Slide Number 1Magnitude SystemSlide Number 3Slide Number 4InitiallySlide Number 6Slide Number 7Slide Number 8Why are Telescopes better than your eyes?Slide Number 10Slide Number 11Slide Number 12Slide Number 13All large modern telescopes are reflectorsIt isAlso,AlsoSlide Number 18AlsoSize of a telescopeSlide Number 21Slide Number 22AtmosphereTwinklingSlide Number 25Advantages of space-based telescopesSlide Number 27Slide Number 28Slide Number 29Not all light from a star reaches EarthLight in space can be affected by dustSlide Number 32It does not helpTo measure lightSlide Number 35CCDsHow do they work?Color separation for digital camerasCCDsHubble TelescopeHubble (launched in 1990)Slide Number 42InitiallySlide Number 44JupiterHubble replacementSlide Number 47Any Questions?Astronomy 100Exploring the UniverseTuesday, Wednesday, ThursdayTom [email protected] System• Brighter –lower numberhttp://www.astronomynotes.com/starprop/appmag.gif4 Vestabrightest asteroidMagnitude difference Relative intensity0 11 2.512 6.313 15.84 39.85 10010 10415 106Initially• Everybody observed with their eyesFigure 7.1Figure 7.2aParallel light LensFigure 7.2bWhy are Telescopes better than your eyes?• They can observe light in different wavelength regions (eyes can only see visible light)• They can collect more light than eyes•They can be built to compensate for the distorting effects of the atmosphereFigure 7.6Refracting telescopeReflecting TelescopeReflecting TelescopesResulting image invertedAll large modern telescopes are reflectors• Since light passes through the lens of a refracting telescope,• You need to make the lens from clear, high-quality glass with precisely shaped surfacesIt is• Its easier to make a high-quality mirror than a lensAlso,• Large lenses are extremely heavyAlso• Lens focuses red and blue light slightly differently• Called chromatic aberrationhttp://en.wikipedia.org/wiki/File:Lens6a.svg• Light changes speed as it moves from one medium to another• The degree of bending of the light's path depends on the angle that the incident beam of light makes with the surface, and on the ratio between the refractive indices of the two media.• The refractive index of many materials (such as glass) varies with the wavelength of the light usedAlso• Light can be absorbed by the glass as it passes through the glass• Minor problem for visible, but severe for ultraviolet and infrared lightSize of a telescope• Diameter of its primary mirror or lens• Light collecting area is proportional to the diameter squared since• Collecting area = π r2• E.g., 8-meter telescope• Telescope that took image b is twice as big as telescope that took image a• Larger the telescope, more detail can be seena b• Telescope on Mauna Kea (14,000 feet high)• Telescope is Japanese Subaru 8-m telescopeAtmosphere• Atmosphere can absorb light• Atmosphere can scatter light•Atmosphere can distort light (twinkling)Twinkling• Twinkling of stars is caused by moving air currents in the atmosphere. • The beam of light from a star passes through many regions of moving air while on its way to an observer’s eye or telescope. • Each atmospheric region distorts the light slightly for a fraction of a second.Advantages of space-based telescopes• It can be open 24 hours, 7 days of week• Do not have to worry about distorting effects of atmosphere • There is no extra background of light due to scattering of light in the Earth’s atmosphere• Observe in more wavelength regionsFigure 7.20http://www.scienzagiovane.unibo.it/English/radio-window/images/radiazioni-em.jpg• Infrared light absorbed by moleculeshttp://www.ucar.edu/learn/1_3_1.htmNot all light from a star reaches EarthLight in space can be affected by dusthttp://www.ipac.caltech.edu/2mass/outreach/survey.htmlhttp://en.wikipedia.org/wiki/File:Rayleigh_sunlight_scattering.pngIt does not help• That you are closer to the starsTo measure light• In the past, they used photographic plates• Now they use CCDs (charge-coupled devices)• CCD are electronic detectors•CCDs are chips of siliconsFigure 7.5CCDs• CCDs convert light into electronsWilliam Boyle George SmithShared the 2009Physics Nobel Prizefor their discoveryHow do they work?• The CCD is made up of pixels.• As the light falls on each pixel, the photons become electrons due to the photoelectric effect. The photoelectric effect happens when photons of light hit the silicon of the pixel and knock electrons out of place. • These electrons are then stored.• Essentially, the charge in each row is moved from one site to the next, a step at a time. This has been likened to a “bucket row” or human chain, passing buckets of water down a line. • As these buckets of electrons reach the end of the line they are dumped out and measured, and this analog measurement is then turned into a digital value. • Thus, a digital grid is made which describes the image.Color separation for digital cameras• Colored filtersCCDs• CCDs can collect 90% of photons that strike them• Photographic plates can only collect 10% of the photons• CCDs are split into squares called pixels•Data is in electronic formHubble Telescope• Can observe in visible, infrared, and ultraviolet wavelength regions•Named after Edwin Hubble, the father of modern cosmologyHubble (launched in 1990)Telescope is the size of a school bus2.4 m mirrorInitially• Hubble’s primary mirror was polished to the wrong shape • Was too flat at the edges• Was barely 2.3 micrometers out from the required shape (1/50 the width of a human hair)• Images were not focused as well as they could be• Later shuttle mission fixed this problem by installing a number of small mirrorshttp://dayton.hq.nasa.gov/IMAGES/SMALL/GPN-2002-000064.jpgJupiterHubble replacement• The first major components of the new James Webb Space Telescope are now being assembled.• While Hubble is the size of a bus, the new James Webb will be the size of a jetliner. • Will launch in 2014• James Webb is a former NASA administrator during the Apollo program• http://video.pbs.org/video/1456686369/Any