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Refracting Telescopes
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Faint Light Astronomical objects are distant and faint. –Effectively at infinity Light collection is more important than magnification. –Refraction –Reflection The Andromeda Galaxy (M31) is 3° wide. –6 times the moon –Only visible to the unaided eye in very dark conditions
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Refraction Light is bent at the surface between two media. This bending is called refraction. Lenses use refraction to bend and focus light. –focal length and a focal point –Parallel rays from a distant source focal length focal point
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Concentrator A refracting telescope uses lenses to concentrate light from a distant object. –Object light rays nearly parallel –Final image rays also parallel objectivefocal pointeyepiece
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Aperture Lenses collect and concentrate light by refraction. The diameter (D) of the objective lens is the aperture. –Measured in m or mm –Larger apertures for fainter objects The light gathering power (LGP) is related to the area of the lens. –Circular lens: A = ( D 2 )/4 –Relative to eye aperture 5 mm: LGP = D 2 /(5 mm) 2
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F-Stop The brightness of an image is measured by the focal ratio of the focal length to the aperture. –F-number or f-stop = f/D –Dimensionless quantity –Denoted by f/ Lower f-numbers are “faster” and need shorter exposure times. Primary focal length = Aperture diameter x f-number
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Magnification Refracting telescopes magnify images with two lenses. Magnification = Primary focal length Eyepiece focal length objectivefocal pointeyepiece
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Chromatic Aberration Real material refract colors differently. –Blue light bends more than red Compound lenses can compensate for chromatic aberration. Tony and Marilyn Karp Wikipedia
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Yerkes Refractor World’s largest refractor is in Wisconsin. 40 inch aperture, f/19. 63 foot tube. Yerkes 40 inch
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