ASTR 1120ASTR 1120General Astronomy:General Astronomy:Stars & GalaxiesStars & Galaxies!AST CLASS"ODA#Learning from light: temperature (from continuum spectrum) chemical composition (from spectral lines) velocity (from Doppler shift) Detecting light:TELESCOPESMauna Kea Observatories, Big Island, HIMauna Kea Observatories, Big Island, HIOn to TelescopesOn to TelescopesImaging with our EyesImaging with our Eyes••pupilpupil –– allows light to enter the eye allows light to enter the eye••lenslens –– focuses light to create an image focuses light to create an image••retinaretina –– detects the light and generates signals detects the light and generates signalssent to brainsent to brainTelescopes andTelescopes andcameras work muchcameras work muchlike our eyeslike our eyesREFRACTOR (LENSES)REFRACTOR (LENSES)REFLECTOR (MIRRORS)REFLECTOR (MIRRORS)Optical TelescopesOptical Telescopes of Two Typesof Two TypesYerkes Yerkes (1 m)(1 m)WorldWorld’’s Largest Refractors Largest RefractorTwin KeckTwin KeckTelescopes (10 m)Telescopes (10 m)WorldWorld’’s Largest Optical Reflectors Largest Optical ReflectorReflecting Reflecting vs vs RefractingRefracting1. A mirror only needs a high-quality surfacecoating, the rest of the glass doesn’t matter• Surface can be recoated as necessary2. Big lenses are heavy!– Big mirrors are heavy too but they can besupported from the back• Newest telescopes use multiple smaller mirrors3. Lenses focus different colors of light atdifferent placesWhy is the largest reflector ten timesWhy is the largest reflector ten timeslarger than the largest refractor?larger than the largest refractor?A. Metal for the long tube of the refractors is tooexpensive.B. Reflecting telescopes are easier to cleansince their mirrors are exposed.C. Lenses will crack if taken to high altitudes.D. Large mirrors are easier to make accuratelythan large lenses.E. Reflecting telescopes work at morewavelengths.Clicker QuestionClicker QuestionWhy is the largest reflector ten timesWhy is the largest reflector ten timeslarger than the largest refractor?larger than the largest refractor?A. Metal for the long tube of the refractors is tooexpensive.B. Reflecting telescopes are easier to cleansince their mirrors are exposed.C. Lenses will crack if taken to high altitudes.D. Large mirrors are easier to make accuratelythan large lenses.E. Reflecting telescopes work at morewavelengths.Clicker QuestionClicker QuestionSize DOES Matter!Size DOES Matter!1.1.Light-Collecting AreaLight-Collecting Area••Telescopes with a larger collecting area canTelescopes with a larger collecting area cangather a greater amount of light in a shortergather a greater amount of light in a shortertime.time.2.2.Angular Angular ResolutionResolution• Telescopes that are larger are capable oftaking images with greater detail.Light Collecting PowerLight Collecting Power••Think of telescope as a Think of telescope as a ““photon bucketphoton bucket””• Bigger bucket = more photonsmore photons• Amount of light collected is directlyproportional to area– The larger the telescope diameter, the more lightrays it intercepts• Area ! Diameter2– To make up for light collecting power, you can justtake longer imagesAngular ResolutionAngular Resolution for telescopes for telescopes• The amount offine detail thatcan be seen!• Expressed as theangle betweentwo objects thatcan be seen asseparated––SMALLERSMALLERangle angle isBETTERBETTERWATCH OUT!WATCH OUT!––HighHighresolution =resolution =smallsmall angularangularresolutionresolutionDiffraction LimitDiffraction Limit• Theoretical best angularresolution a telescopecan get.• Measured inarcseconds (”)• 1 arcsec (”)• = 1/60 arcminute• = 1/3600 degree• 1 arcsec = angular sizeof a dime placed 2.5miles awayDiffraction LimitDiffraction Limit• " = (2.5 x 105 arcsec) x # / D– (# is light wavelength, D is mirror diameter )is light wavelength, D is mirror diameter )• Better (smaller angles) for shorterwavelengths, or larger telescopes• SBO 16” telescope = 40 cm = 0.4meters = 2/5 meter• Wavelength of green light= 500 nm = 500 x 10-9 m= 5 x 10-7 mAngular resolution (arcseconds)" = (2.5 x 105 arcsec) x (5 x 10-7 meters)/ (0.4 meters)" = 0.3 arcsecondsWatch out for the units!Watch out for the units!They must match forThey must match forwavelength and size ofwavelength and size oftelescope!telescope!Diffraction Limit ExampleDiffraction Limit Example• Keck 10 meter telescope• Wavelength of green light = 500 nm = 500 x 10-9 m= 5 x 10-7 mAngular resolution (arcseconds)" = (2.5 x 105 arcsec) x (5 x 10-7 meters)/ (10 meters)" = 0.01 arcsecondsWatch out for the units!Watch out for the units!They must match forThey must match forwavelength and size ofwavelength and size oftelescope!telescope!Another Diffraction Limit ExampleAnother Diffraction Limit Example• What is the diffraction-diffraction-limited resolution of yourlimited resolution of youreyeeye (~ 0.5 cm aperture) ata wavelength of 500 nm(yellow light)?A) 0.25 arcsecB) 2.5 arcsecC) 25 arcsecD) 2500 arcsec (0.7 degree) Practice QuestionPractice Question• Diffraction limit Resolution =" = (2.5 x 105 arcsec) x (500 x 10-9 meters) / 0.5 cm" = (2.5 x 105 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 screwsOur Atmospheric screwsviewing up!viewing up!• Light Pollution– 90% of the Earth’s population can not see the MilkyWay on the average nightHow many light bulbs does it takeHow many light bulbs does it taketo screw up an astronomer?to screw up an astronomer?• An immediately curablepollution: simply turn thelights off!• Several famousobservatories are nowuseless…Los Angeles basin viewfrom Mt. WilsonObservatory, 1908 and1998Other sources of disturbance:Other sources of disturbance:Atmospheric turbulenceAtmospheric turbulence• AtmosphericTurbulence• Very dependent onlocal conditions• Lousy in Boulder,where wind “breaks”like a wave over townBad seeing Good seeing..Sites in Hawaii, Arizona,Sites in Hawaii, Arizona,Chile, Canary IslandsChile, Canary Islands….DARKDARKDRYDRYCALMCALMHIGHHIGH• Mauna Kea, Big Island ofHawaii, 14,000’ elevation,middle of the Pacific• Dry, high, dark andisolated. Best on theplanet?• Even in the best placesthough, seeing is