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Optical telescopes reflectors reflecting telescopes

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Optical Telescopes: ReflectorsReflecting telescopes use mirrors.There areprimaryandsecondarymirrors.Focal length is determined by the path the light takes reflecting off the mirrors.Reflectors have advantages over refractors.No chromatic aberration.Bigger telescopes due to increased focal length in the same amount of physical spaceand no need for massive lenses.The largest telescopes in the world are reflectorsOptical and Atmospheric LimitationsResolution= smallest details that can be separated.The longer the focal length, the better the separation of two objects or features.Diffraction, or blurring of an image, sets the best possible resolution.Thediffraction limitdepends on the ratio of wavelength to telescope aperture.Earth’s atmosphere degrades images.Astronomical seeinglimits resolution due to the atmosphere’s turbulence.Space-based telescopes do not have this problem.Adaptive opticscan help correct for this atmospheric distortion.Earth-based image quality can compete with the Hubble Space Telescope in the visiblespectrum.Collecting LightThe eye collects light and focuses an image.Suffers from poor angular resolution.The faintest we can see is limited by:Integration time: the time over which the eye can add up photons.Quantum efficiency: the likelihood that a photon falling on the retina willproduce a response.Photography opened the door to modern astronomy.Captured images on photographic plates.Increased integration time comes with longer exposures.Expensive, slow, and messy.Electronic detectors record photons onpixels.Photons create a signal in the array.CCDs = charge-coupled devices(such as digital cameras).The electronically recorded images can greatly exceed photographs in quality.
CCD is the astronomer’s detector of choice.SpectrographsSpectrographsdisperse the incoming light into its component wavelengths.Lets astronomers study thespectrumof anobject’s lightBeyond Visible LightThe atmosphere does not transmit all light.Nearly all X-ray, ultraviolet, and infrared wavelengths are blocked.Space-based telescopes are needed for these wavelengths.Radio TelescopesRadio telescopesare large, steerable parabolic dishes with antennas.Allow astronomers to study radio waves.Wavelengths of a centimeter to about 10 meters.Radio waves can pass through gas and dustSingle radio telescopes have poor resolution due to the long wavelengths.Interferometric arrayscombine the signals from many telescopes, increasing resolutionBeyond the AtmosphereAirborne observatories: raise the telescope above clouds and water vapor (infraredastronomy).Satellites in orbit: detect wavelengths that the atmosphere blocks (ultraviolet and X-rays).Can produce very sharp images (e.g., the Hubble Space Telescope).Spacecraft have visited or flown by all the classical planets.Flybysandorbitersobserve from afar.

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Term
Fall
Professor
N/A
Tags
Astronomy, Light, Gravitational Force

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