PHY 2049: Chapter 36 1Wave Interference and DiffractionPart 3: Telescopes and InterferometryPHY 2049Physics 2 with CalculusPHY 2049: Chapter 36 2Telescopes: Purpose is Light CollectionÎPupil of eye D ≈ 8mm (in very dim light)ÎLargest telescope (Keck) has D = 10mÎRatio of areas = (10/0.008)2= 1.5 × 106Can collect light for hours rather than 0.1 sec More sensitive light collectors (CCD arrays) Thus telescopes are several billiontimes more sensitiveÎCan see near the end of the known universePHY 2049: Chapter 36 3Telescope ConstructionÎAll 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 movingPHY 2049: Chapter 36 4Keck Telescope (D = 10 m)PHY 2049: Chapter 36 5Keck Hexagonal Mirror (36 Segments)PHY 2049: Chapter 36 6Keck Primary Mirror (Each segment 1.8 m)HolePHY 2049: Chapter 36 7Main Limitation on Earth: AtmosphereÎAir 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 mountainsΓAdaptive optics” just beginning to offset this distortionPHY 2049: Chapter 36 8Diffraction Through Circular OpeningIntensity of light after passingthrough a circular opening.Spreading caused by diffraction.PHY 2049: Chapter 36 9Theoretical Performance Limit: DiffractionÎLight 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 Δθ = 1.22λ/D (D = diameter)PHY 2049: Chapter 36 10Example: Optical TelescopesÎKeck telescope: D = 10m, λ = 550nm Δθ = 1.22 × 550 × 10-9/ 10 = 6.7 × 10-8 rad = 0.014” Compare this to 0.25” – 0.5” from atmosphereÎHubble space telescope: D = 2.4m, λ = 550nm Δθ = 1.22 × 550 × 10-9/ 2.4 = 2.8 × 10-7 rad = 0.058” But actually can achieve this resolution!ÎRayleigh criterion Two objects separated by Δθ < 1.22λ/D cannot be distinguished An approximate rule, shows roughly what is possiblePHY 2049: Chapter 36 11Single StarUnits in multiples of λ/DPHY 2049: Chapter 36 12Two Stars: Separation = 2.0Units in multiples of λ/DPHY 2049: Chapter 36 13Two Stars: Separation = 1.5Units in multiples of λ/DPHY 2049: Chapter 36 14Two Stars: Separation = 1.22Units in multiples of λ/DPHY 2049: Chapter 36 15Two Stars: Separation = 1.0Units in multiples of λ/DPHY 2049: Chapter 36 16Two Stars: Separation = 0.8Units in multiples of λ/DPHY 2049: Chapter 36 17Two Stars: Separation = 0.6Units in multiples of λ/DPHY 2049: Chapter 36 18Two Stars: Separation = 0.4Units in multiples of λ/DPHY 2049: Chapter 36 19Single StarUnits in multiples of
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