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. ii rr tt

4. Radius of Curvature 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 R f

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 objectivefocal pointeyepiece

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 = (  D 2 )/4 –Intensification relative to eye aperture 5 mm: LGP = D 2 /(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. Fraunhofer Diffraction A single narrow slit creates diffraction. –No minimum for m = 0

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

10. Angular Resolution 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 J 1

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 objectivefocal pointeyepiece

12. Magnification The eyepiece magnifies the intermediate image. The total magnification is the product from both lenses. objectivefocal pointeyepiece

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

14. Chromatic Aberration 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. Airn(589 nm) =1.00029 Crown glass1.52 Flint glass1.66

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

16. Curved Mirror Light that begins at one focus of an elliptical mirror converges at the other focus. –A parabola for a focus at infinity 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. primary mirror secondary diagonal 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 eyepiece Schmidt corrector lens

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. Telescope Advantages 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

25. Altazimuth Mount Telescope mounts should permit two directions of motion. Altazimuth mounts directly control altitude and azimuth. azimuth control altitude 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. polar axis declination 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.Hubble It has no atmosphere to peer through. The onboard computer gives it enhanced optics. There are four different cameras for different views.

30. Infrared and Ultraviolet 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 M81 from GALEX