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Optical Telescope

Optical Telescope. Faint Light. Astronomical objects are distant and faint. Effectively at infinity Light collection is more important than magnification. Refraction Reflection. The Andromeda Galaxy (M31) subtends 3 ° . 6 times the moon

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Optical Telescope

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  1. Optical Telescope

  2. Faint Light • Astronomical objects are distant and faint. • Effectively at infinity • Light collection is more important than magnification. • Refraction • Reflection • The Andromeda Galaxy (M31) subtends 3°. • 6 times the moon • Only visible to the unaided eye in very dark conditions

  3. Refraction • Light is bent at the surface between two media. • Index of refraction n • Refraction is governed by Snell’s law. qi qt qr

  4. Lenses shaped like parts of spheres are easy to make. Easy to calculate rays Use Snell’s Law on a small part of a sphere. Radius of curvature R Focal length f Index for air is 1 Radius of Curvature f R

  5. Refracting Telescope • A refracting telescope is designed to concentrate light from a distant object. • Object light rays nearly parallel • Final image rays also parallel objective focal point eyepiece

  6. Aperture • Lenses collect and concentrate light. • 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 = (D2)/4 • Intensification relative to eye aperture 5 mm: LGP = D2/(5 mm)2

  7. 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.

  8. A single narrow slit creates diffraction. No minimum for m = 0 Fraunhofer Diffraction

  9. Fraunhofer patterns are symmetric around the opening. A circular hole produces rings around a central maximum. 84% of energy in center Airy Disk

  10. The limit of resolution is set by the aperture. The Rayleigh criterion is calculated from the first minimum of the Airy disk. Aperture radius a Wavenumber k Bessel function J1 Angular Resolution

  11. Tube Length • The intermediate image at the focal point is a real image. • Long tube accommodates long focal length • Parallel ray image related to the focal length objective focal point eyepiece

  12. Magnification • The eyepiece magnifies the intermediate image. • The total magnification is the product from both lenses. objective focal point eyepiece

  13. Yerkes Refractor • The world’s largest refractor is in Wisconsin. • 40 inch aperture, f/19 • 63 foot tube Yerkes 40 inch

  14. The index of refraction depends on the wavelength. Longer wavelengths - lower indexes Blue light bends more than red Compound lenses can compensate for chromatic aberration. Chromatic Aberration • Air n(589 nm) =1.00029 • Crown glass 1.52 • Flint glass 1.66

  15. A spherical surface does not focus all parallel lines to the same point. Aspheric lenses can be used to correct the aberration . Spherical Aberration f

  16. Light that begins at one focus of an elliptical mirror converges at the other focus. A parabola for a focus at infinity focus Curved Mirror focus

  17. Parabolic Mirror • A perfect parabolic mirror has a focal length like a lens. • All wavelengths are focused to the same point. • No chromatic aberration • The size of the mirror dish is the aperture. focal length focal point

  18. Newtonian Reflector • For viewing ray should be parallel on exit. • Combined primary mirror and eyepiece • The reflecting telescope is cheaper, because a mirror is easier to make than a lens for a given size. secondary diagonal mirror primary mirror eyepiece

  19. Schmidt-Cassegrain Reflector • A Cassegrain focus uses a flat mirror to make the tube up to three times longer. • Spherical aberration from extra mirror • Aspheric Schmidt lens corrects aberration Schmidt corrector lens eyepiece

  20. Keck Reflector • World’s largest reflector is in Hawaii. • 400 inch aperture, f/1.75 • Focal length 57.4 feet. • Telescope height 81 feet. Keck Observatory

  21. Coma • Parabolic mirrors focus precisely for rays parallel to the central axis. • The distortion for off axis objects is called coma. • Greatest for low f-numbers • Lenses can correct for the coma. Starizona.com

  22. Atmospheric Absorption • The atmosphere absorbs radiation, except at visible light, infrared, and radio frequencies.

  23. Adaptive Optics • The moving atmosphere disturbs images. • Wavefront distortions • Real time corrections are made by feedback to a deformable mirror. • Sample wavefront from beam splitter • Measure distortion • Compute necessary compensation for mirrors

  24. REFRACTOR Superb resolution Good for detail Rugged alignment Transports well REFLECTOR Inexpensive optics Large aperture Good for dim objects Uniform treatment of colors SCHMIDT-CASSEGRAIN Portable size Combines best optical qualities Good for photography Telescope Advantages

  25. Altazimuth Mount • Telescope mounts should permit two directions of motion. • Altazimuth mounts directly control altitude and azimuth. altitude control azimuth control

  26. Equatorial Mount • Altazimuth mounts do not track with the star’s movement. • Equatorial mounts are oriented to the pole. • Allows control of declination and right ascension. declination axis polar axis

  27. Charge-Coupled Device • The CCD is an array of photosensitive semiconductor capacitors. • Charge stored proportional to light intensity • Transfers charge as a shift register • Amplifier on last capacitor converts charge to voltage Hammamatsu.com

  28. Telescope CCDs • CCDs are sensitive to light from ultraviolet to infrared. • CCDs are very efficient. • Can be sensitive to individual photons • Sensitivity to thermal noise and cosmic rays can blur an image. • Multiple exposures are averaged to get correct image. • Dark frame closed shutter

  29. Hubble Space Telescope • The Hubble is an orbiting reflector telescope. • It has no atmosphere to peer through. • The onboard computer gives it enhanced optics. • There are four different • cameras for different views.

  30. Infrared is absorbed by water vapor. Observe at high altitude Satellite telescopes avoid the atmosphere. IRAS (1983) - first evidence of planets around other stars Spitzer Space Telescope (2003-9). Ultraviolet is largely absorbed by the atmosphere. Requires satellites HST, GALEX Infrared and Ultraviolet M81 from GALEX

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