ASTR 1120 On to Telescopes General Astronomy: Stars & Galaxies � AST CLASS Learning from light: temperature (from continuum spectrum) chemical composition (from spectral lines) velocity (from Doppler shift) � ODA � Detecting light: TELESCOPES Mauna Kea Observatories, Big Island, HI Imaging with our Eyes Telescopes and • pupil – allows light to enter the eye cameras work much • lens – focuses light to create an image like our eyes • retina – detects the light and generates signals sent to brain
World’s Largest Refractor Optical Telescopes of Two Types REFRACTOR (LENSES) Yerkes (1 m) REFLECTOR (MIRRORS) World’s Largest Optical Reflector Reflecting vs Refracting 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 Twin Keck Telescopes (10 m)
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 A. Metal for the long tube of the refractors is too expensive. expensive. B. Reflecting telescopes are easier to clean B. Reflecting telescopes are easier to clean since their mirrors are exposed. since their mirrors are exposed. C. Lenses will crack if taken to high altitudes. C. Lenses will crack if taken to high altitudes. D. Large mirrors are easier to make accurately D. Large mirrors are easier to make accurately than large lenses. than large lenses. E. Reflecting telescopes work at more E. Reflecting telescopes work at more wavelengths. wavelengths. Size DOES Matter! Light Collecting Power 1. Light-Collecting Area • Think of telescope as a “photon bucket” Telescopes with a larger collecting area can • • Bigger bucket = more photons gather a greater amount of light in a shorter • Amount of light collected is directly time. proportional to area 2. Angular Resolution – The larger the telescope diameter, the more light rays it intercepts • Telescopes that are larger are capable of • Area � Diameter 2 taking images with greater detail. – To make up for light collecting power, you can just take longer images
Angular Resolution for telescopes Diffraction Limit • The amount of fine detail that • Theoretical best angular can be seen! resolution a telescope can get. • Expressed as the angle between • Measured in two objects that arcseconds (”) can be seen as separated – SMALLER • 1 arcsec (”) angle is • = 1/60 arcminute BETTER • = 1/3600 degree WATCH OUT! – High resolution = • 1 arcsec = angular size small angular of a dime placed 2.5 resolution miles away Diffraction Limit Diffraction Limit Example • SBO 16” telescope = 40 cm = 0.4 meters = 2/5 meter • � = (2.5 x 10 5 arcsec) x � / D • Wavelength of green light – ( � is light wavelength, D is mirror diameter ) = 500 nm = 500 x 10 -9 m = 5 x 10 -7 m • Better (smaller angles) for shorter Angular resolution (arcseconds) wavelengths, or larger telescopes � = (2.5 x 10 5 arcsec) x (5 x 10 -7 meters) / (0.4 meters) Watch out for the units! � = 0.3 arcseconds They must match for wavelength and size of telescope!
Practice Question Another Diffraction Limit Example • What is the diffraction- • Keck 10 meter telescope limited resolution of your • Wavelength of green light eye (~ 0.5 cm aperture) at = 500 nm = 500 x 10 -9 m a wavelength of 500 nm = 5 x 10 -7 m (yellow light)? A) 0.25 arcsec Angular resolution (arcseconds) B) 2.5 arcsec � = (2.5 x 10 5 arcsec) x (5 x 10 -7 meters) / (10 meters) C) 25 arcsec Watch out for the units! � = 0.01 arcseconds They must match for D) 2500 arcsec (0.7 degree) wavelength and size of telescope! Our Atmospheric screws viewing up! • Diffraction limit Resolution = • Light Pollution � = (2.5 x 10 5 arcsec) x (500 x 10 -9 meters) / 0.5 cm – 90% of the Earth’s population can not see the Milky � = (2.5 x 10 5 arcsec) x (5 x 10 -7 meters) / 5 x 10 -3 meters Way on the average night (note the change in units!) � = 25 arcsec! ( In reality, the eye can only do about 100 arcsec at best = a dime 40 meters away)
How many light bulbs does it take Other sources of disturbance: to screw up an astronomer? Atmospheric turbulence • Atmospheric • An immediately curable Turbulence pollution: simply turn the lights off! • Very dependent on • Several famous local conditions observatories are now useless… • Lousy in Boulder, where wind “breaks” Los Angeles basin view Bad seeing Good seeing from Mt. Wilson like a wave over town Observatory, 1908 and 1998 The Quest for Good DARK Weather and Seeing DRY . CALM HIGH • Mauna Kea, Big Island of Hawaii, 14,000’ elevation, middle of the Pacific Sites in Hawaii, Arizona, • Dry, high, dark and Chile, Canary Islands…. isolated. Best on the planet? • Even in the best places though, seeing is typically � ~ 0.3-0.5 arcsec
Adaptive Optics to the Adaptive Optics to the Rescue! Rescue! NEPTUNE! • 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 Images from the Keck Observatory courtesy of the NSF Center for Adaptive Optics Atmospheric Absorption of “Light” Radio telescopes • Earth’s atmosphere absorbs most types of light • RADIO WAVES : most (not entirely bad, or we would be dead!) • Only visible, radio, some IR, and some UV light get get through through to the ground – Thus radio telescopes are built on the ground • Weather is not an issue - radio waves come right through the clouds • But poor angular resolution – Why? • VERY long wavelengths!
Interferometry Can we go even bigger? YES! • Join multiple telescopes together to simulate one • Very Large Baseline Array: VLBA is an array of ten 25-meter large telescope. telescopes • Only perfected at radio wavelengths: Very Large • Resolutions as small as 0.001 Array (VLA) in New arcseconds for radio light Mexico has 27 dishes • Other observing campaigns use across a 40 km valley observations from around the – D=40 km = 4 x 10 4 m world, synchronized by atomic clocks • Recent initial success using the two Keck Space interferometry is coming…. telescopes as an infrared interferometer. Infrared Telescopes For other wavelengths we have to get above • INFRARED can be the atmosphere absorbed by molecules like H 2 0, CO 2 , CO, etc. • UV, X-rays, Gamma Rays • Absorption is in specific wavebands, leaving – These all have enough energy to ionize electrons in “windows” where we can atoms or break apart see through the molecules atmosphere • Heavily absorbed by the atmosphere • Methods: balloons, • Combination of ground- based, airplane, balloon, rockets, and the Space rockets, satellite Shuttle
Hubble Space Telescope: NASA’s most famous observatory • Launched in 1990 – Error in mirror made blurry images • Corrective optics installed in 1993 (Ball Aerospace here in Boulder) • Small (only 2.5 meters) but diffraction-limited • Low orbit accessible by Shuttle, refurbishing missions mean long lifetime (1990 to 2008+) • $5 billion over 20 years = 10-100 times more than ground-based telescope NASA’s Great Observatories X-ray telescopes Spitzer Space Telescope Infrared • Difficult to focus X-rays; They penetrate or are Compton Gamma Ray Observatory absorbed • Glancing angles scatter X-rays, bringing them to a focus to make an image Hubble Space Telescope UV/Visible • Always false color! Chandra X-Ray Observatory
Imaging (Digital with CCDs) Instruments in the Focal Plane How astronomers use light collected by a telescope: 1. Imaging • Filters are placed in front – use camera to take pictures (images) of camera to allow only – photometry � measure amount and color certain colors to be (with filters) of light from object imaged 2. Spectroscopy • Single color images are – use spectrograph to separate light in detail superimposed to form into its different wavelengths (colors) “true color” images. 3. Timing – measure how amount of light changes with time (sometimes in a fraction of a second) Spectroscopy – analyzing the light Timing • Spectrograph reflects light off a diffraction grating : Diffraction finely ruled, smooth Light from grating breaks surface only one star light into • Light (by enters spectrum interference) disperses into colors Detector • This spectrum is records recorded by digital • A light curve represents a series of brightness spectrum CCD detector measurements made over a period of time
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