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lecture4 - Large, Modern Telescopes ! Why large ?! 2...

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Large, Modern Telescopes Why large ? 2 reasons: Light gathering power increases as area ( ! r 2 ) -- Palomar 200” tel. can observe (200/14) 2 ~200 times faster than the Robinson 14in tel. Angular resolution varies as 1.2 " /D “diffraction limit” of tel. : ~ " /D ==> at " = 500 nm w/ 5m (200in) aperture, angular resolution ~ 0.02 arcsec (!) a dime at 100 km distance ! from the ground, for " < 10 microns (10,000nm) , the diffraction limit is never achieved without drastic measures. Variable refraction in the atmosphere ==> “seeing” w/ ~ 0.5 - 1 arcsec Must go to space or use adaptive optics to correct for seeing aberrations (image distortions in optics of tel.) : chromatic -- due to index of refraction depending on wavelength of light -- problem for refractors ==> focus depends on " (see color fringes) spherical -- reFector surface should be parabolic but is not (due to gravity) => light focus at different distances across aperture most large tel. are reFectors (not refractors) : can support mirror across entire back glass does not have to be perfect in interior no chromatic aberrations only choice at radio wavelengths Telescope support need to point to desired declination and then track steadily as celestial sphere rotates above earth classic solution : 2 axes -- polar and equatorial modern solution :
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lecture4 - Large, Modern Telescopes ! Why large ?! 2...

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