Telescopes PHY2054: Chapter 25 26
Main purposes Telescopes Resolution of closely spaced objects Light collection (measure spectra, see distant and dim objects) Resolution through magnification mθ = fobjective / feyepiece Light collection: compare to eye (discussed in textbook) Pupil of eye D 8mm (in very dim light) Largest telescope (Keck) has D = 10m Ratio of areas = (10/0.008) 2 = 1.5 10 6 Can collect light for hours rather than 0.1 sec More sensitive light collectors (CCD arrays) Large telescopes favored by several billion over eye! Can see near the end of the known universe PHY2054: Chapter 25 27
Stellar Spectra (telescope + diffraction grating) Spectrum of star Helium Mercury PHY2054: Chapter 25 28
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 Two main components in adaptive optics Mechanism to measure wave distortion above mirror Actuators under mirror move surface to offset distortions Easier for infrared wavelengths than for optical wavelengths PHY2054: Chapter 25 29
Adaptive Optics: Gemini Telescope Gemini = twins D = 8.1 m Hawaii, Chile Both outfitted with adaptive optics PHY2054: Chapter 25 30
Adaptive Optics in Infrared (936 nm) 9 better! PHY2054: Chapter 25 31
Gemini North Images (7x Improvement) Resolution = 0.6 Resolution = 0.09 PHY2054: Chapter 25 32
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 Δθ = 1.22λ/D (D = diameter) Intensity vs angle 1.22 λ/d 1.22 λ/d θ in units of λ/d PHY2054: Chapter 25 33
Example: Optical Telescopes Keck 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 atmosphere Hubble 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! Rayleigh criterion Two objects separated by Δθ < 1.22λ/D cannot be distinguished An approximate rule, shows roughly what is possible PHY2054: Chapter 25 34
Pluto and Its Moon Charon Gemini North w/ adaptive optics (0.083 resolution) 1999 image Hubble image PHY2054: Chapter 25 35
Pluto Seen by Keck w/ Adaptive Optics Resolution = 0.035 λ = 1.6μm (2007) PHY2054: Chapter 25 36
New Pluto Moons Seen with Adaptive Optics Charon Nix and Hydra Resolution = 0.035 λ = 1.6μm (2007) Star trails PHY2054: Chapter 25 37
Single Star Units in multiples of λ/d PHY2054: Chapter 25 38
Two Stars: Separation = 2.0 λ/d Units in multiples of λ/d PHY2054: Chapter 25 39
Two Stars: Separation = 1.5 λ/d Units in multiples of λ/d PHY2054: Chapter 25 40
Two Stars: Separation = 1.22 λ/d Units in multiples of λ/d PHY2054: Chapter 25 41
Two Stars: Separation = 1.0 λ/d Units in multiples of λ/d PHY2054: Chapter 25 42
Two Stars: Separation = 0.8 λ/d Units in multiples of λ/d PHY2054: Chapter 25 43
Two Stars: Separation = 0.6 λ/d Units in multiples of λ/d PHY2054: Chapter 25 44
Two Stars: Separation = 0.4 λ/d Units in multiples of λ/d PHY2054: Chapter 25 45
Single Star Units in multiples of λ/d PHY2054: Chapter 25 46
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 Δθ = 1.22λ/D (D = diameter of dish or mirror) Two telescope resolution Δθ ~ λ/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 PHY2054: Chapter 25 47
Radiotelescope at Mauna Kea (Hawaii) PHY2054: Chapter 25 48
Interferometry using widely spaced radiotelescopes PHY2054: Chapter 25 49
Two radiotelescopes Example of Interferometry D = 50m Separated by diameter of earth = 12,700 km 6 GHz radio waves, λ = 5 cm Single telescope resolution Δθ = 1.22λ/D = 1.22 0.05 / 50 = 0.0012 rad = 200 Two telescope resolution Δθ ~ λ/d = 0.05 / 1.27 10 7 = 4 10-9 rad = 0.0004 Compare to 0.25 for best earthbound telescope, 0.06 for Hubble VLBI: Very Long Baseline Interferometry PHY2054: Chapter 25 50
Worldwide VLBI Radiotelescope Network PHY2054: Chapter 25 51
Spaced Based Interferometry: VSOP VSOP (VLBI Space Observatory Programme) http://www.vsop.isas.ac.jp/ PHY2054: Chapter 25 52
VLBI Using Satellite (λ = 6cm) Quasar: VLBI ground only Quasar: VLBI ground plus space PHY2054: Chapter 25 53
VLBI Using Satellite (λ = 17cm) Quasar: VLBI ground only Quasar: VLBI ground plus space Space based ~ 30,000 km baseline PHY2054: Chapter 25 54