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

 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 2

 The dynamic range sun/planet is very high:  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 3

 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 4 fffff planet Star (on axis) telescope pupil1st focuspupil imagefinal focus apodizerLyot stop / adaptive optics

 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 5 Euro 50 design

 Spergel, Kasdin and Vanderbei (2003-4) optimised aperture shape  Scatter even less light along diagonals  Lossy in light, efficient in reordering it 6 power spectrum pupilpoint-spread function

 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 7

 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 8 Polishing errors, Subaru Fixed diffraction pattern (log scale) 14 nm phase errors corrigible by extreme adaptive optics

 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 9

 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 10 collimated laser beam spatial light modulator binary grid beam blocker +1 0 camera σ φ ≈λ

 Notice shape of diffraction rings  Phase errors (system + atmosphere) σ φ ≈λ log (magnitude) scale aperture 11

 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 12

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

 The modulated intensity at x, y is 14 planet signal stellar signallocal position angle rotation angle I a I b I c I d sine term cosine termshot noise

 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? 15

 Look at specific pixels during rotations: 1.Not a single period 2.Not a single shape 3.Not a single phase 4.Noisy a.Atmosphere b.Sky background c.Poisson noise d.Read noise 16 off-planet bright planet 40 steps/revolution

sky planet  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) 17

 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 18

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

 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 20 average (~long exposure) faintest (31%) faintest (11%-31%)

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22 fffff planet Star (on axis) telescope pupil1st focuspupil imagefinal focus apodizerLyot 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

23 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