Fainter and closer: finding planets by symmetry breaking

Abstract

Imaging of planets is very difficult, due to the glare from their nearby, much brighter suns. Static and slowly-evolving aberrations are the limiting factors, even after application of adaptive optics. The residual speckle pattern is highly symmetrical due to diffraction from the telescope's aperture. We suggest to break this symmetry and thus to locate planets hidden beneath it. An eccentric pupil mask is rotated to modulate the residual light pattern not removed by other means. This modulation is then exploited to reveal the planet's constant signal. In well-corrected ground-based observations we can reach planets six stellar magnitudes fainter than their sun, and only 2-3 times the diffraction limit from it. At ten times the diffraction limit, we detect planets 16 magnitudes fainter. The stellar background drops by five magnitudes. The trouble with planetsPlanets in other stellar systems (exoplanets) are difficult to discover, and even more difficult to image. This is because there is a very large intensity difference between them and their parent stars, which can be anything from 10 -4 in the infrared down to 10 -10 and fainter in the visible regime, depending on the physical properties of the system [1,2]. Light from the star is scattered off-axis by the atmosphere, the aperture of the telescope, the secondary mirror holders (spiders) and other optical elements in the path to the imaging camera. Techniques such as adaptive optics (AO) and coronagraphy aim to remove this effect by concentrating light back on the optical axis and removing it [3]. All future ground-based high-contrast imaging systems will employ coronagraphs and AO [4,5].It has been shown through simulations and experiments that the limiting factors in highcontrast imaging from ground and space are static and quasi-static speckles [6][7][8]. While residual atmospheric aberrations after AO correction are random and will average out over time, these persistent speckles will stand out against the AO-corrected stellar halo and masquerade as faint sources even after long integrations. This is why future efforts to directly image planets will also entail post-processing schemes based on the concept of point spread function (PSF) subtraction. Proposed approaches utilize PSF estimates provided by on-sky rotation [9], as well as spectral [10,11] and polarization-based [12] discrimination between the light coming from the parent star and the companion. The problem with some of these techniques is that the PSF-subtracted images still contain static speckles (at a lower brightness level than in the direct images) due to errors in PSF estimation. These errors arise due to the inherent sensitivity of PSF subtraction to changes in "seeing", mechanical flexure or the introduction of extra imaging channels. Recently a detection algorithm has been proposed which relies only on multiple exposures and statistical properties of AO-corrected intensity [13].

Review of small-angle coronagraphic techniques in the wake of ground-based second-generation adaptive optics systems

“…The idea here is to use the control system or part of the coronagraph itself to provide a known modulation, such as phase probes with the deformable mirror, 56, 57 fringes through controlled leaks at the Lyot stop level (such as with the self-coherent camera 58 ), or even modulation of the pupil shape. 59 Indeed, as rightfully argued in Ref. 56: focal plane detection coupled with common-path use of the self-coherent nature of each stellar photon through, e.g., successive images, plus a knowledge of the perturbations applied, should allow one to remove most of the speckle field in post-processing.…”

Adaptive pupil masking for quasi-static speckle suppression

“…The actual fraction can be chosen and optimized by the user depending on system specific parameters. Here we choose 15 per cent as this is consistent with the throughput of many coronagraphic systems (Ribak & Gladysz ).…”

“…Several other interesting and inventive techniques are also being developed in order to further enhance the probability of detecting faint companions. Ribak & Gladysz () demonstrate that it is possible to enhance the contrast ratio by placing a rotating eccentric mask in the pupil plane. This breaks the symmetry between the telescope pupil and the focal plane causing the quasi‐static speckles to move; the companion will however stay fixed at the same location.…”

Vectorial shearing interferometer applied in the faint sources detection

“…It´s said that a planet is hidden when the light from the star is brighter than the reflected and emitted light from the planet. This is why, in order to detect this hidden planet, it becomes necessary to be able to distinguish or separate the light coming from the star and the one coming from the planet [3,4].…”