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Detecting planets by symmetry breaking

Detecting planets by symmetry breaking. Erez Ribak 1,3 and Szymon Gladysz 2,3 1 Physics, Technion, Haifa, Israel 2 European Southern Observatory 3 Research performed at NUI, Galway Thanks to Ruth Mackay for helping with the lab experiment and to Chris Dainty for his full support.

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Detecting planets by symmetry breaking

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  1. Detecting planets by symmetry breaking Erez Ribak1,3 and Szymon Gladysz2,3 1 Physics, Technion, Haifa, Israel 2 European Southern Observatory 3 Research performed at NUI, Galway Thanks to Ruth Mackay for helping with the lab experiment and to Chris Dainty for his full support

  2. Why can’t we see planets? • They are too faint • They are too close to their mother suns • They are too far away, we get only the nearby ones • Many excuses, but we still want to see them • Most planets detected by photometry, not imaging

  3. Difficulties in imaging • The dynamic range sun/planet is very high: 105-1011 • The atmosphere scatters stellar light onto planet • First method: adaptive optics • Reducing atmospheric phase errors • Second-order effects still disturbing • Second method: extreme adaptive optics • Correcting for amplitude errors • Employing more deformable mirrors • More degrees of freedom • Correction for atmospheric depth effects

  4. Coronagraphy • Removing all stellar flux within 2.44λ/D • Blocking stellar central spot • Scattering light out by optical vortices • Nulling on-axis interferometry • Blocking diffraction from aperture, spider • Simple Lyot stop downwards from scatterer • Advanced aperture design: edges, spider, more telescope pupil 1st focus pupil image final focus apodizer Lyot stop /adaptive optics planet Star (on axis) f f f f f

  5. Telescope aperture • First brought into play by Herschel • Hexagonal pupil shape • Discovery of Sirius B (Barnard, 1909) • van Albada (1930s) used shaped pupils • Watson et al, Nisenson and Papaliolios (~1991) re-examined square apertures • Star light is concentrated along axes • Planet best visible along diagonals • Stellar signal drops as (sin r / r)4 Euro 50 design

  6. Aperture design • Spergel, Kasdin and Vanderbei (2003-4) optimised aperture shape • Scatter even less light along diagonals • Lossy in light, efficient in reordering it power spectrum pupil point-spread function

  7. Aperture phase • Ground and space observations suffer from wave front phase errors • Relatively easy to fix by adaptive optics • Strong, nearby reference signal • Extreme adaptive optics correct amplitude errors and second order phase errors • Even combining coronagraphy and adaptive optics still leaves residual but detrimental stellar light leakage

  8. Nature of clutter • The nearby star creates a fixed pattern (even after adaptive optics correction) • The pattern still shows traces of Airy rings • The pattern has high rotational symmetry 14 nm phase errors corrigible by extreme adaptive optics Polishing errors, Subaru Fixed diffraction pattern (log scale)

  9. Breaking the symmetry • If the symmetry is created by the aperture shape, modify this shape • If the modification is insufficient, modify the modification • Turn the modified pupil around • If turning is not enough, remodify pupil shape • We chose a mechanically simple solution • Block the side of the telescope pupil (or its copy) • Rotate the occluding mask

  10. Point Spread Function • The PSF is the power spectrum of the pupil • By breaking the pupil symmetry, the PSF loses rotational symmetry • By rotating the occluding mask, the PSF rotates • Laboratory experiment +1 0 -1 collimated laser beam beam blocker camera spatial light modulatorbinary grid σφ≈λ

  11. Lab simulation • Notice shape of diffraction rings • Phase errors (system + atmosphere) σφ≈λ log (magnitude) scale aperture

  12. Changing pupil size • As the pupil rotates, Airy rings shrink/expand • The zero intensity rings sweeps in/out • As a zero ring passes by planet, it will become visible

  13. Periodic signal The intensity at image position x, y, time (frame) tis stellar signal planet signal unknown position phase angle mask rotation angle

  14. Stellar signal • The modulated intensity at x, yis stellar signal local position angle planet signal rotation angle Ia Ib Ic Id cosine term shot noise sine term

  15. Planet and star • Divide the planet by stellar signal in each pixel • Different statistics (atmosphere, Poisson) • Median-filtered image • Only good when planet>modulation, at the rim • Other options?

  16. Time traces • Look at specific pixels during rotations: • Not a single period • Not a single shape • Not a single phase • Noisy • Atmosphere • Sky background • Poisson noise • Read noise bright planet off-planet 40 steps/revolution

  17. Removing near-periodic signal • suggested methods • Fit typical period, subtract • Time domain • Frequency domain • Search for different statistics • Rely on atmosphere vs. Poisson vs. Gaussian • Wait until minima occur, at whatever angle • Only sky background and planet will show • Similar to dark speckle (but with active nudge) planet sky

  18. Simulations • Modelled SPHERE, the planet finder for the VLT, with the PAOLA AO package • Created AO-corrected wave fronts • Added static aberrations from a mirror error map, σ = 20 nm • Added a realization of an f -2 spectrum from additional optical components, σ = 10 nm • The coronagraphic module was not included • λ =1600±15 nm (methane band), but Spectral Differential Imaging was not employed • The global tip and tilt were left in • Generated 200 short (dt = 0.1 s or 0.5 s) exposures • Between exposures the eccentric aperture was rotated by 9°, total 5 cycles per data-cube • Asterism of a primary star and 36 planets, all with same PSF • The star was magnitude 4, the planets dropping from 12 to 20 outwards • Added Poisson, background (14 mag arcsec-2) and readout noise (10 e-) • The focal-plane sampling was 0.25λ/D (D = 8.2 m) • The planets were placed in a central cross, spaced by 10 pixels, or 2.5λ/D , or 0.1'‘ • Planets locations nearly coincide with the Airy rings

  19. Faintest intensities • 31% side obscuration • Integrations of 100ms, 9° steps, total 200 steps • Data-cube is 200×200×200. In each (x, y) pixel: average (~long exposure) faintest average of 3 faintest (limitation of dark speckle: the ensemble faintest can be too faint, even zero)

  20. Removing another redundancy • This 31% side obscuration did not uncover all pixels • Repeat with other values: 11%, 16%, 21%, 26%, 31% • Now data-cube has 200×200×1000 values • For weakest occurrence: keep only 200×200 minima average (~long exposure) faintest (31%) faintest (11%-31%)

  21. Comparison

  22. Realisation telescope pupil 1st focus pupil image final focus apodizer Lyot stop /adaptive optics • Changing the occulting aperture size and rotating it at the same time • The blocked portion grows from 0 up to 24% of the diameter • The cycle repeats at 7/3 times the rotation speed • Employ planetary gear: non-circular or axis-displacing planet Star (on axis) f f f f f

  23. Summary • A simple rotating mask removes symmetries of the pupil • Main limitation is short exposure • Data analysis: • Averaging over cycles (yet) unsuccessful • Finding data-cube minima is prone to statistics of extrema • Higher contrast achievable with star apodiser (not included) • Next: • Combine Airy ring wobble by aperture and by λ(Thatte) • Laboratory white-light experiments • Observatory tests with AO system

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