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Register a new userLaboratory Demonstration of Spatial Linear Dark Field Control For Imaging Extrasolar Planets in Reflected Light
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Imaging planets in reflected light, a key focus of future NASA missions and ELTs, requires advanced wavefront control to maintain a deep, temporally correlated null of stellar halo -- i.e. a dark hole -- at just several diffraction beam widths. Using the Ames Coronagraph Experiment testbed, we present the first laboratory tests of Spatial Linear Dark Field Control (LDFC) approaching raw contrasts ($sim$ 5$times$10$^{-7}$) and separations (1.5--5.2 $lambda$/D) needed to image jovian planets around Sun-like stars with space-borne coronagraphs like WFIRST-CGI and image exo-Earths around low-mass stars with future ground-based 30m class telescopes. In four separate experiments and for a range of different perturbations, LDFC largely restores (to within a factor of 1.2--1.7) and maintains a dark hole whose contrast is degraded by phase errors by an order of magnitude. Our implementation of classical speckle nulling requires a factor of 2--5 more iterations and 20--50 DM commands to reach contrasts obtained by spatial LDFC. Our results provide a promising path forward to maintaining dark holes without relying on DM probing and in the low-flux regime, which may improve the duty cycle of high-contrast imaging instruments, increase the temporal correlation of speckles, and thus enhance our ability to image true solar system analogues in the next two decades.
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Imaging rocky planets in reflected light, a key focus of future NASA missions and ELTs, requires advanced wavefront control to maintain a deep, temporally correlated null of stellar halo at just several diffraction beam widths. We discuss development of Linear Dark Field Control (LDFC) to achieve this aim. We describe efforts to test spatial LDFC in a laboratory setting for the first time, using the Ames Coronagraph Experiment (ACE) testbed. Our preliminary results indicate that spatial LDFC is a promising method focal-plane wavefront control method capable of maintaining a static dark hole, at least at contrasts relevant for imaging mature planets with 30m-class telescopes.
The ESA PLATO space mission is devoted to unveiling and characterizing new extrasolar planets and their host stars. This mission will encompass a very large field of view, granting it the potential to survey up to one million stars depending on the final observation strategy. The telemetry budget of the spacecraft cannot handle transmitting individual images for such a huge stellar sample at the right cadence, so the development of an appropriate strategy to perform on-board data reduction is mandatory. We employ aperture photometry to produce stellar light curves in flight. Our aim is thus to find the mask model that optimizes the scientific performance of the reduced data. We considered three distinct aperture models: binary mask, weighted Gaussian mask, and weighted gradient mask giving lowest noise-to-signal ratio, computed through a novel direct method. An innovative criterion was adopted for choosing between different mask models. We designated as optimal the model providing the best compromise between sensitivity to detect true and false planet transits. We determined the optimal model based on simulated noise-to-signal ratio and frequency of threshold crossing events. Our results show that, although the binary mask statistically presents a few percent higher noise-to-signal ratio compared to weighted masks, both strategies have very similar efficiency in detecting legitimate planet transits. When it comes to avoiding spurious signals from contaminant stars however the binary mask statistically collects considerably less contaminant flux than weighted masks, thereby allowing the former to deliver up to $sim$30% less false transit signatures at $7.1sigma$. Our proposed approach for choosing apertures has been proven to be decisive for the determination of a mask model capable to provide near maximum planet yield and substantially reduced occurrence of false positives.
A compact device lifted over the ground surface might be used to observe optical radiation of extensive air showers (EAS). Here we consider spatial and temporal characteristics of Vavilov-Cherenkov radiation (Cherenkov light) reflected from the snow surface of Lake Baikal, as registered by the SPHERE-2 detector. We perform detailed full direct Monte Carlo simulations of EAS development and present a dedicated highly modular code intended for detector response simulations. Detector response properties are illustrated by example of several model EAS events. The instrumental acceptance of the SPHERE-2 detector was calculated for a range of observation conditions. We introduce the concept of composite model quantities, calculated for detector responses averaged over photoelectron count fluctuations, but retaining EAS development fluctuations. The distortions of EAS Cherenkov light lateral distribution function (LDF) introduced by the SPHERE-2 telescope are understood by comparing composite model LDF with the corresponding function as would be recorded by an ideal detector situated at the ground surface. We show that the uncertainty of snow optical properties does not change our conclusions, and, moreover, that the expected performance of the SPHERE experiment in the task of cosmic ray mass composition study in the energy region $sim$10 PeV is comparable with other contemporary experiments. Finally, we compare the reflected Cherenkov light method with other experimental techniques and briefly discuss its prospects.
Wavefront stabilization is a fundamental challenge to high contrast imaging of exoplanets. For both space and ground observations, wavefront control performance is ultimately limited by the finite amount of starlight available for sensing, so wavefront measurements must be as efficient as possible. To meet this challenge, we propose to sense residual errors using bright focal-plane speckles at wavelengths outside the high contrast spectral bandwidth. We show that a linear relationship exists between the intensity of the bright out-of-band speckles and residual wavefront aberrations. An efficient linear control loop can exploit this relationship. The proposed scheme, referred to as Spectral Linear Dark Field Control (spectral LDFC), is more sensitive than conventional approaches for ultra-high contrast imaging. Spectral LDFC is closely related to, and can be combined with, the recently proposed spatial LDFC which uses light at the observation wavelength but located outside of the high contrast area in the focal plane image. Both LDFC techniques do not require starlight to be mixed with the high contrast speckle field, so full-sensitivity uninterrupted high contrast observations can be conducted simultaneously with wavefront correction iterations. We also show that LDFC is robust against deformable mirror calibration errors and drifts, as it relies on detector response stability instead of deformable mirror stability. LDFC is particularly advantageous when science acquisition is performed at a non-optimal wavefront sensing wavelength, such as nearIR observations of planets around solar-type stars, for which visible-light speckle sensing is ideal. We describe the approach at a fundamental level and provide an algorithm for its implementation. We demonstrate, through numerical simulation, that spectral LDFC is well-suited for picometer-level cophasing of a large segmented space telescope.
In this paper, we review the various ways in which an infrared stellar interferometer can be used to perform direct detection of extrasolar planetary systems. We first review the techniques based on classical stellar interferometry, where (complex) visibilities are measured, and then describe how higher dynamic ranges can be achieved with nulling interferometry. The application of nulling interferometry to the study of exozodiacal discs and extrasolar planets is then discussed and illustrated with a few examples.
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