PHY2049ch36B%284-19-10%29

PHY2049ch36B(4-19- - Wave Interference and Diffraction Part 3 Telescopes and Interferometry PHY 2049 Physics 2 with Calculus PHY 2049 Chapter 36 1

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Unformatted text preview: Wave Interference and Diffraction Part 3: Telescopes and Interferometry PHY 2049 Physics 2 with Calculus PHY 2049: Chapter 36 1 Telescopes: Purpose is Light Collection Pupil of eye D 8mm (in very dim light) 106 Largest telescope (Keck) has D = 10m Ratio of areas = (10/0.008)2 = 1.5 Can collect light for hours rather than 0.1 sec More sensitive light collectors (CCD arrays) Thus telescopes are several billion times more sensitive Can see near the end of the known universe PHY 2049: Chapter 36 2 Telescope Construction All large telescopes are reflectors: Why? Mirror only needs single high quality surface (lens needs perfect volume since light passes through it) No chromatic aberration (no lens for refracting) Full support for mirror, no distortion from moving PHY 2049: Chapter 36 3 Keck Telescope (D = 10 m) PHY 2049: Chapter 36 4 Keck Hexagonal Mirror (36 Segments) PHY 2049: Chapter 36 5 Keck Primary Mirror (Each segment 1.8 m) Hole PHY 2049: Chapter 36 6 Main Limitation on Earth: Atmosphere Air cells in atmosphere Air cells above telescope mirror cause distortion of light Best performance is 0.25” – 0.5” resolution on the ground This is why telescopes are sited on high mountains “Adaptive optics” just beginning to offset this distortion PHY 2049: Chapter 36 7 Diffraction Through Circular Opening Intensity of light after passing through a circular opening. Spreading caused by diffraction. PHY 2049: Chapter 36 8 Theoretical Performance Limit: Diffraction Light rays hitting mirror spread due to diffraction These rays interfere, just like for single slit Calculation a little different because of circular shape Angle of spread = 1.22 /D (D = diameter) PHY 2049: Chapter 36 9 Example: Optical Telescopes Keck telescope: D = 10m, = 550nm = 1.22 550 10-9 / 10 = 6.7 10-8 rad = 0.014” Compare this to 0.25” – 0.5” from atmosphere Hubble space telescope: D = 2.4m, = 550nm = 1.22 550 10-9 / 2.4 = 2.8 10-7 rad = 0.058” But actually can achieve this resolution! Rayleigh criterion Two objects separated by < 1.22 /D cannot be distinguished An approximate rule, shows roughly what is possible PHY 2049: Chapter 36 10 Single Star Units in multiples of /D PHY 2049: Chapter 36 11 Two Stars: Separation = 2.0 Units in multiples of /D PHY 2049: Chapter 36 12 Two Stars: Separation = 1.5 Units in multiples of /D PHY 2049: Chapter 36 13 Two Stars: Separation = 1.22 Units in multiples of /D PHY 2049: Chapter 36 14 Two Stars: Separation = 1.0 Units in multiples of /D PHY 2049: Chapter 36 15 Two Stars: Separation = 0.8 Units in multiples of /D PHY 2049: Chapter 36 16 Two Stars: Separation = 0.6 Units in multiples of /D PHY 2049: Chapter 36 17 Two Stars: Separation = 0.4 Units in multiples of /D PHY 2049: Chapter 36 18 Single Star Units in multiples of /D PHY 2049: Chapter 36 19 Gemini Telescope w/ Adaptive Optics Gemini = “twins” D = 8.1 m Hawaii, Chile Both outfitted with adaptive optics PHY 2049: Chapter 36 20 Adaptive Optics in Infrared (936 nm) 9 better! PHY 2049: Chapter 36 21 Pluto and Its Moon Pluto and its moon Charon (0.083” resolution) PHY 2049: Chapter 36 22 Gemini North Images (7x Improvement) Resolution = 0.6” Resolution = 0.09” PHY 2049: Chapter 36 23 Keck Adaptive Optics Images of Uranus PHY 2049: Chapter 36 24 Keck Star Image with Adaptive Optics PHY 2049: Chapter 36 25 Interferometry Basic interferometer discussed in Chap. 35 (p. 978) Michaelson interferometer Change in path lengths causes interference Adjusted so that waves lined up initially Insert material of thickness d in path 1 Before: # waves = Ninit = 2d/ After: # waves = Nfinal = 2dn/ # “fringes” = (2d/ ) (n 1) Technique used for very precise length measurements Also used to prove that there is no “ether” carrying E&M waves PHY 2049: Chapter 36 26 Interferometry: Multiple Radiotelescopes Combine information from multiple radiotelescopes Atomic clocks to keep time information (time = phase) Each telescope records signals on tape with time stamp Tapes brought to “correlator” to build synthetic image Single telescope resolution = 1.22 /D (D = diameter of dish or mirror) Two telescope resolution ~ /D (D = distance between telescopes) Spectacular improvement in resolution Diameter of dish ~ 20 – 50m Distance between two dishes ~ 12,000 km (diameter of earth) Improvement is factor of ~ 200,000 – 500,000 PHY 2049: Chapter 36 27 Example of Interferometry Two radiotelescopes D = 50m Separated by diameter of earth = 12,700 km 6 GHz radio waves, = 5 cm Single telescope resolution = 1.22 /D = 1.22 0.05 / 50 = 0.0012 rad = 200” Two telescope resolution ~ /D = 0.05 / 1.27 107 = 4 10-9 rad = 0.0004” Compare to 0.25” for best earthbound telescope, 0.06” for Hubble PHY 2049: Chapter 36 28 Radiotelescope (Mauna Kea) PHY 2049: Chapter 36 29 Spaced Based Interferometry: Japan VSOP (VLBI Space Observatory Programme) http://www.vsop.isas.ac.jp/ PHY 2049: Chapter 36 30 VLBI Using Satellite ( = 6cm) Quasar: VLBI ground only Quasar: VLBI ground plus space PHY 2049: Chapter 36 31 VLBI Using Satellite ( = 17cm) Quasar: VLBI ground only Quasar: VLBI ground plus space Space based ~ 30,000 km baseline PHY 2049: Chapter 36 32 ...
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This note was uploaded on 04/23/2010 for the course PHY 2049 taught by Professor Any during the Spring '08 term at University of Florida.

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