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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

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

Why can t we see planets
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

Difficulties in imaging
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


  • 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


Lyot stop /adaptive optics


Star (on axis)






Telescope aperture
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

Aperture design
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


point-spread function

Aperture phase
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

Nature of clutter
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)

Breaking the symmetry
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

Point spread function
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




collimated laser beam

beam blocker


spatial light modulatorbinary grid


Lab simulation
Lab simulation

  • Notice shape of diffraction rings

  • Phase errors (system + atmosphere) σφ≈λ

    log (magnitude) scale


Changing pupil size
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

Periodic signal
Periodic signal

The intensity at image position x, y, time (frame) tis

stellar signal

planet signal

unknown position phase angle

mask rotation angle

Stellar signal
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

Planet and star
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?

Time traces
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


40 steps/revolution

Removing near periodic signal
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)




  • 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

Faintest intensities
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)

Removing another redundancy
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%)


telescope pupil

1st focus

pupil image

final focus


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


Star (on axis)







  • 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