon bucket photon bucket” ” • • Bigger bucket = more photons more photons • Amount of light collected is directly proportional to area – The larger the telescope diameter, the more light rays it intercepts • Area � Diameter 2 – To make up for light collecting power, you can just take longer images
Angular Resolution for telescopes for telescopes Angular Resolution • The amount of fine detail that can be seen! • Expressed as the angle between two objects that can be seen as separated – SMALLER SMALLER – angle is angle BETTER BETTER WATCH OUT! WATCH OUT! – High High – resolution = resolution = small angular angular small resolution resolution
Diffraction Limit Diffraction Limit • Theoretical best angular resolution a telescope can get. • Measured in arcseconds (”) • 1 arcsec (”) • = 1/60 arcminute • = 1/3600 degree • 1 arcsec = angular size of a dime placed 2.5 miles away
Diffraction Limit Diffraction Limit • � = (2.5 x 10 5 arcsec) x � / D – ( � is light wavelength, D is mirror diameter ) is light wavelength, D is mirror diameter ) • Better (smaller angles) for shorter wavelengths, or larger telescopes
Diffraction Limit Example Diffraction Limit Example • SBO 16” telescope = 40 cm = 0.4 meters = 2/5 meter • Wavelength of green light = 500 nm = 500 x 10 -9 m = 5 x 10 -7 m Angular resolution (arcseconds) � = (2.5 x 10 5 arcsec) x (5 x 10 -7 meters) / (0.4 meters) Watch out for the units! Watch out for the units! � = 0.3 arcseconds They must match for They must match for wavelength and size of wavelength and size of telescope! telescope!
Another Diffraction Limit Example Another Diffraction Limit Example • Keck 10 meter telescope • Wavelength of green light = 500 nm = 500 x 10 -9 m = 5 x 10 -7 m Angular resolution (arcseconds) � = (2.5 x 10 5 arcsec) x (5 x 10 -7 meters) / (10 meters) Watch out for the units! Watch out for the units! � = 0.01 arcseconds They must match for They must match for wavelength and size of wavelength and size of telescope! telescope!
Practice Question Practice Question • What is the diffraction- diffraction- limited resolution of your limited resolution of your eye (~ 0.5 cm aperture) at eye a wavelength of 500 nm (yellow light)? A) 0.25 arcsec B) 2.5 arcsec C) 25 arcsec D) 2500 arcsec (0.7 degree)
• Diffraction limit Resolution = � = (2.5 x 10 5 arcsec) x (500 x 10 -9 meters) / 0.5 cm � = (2.5 x 10 5 arcsec) x (5 x 10 -7 meters) / 5 x 10 -3 meters (note the change in units!) � = 25 arcsec! ( In reality, the eye can only do about 100 arcsec at best = a dime 40 meters away)
Our Atmospheric screws Our Atmospheric screws viewing up! viewing up! • Light Pollution – 90% of the Earth’s population can not see the Milky Way on the average night
How many light bulbs does it take How many light bulbs does it take to screw up an astronomer? to screw up an astronomer? • An immediately curable pollution: simply turn the lights off! • Several famous observatories are now useless… Los Angeles basin view from Mt. Wilson Observatory, 1908 and 1998
Other sources of disturbance: Other sources of disturbance: Atmospheric turbulence Atmospheric turbulence • Atmospheric Turbulence • Very dependent on local conditions • Lousy in Boulder, where wind “breaks” Bad seeing Good seeing like a wave over town
DARK DARK DRY DRY . . CALM CALM HIGH HIGH Sites in Hawaii, Arizona, Sites in Hawaii, Arizona, Chile, Canary Islands…. Chile, Canary Islands
The Quest for Good The Quest for Good Weather and Seeing Weather and Seeing • Mauna Kea, Big Island of Hawaii, 14,000’ elevation, middle of the Pacific • Dry, high, dark and isolated. Best on the planet? • Even in the best places though, seeing is typically � ~ 0.3-0.5 arcsec
Adaptive Optics to the to the Adaptive Optics Rescue! Rescue! • Use a laser to create an artificial star and correct for the distortion caused by earth’s atmosphere – If you bounce the incoming light off a “deformable mirror” the light comes off corrected • Its like reversing the effect of a funhouse mirror
Adaptive Optics to the to the Adaptive Optics Rescue! Rescue! NEPTUNE! NEPTUNE! Images from the Keck Observatory courtesy of the NSF Center for Adaptive Optics
Atmospheric Absorption of “ “Light Light” ” Atmospheric Absorption of • Earth’s atmosphere absorbs most types of light (not entirely bad, or we would be dead!) • Only visible, radio, some IR, and some UV light get through to the ground
Radio telescopes Radio telescopes • RADIO WAVES RADIO WAVES : most • most get through get through – Thus radio telescopes are Thus radio telescopes are – built on the ground built on the ground • Weather is not an issue - • Weather is not an issue - radio waves come right radio waves come right through the clouds through the clouds • But poor angular • But poor angular resolution resolution – Why? Why? – • VERY long wavelengths! VERY long wavelengths! •
Interferometry Interferometry • Join multiple telescopes together to simulate simulate one large telescope. • Only perfected at radio wavelengths: Very Large Array (VLA) in New Mexico has 27 dishes across a 40 km valley – D=40 km = 4 x 10 4 m • Recent initial success using the two Keck telescopes as an infrared interferometer.
Can we go even bigger? YES! Can we go even bigger? YES! • Very Large Baseline Array: VLBA is an array of ten 25-meter telescopes • Resolutions as small as 0.001 arcseconds for radio light • Other observing campaigns use observations from around the world, synchronized by atomic clocks Space interferometry is coming….
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