TheThe FantasticalFantastical WorldWorld ofof AdaptiveAdaptive OpticsOptics A multimedia presentation of the physics and technology of adaptive optics James W. Beletic Senior Director, Astronomy & Civil Space 400 Years of the Telescope 1609 - First astronomical use of the telescope ~2 cm diameter aperture Firenze, Italia Galileo Galilei (1564–1642) PontePonte VecchioVecchio UffiziUffizi MuseoMuseo PalazzoPalazzo GalileoGalileo VecchioVecchio ESI 2011 – Adaptive Optics 2 400 Years of the Telescope We have come a long way…….ESO 8-meter telescope ESI 2011 – Adaptive Optics 3 ESI 2011 – Adaptive Optics 4 400 Years of the Telescope 2009 - 17 telescopes with 6.5-meter aperture or larger ESI 2011 – Adaptive Optics 5 400 Years of the Telescope 2009 - 17 telescopes with 6.5-meter aperture or larger KeckKeck –– two two 10-m10-m LBTLBT –– twin twin 8.4-m8.4-m HETHET 9.2-m9.2-m (effective)(effective) GrantecanGrantecan 10.4-m10.4-m Subaru 8.2-m Subaru 8.2-m GeminiGemini 8-m8-m MMTMMT 6.5-m6.5-m GeminiGemini 8-m8-m SALTSALT 10-m10-m (eff.)(eff.) ESOESO VLTVLT –– four four 8.2-m8.2-m CarnegieCarnegie MagellanMagellan –– two two 6.5-m6.5-m ESI 2011 – Adaptive Optics 6 The Electromagnetic Spectrum ESI 2011 – Adaptive Optics 7 400 Years of the Telescope The era of the Extremely Large Telescopes (ELTs) is imminent GMT 24.5-m TMT 359 m2 30-m 707 m2 Existing Large E-ELT Telescopes 42-m 2 944 m of 2 collecting area 1385 m 3 6.5-m 9 8-m 5 10-m ESI 2011 – Adaptive Optics 8 Why bigger telescopes ? See fainter objects Light collection area = π r2 Resolve finer detail Angular resolution = 1.22 λ / D λ = wavelength of light D = diameter of telescope aperture r = radius of telescope aperture = D / 2 13 milliarcsec is the apparent size of a football in Moscow as seen from Madrid ESI 2011 – Adaptive Optics 9 Understanding the performance of optical telescopes ESI 2011 – Adaptive Optics 10 Introduction to Fourier Optics Interferometric Imaging in Astronomy by Joseph W. Goodman by Francois Roddier (3rd edition 2005, first published in 1968) (Physics Reports, 1988) (Vol. 170, No. 2, pp. 97-166) ESI 2011 – Adaptive Optics 11 Propagation of Light Only need the electric field to understand telescope optics ESI 2011 – Adaptive Optics 12 Wave model of image formation Shui Kwok’s animation ESI 2011 – Adaptive Optics 13 Phasor Representation of EM Wave ω = 2πf f = frequency Direction of Propagation Increasing phase Increasing time 180° 0° phase (π radians) ▬ Electric Field + • • ESI 2011 – Adaptive Optics 14 Huygens-Fresnel Principle of Wave Propagation Christiaan Huygens Augustin-Jean Fresnel (1629–1695) (1788–1827) ESI 2011 – Adaptive Optics 15 Diffraction-Limited Resolution Image Plane Optical • Axis ESI 2011 – Adaptive Optics 16 Diffraction-Limit Phasor Distribution E-field Amplitude D • λ / D • Intensity E-field Amplitude (Amplitude2) 2 λ / D • ESI 20112009 – Adaptive Optics – James Beletic 17 Diffraction-Limited Resolution Square Circular Aperture Aperture Airy Diffraction Pattern Intensity Zeroes of function First zero, diffraction limit Sir George Biddell Airy (1801–1892) 1.00 0.75 Intensity & 0.50 Encircled Energy 0.25 0.00 First zero at λ / D First zero at 1.22 λ / D Strehl Ratio Measure of the quality of imaging system The Strehl ratio is the ratio of the observed peak intensity at the detection plane of a telescope or other imaging system from a point source compared to the theoretical maximum peak intensity of a perfect imaging system working at the diffraction limit. ESI 2011 – Adaptive Optics 19 Square Aperture - no distortions Wavefront (rms = 0 wave) Point Spread Function +2λ OPD -2λ Power Only DC power -3λ 0 +3λ OPD Histogram Spatial frequency (cycles/m) Power Spectrum of Wavefront Surface -λ 0 +λ Strehl = 1.27 OPD Histogram with phase wrapping relative to circular aperture ESI 2011 – Adaptive Optics 20 Circular Aperture - no distortions Wavefront (rms = 0 wave) Point Spread Function +2λ OPD -2λ Power Only DC power -3λ 0 +3λ OPD Histogram Spatial frequency (cycles/m) Power Spectrum of Wavefront Surface -λ 0 +λ Strehl = 1.00 OPD Histogram with phase wrapping ESI 2011 – Adaptive Optics 21 SPEC Mirror 1 Mirror 2 Mirror 3 Mirror 4 R. curvature (mm) 28800+-100 28762.9 28760.0 28762.6 28759.2 WFE RMS (nm) N/A 42 39 35 17 θ RMS (arc secs) N/A 0.080 0.074 0.087 0.062 CIR @ r0=500mm >0.82(*) 0.875 0.898 0.893 0.975 CIR @ r0=250mm N/A 0.935 0.951 0.935 0.981 Strehl >0.25(*) 0.762 0.791 0.824 0.953 (*) λ=500 nm - Very high spatial frequency errors ~3-7 nm RMS (wavefront) - Microroughness < 20 Å - Correction forces typically ~80 N (spec <120 N) - Matching error measured by direct Hartmann test, negligible (below measurement accuracy) - All radii of curvature within 3.7 mm ESO VLT 8.2-m telescope Circular Aperture - white noise Wavefront (PV= 0.4 wave, rms = 0.05 wave) Point Spread Function +2λ OPD -2λ Power Equal power at all spatial frequencies -3λ 0 +3λ OPD Histogram Spatial frequency (cycles/m) Power Spectrum of Wavefront Surface -λ 0 +λ Strehl = 0.91 OPD Histogram with phase wrapping ESI 2011 – Adaptive Optics 23 Circular Aperture - white noise Wavefront (PV = 0.81 wave, rms = 0.10 wave) Point Spread Function +2λ OPD -2λ Power Equal power at all spatial frequencies -3λ 0 +3λ OPD Histogram Spatial frequency (cycles/m) Power Spectrum of Wavefront Surface -λ 0 +λ Strehl = 0.67 OPD Histogram with phase wrapping ESI 2011 – Adaptive Optics 24 Circular Aperture - white noise Wavefront (PV = 1.21 waves, rms = 0.15 wave) Point Spread Function +2λ OPD -2λ Power Equal power at all spatial frequencies -3λ 0 +3λ OPD Histogram Spatial frequency (cycles/m) Power Spectrum of Wavefront Surface -λ 0 +λ Strehl = 0.41 OPD Histogram with phase wrapping ESI 2011 – Adaptive Optics 25 Circular Aperture - white noise Wavefront (PV = 1.61 waves, rms = 0.20 wave) Point Spread Function +2λ OPD -2λ Power Equal power at all spatial frequencies -3λ 0 +3λ OPD Histogram Spatial frequency (cycles/m) Power Spectrum of Wavefront Surface -λ 0 +λ Strehl = 0.21 OPD Histogram with phase wrapping ESI 2011 – Adaptive Optics 26 Circular Aperture - white noise Wavefront (PV = 2.01 waves, rms = 0.25 wave) Point Spread Function +2λ OPD -2λ Power Equal power at all spatial frequencies -3λ 0 +3λ OPD Histogram Spatial frequency (cycles/m) Power Spectrum of Wavefront Surface -λ 0 +λ Strehl = 0.09 OPD Histogram with phase wrapping ESI 2011 – Adaptive Optics 27 Circular Aperture - white noise Wavefront (PV = 2.42 waves, rms = 0.30 wave) Point Spread Function +2λ OPD -2λ Power Equal power at all spatial frequencies -3λ 0 +3λ OPD Histogram Spatial frequency (cycles/m) Power Spectrum of Wavefront Surface -λ 0 +λ Strehl = 0.03 OPD Histogram with phase wrapping ESI 2011 – Adaptive Optics 28 Circular Aperture - white noise Wavefront (PV = 2.82 waves, rms = 0.35 wave) Point Spread Function +2λ OPD -2λ Power Equal power at all spatial frequencies -3λ 0 +3λ OPD Histogram Spatial frequency (cycles/m) Power Spectrum of Wavefront Surface -λ 0 +λ Strehl = 0.01 OPD Histogram with phase wrapping ESI 2011 – Adaptive Optics 29 Circular Aperture - white noise Wavefront (PV = 3.22 waves, rms = 0.40 wave) Point Spread Function +2λ OPD -2λ Power Equal power at all spatial frequencies -3λ 0 +3λ OPD Histogram Spatial frequency (cycles/m) Power Spectrum of Wavefront Surface -λ 0 +λ Strehl = 0.00 OPD Histogram with phase wrapping ESI 2011 – Adaptive Optics 30 Circular Aperture - white noise Wavefront (PV = 3.63 waves, rms = 0.45 wave) Point Spread Function +2λ OPD -2λ Power Equal power at all spatial frequencies -3λ 0 +3λ OPD Histogram Spatial frequency (cycles/m) Power Spectrum of Wavefront Surface -λ 0 +λ Strehl = 0.00 OPD Histogram with phase wrapping ESI 2011 – Adaptive Optics 31 Point Spread Function Mirror 4 WFE RMS (nm) 17 Strehl 0.953 (*) λ=500 nm ESO VLT 8.2-m telescope Strehl = 0.91 ESI 2011 – Adaptive Optics 32 Atmospheric Blurring The bane of ground-based astronomy Long exposure image is called the “seeing disk” Long exposure image Binary star pair 100 Her, 14 arc sec separation (Vmag = 6.0) ESI 2011 – Adaptive Optics 10 msec frame time 33 Resolution of Ground-based telescopes If the Theory of making Telescopes could at length be fully brought in Practice, yet there would be certain Bounds beyond which Telescopes could not perform. For the Air through which we look upon the Stars, is in a perpetual Tremor; as may be seen by the tremulous Motion of Shadows cast from high Towers, and by the twinkling of the fix’d Stars… And all these illuminated Points constitute one broad lucid Point, composed of those many trembling Points confusedly and insensibly mixed with one another by very short and swift Tremors, and thereby cause Isaac Newton (1643 1727) – the Star to appear broader than it is… The only Remedy is a most serene and quiet Air, such as may perhaps be found on the tops of the highest Mountains above the grosser Clouds. Isaac Newton, Opticks, 1704 ESI 2011 – Adaptive Optics 34 Atmospheric Seeing ESO Paranal Observatory Seeing statistics for 1999-2004 Long exposure image the “seeing disk” Short exposure image (1/100 sec) Full Width Half Maximum (arc sec) 0.5 µm, zenith ESI 2011 – Adaptive Optics 35 The Devil behind atmospheric distortions ESI 2011 – Adaptive Optics 36 Velocity of light • Velocity v of light through any medium v = c / n c = speed of light in a vacuum (3.28×108m/s) n = index of refraction • Index of refraction of air ~ 1.0003 Atmospheric distortions are due to temperature fluctuations • Refractivity of air N ≡ (n −1) ×106 = 77.6 (1+ 7.5210−3 λ −2)× (P /T) where P = pressure in millibars, T = temp.