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Register a new userThe polarization-encoded self-coherent camera
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Steven P. Bos
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The exploration of circumstellar environments by means of direct imaging to search for Earth-like exoplanets is one of the challenges of modern astronomy. One of the current limitations are evolving non-common path aberrations (NCPA) that originate from optics downstream of the main wavefront sensor. The self-coherent camera (SCC) is an integrated coronagraph and focal-plane wavefront sensor that generates wavefront information-encoding Fizeau fringes in the focal plane by adding a reference hole (RH) in the Lyot stop. Here, we aim to show that by featuring a polarizer in the RH and adding a polarizing beamsplitter downstream of the Lyot stop, the RH can be placed right next to the pupil. We refer to this new variant of the SCC as the polarization-encoded self-coherent camera (PESCC). We study the performance of the PESCC analytically and numerically, and compare it, where relevant, to the SCC. We show analytically that the PESCC relaxes the requirements on the focal-plane sampling and spectral resolution with respect to the SCC by a factor of 2 and 3.5, respectively. Furthermore, we find via our numerical simulations that the PESCC has effectively access to $sim$16 times more photons, which improves the sensitivity of the wavefront sensing by a factor of $sim4$. We also show that without additional measurements, the RH point-spread function (PSF) can be calibrated using PESCC images, enabling coherent differential imaging (CDI) as a contrast-enhancing post-processing technique for every observation. In idealized simulations (clear aperture, charge two vortex coronagraph, perfect DM, no noise sources other than phase and amplitude aberrations) and in circumstances similar to those of space-based systems, we show that WFSC combined with CDI can achieve a $1sigma$ raw contrast of $sim3cdot10^{-11}- 8 cdot 10^{-11}$ between 1 and 18 $lambda / D$.
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Direct imaging and spectral characterization of exoplanets using extreme adaptive optics (ExAO) is a key science goal of future extremely large telescopes and space observatories. However, quasi-static wavefront errors will limit the sensitivity of this endeavor. Additional limitations for ground-based telescopes arise from residual AO-corrected atmospheric wavefront errors, generating millisecond-lifetime speckles that average into a halo over a long exposure. A solution to both of these problems is to use the science camera of an ExAO system as a wavefront sensor to perform a fast measurement and correction method to minimize these aberrations as soon as they are detected. We develop the framework for one such method based on the self-coherent camera (SCC) to be applied to ground-based telescopes, called Fast Atmospheric SCC Technique (FAST). We show that with the use of a specially designed coronagraph and coherent differential imaging algorithm, recording images every few milliseconds allows for a subtraction of atmospheric and static speckles while maintaining a close to unity algorithmic exoplanet throughput. Detailed simulations reach a contrast close to the photon noise limit after 30 seconds for a 1 % bandpass in H band on both 0$^text{th}$ and 5$^text{th}$ magnitude stars. For the 5th magnitude case, this is about 110 times better in raw contrast than what is currently achieved from ExAO instruments if we extrapolate for an hour of observing time, illustrating that sensitivity improvement from this method could play an essential role in the future detection and characterization of lower mass exoplanets.
The two main advantages of exoplanet imaging are the discovery of objects in the outer part of stellar systems -- constraining models of planet formation --, and its ability to spectrally characterize the planets -- information on their atmosphere. It is however challenging because exoplanets are up to 1e10 times fainter than their star and separated by a fraction of arcsecond. Current instruments like SPHERE/VLT or GPI/Gemini detect young and massive planets because they are limited by non-common path aberrations (NCPA) that are not corrected by the adaptive optics system. To probe fainter exoplanets, new instruments capable of minimizing the NCPA is needed. One solution is the self-coherent camera (SCC) focal plane wavefront sensor, whose performance was demonstrated in laboratory attenuating the starlight by factors up to several 1e8 in space-like conditions at angular separations down to 2L/D. In this paper, we demonstrate the SCC on the sky for the first time. We installed an SCC on the stellar double coronagraph (SDC) instrument at the Hale telescope. We used an internal source to minimize the NCPA that limited the vortex coronagraph performance. We then compared to the standard procedure used at Palomar. On internal source, we demonstrated that the SCC improves the coronagraphic detection limit by a factor between 4 and 20 between 1.5 and 5L/D. Using this SCC calibration, the on-sky contrast is improved by a factor of 5 between 2 and 4L/D. These results prove the ability of the SCC to be implemented in an existing instrument. This paper highlights two interests of the self-coherent camera. First, the SCC can minimize the speckle intensity in the field of view especially the ones that are very close to the star where many exoplanets are to be discovered. Then, the SCC has a 100% efficiency with science time as each image can be used for both science and NCPA minimization.
High contrast imaging and spectroscopy provide unique constraints for exoplanet formation models as well as for planetary atmosphere models. But this can be challenging because of the planet-to-star small angular separation and high flux ratio. Recently, optimized instruments like SPHERE and GPI were installed on 8m-class telescopes. These will probe young gazeous exoplanets at large separations (~1au) but, because of uncalibrated aberrations that induce speckles in the coronagraphic images, they are not able to detect older and fainter planets. There are always aberrations that are slowly evolving in time. They create quasi-static speckles that cannot be calibrated a posteriori with sufficient accuracy. An active correction of these speckles is thus needed to reach very high contrast levels (>1e7). This requires a focal plane wavefront sensor. Our team proposed the SCC, the performance of which was demonstrated in the laboratory. As for all focal plane wavefront sensors, these are sensitive to chromatism and we propose an upgrade that mitigates the chromatism effects. First, we recall the principle of the SCC and we explain its limitations in polychromatic light. Then, we present and numerically study two upgrades to mitigate chromatism effects: the optical path difference method and the multireference self-coherent camera. Finally, we present laboratory tests of the latter solution. We demonstrate in the laboratory that the MRSCC camera can be used as a focal plane wavefront sensor in polychromatic light using an 80 nm bandwidth at 640 nm. We reach a performance that is close to the chromatic limitations of our bench: contrast of 4.5e-8 between 5 and 17 lambda/D. The performance of the MRSCC is promising for future high-contrast imaging instruments that aim to actively minimize the speckle intensity so as to detect and spectrally characterize faint old or light gaseous planets.
Current and future high contrast imaging instruments aim to detect exoplanets at closer orbital separations, lower masses, and/or older ages than their predecessors, with the eventual goal of directly detecting terrestrial-mass habitable-zone exoplanets. However, continually evolving speckles in the coronagraphic science image still limit state-of-the-art ground-based exoplanet imaging instruments to contrasts at least two orders of magnitude worse than what is needed to achieve this goal. For ground-based adaptive optics (AO) instruments it remains challenging for most speckle suppression techniques to attenuate both the dynamic atmospheric and quasi-static instrumental speckles. We have proposed a focal plane wavefront sensing and control algorithm to address this challenge, called the Fast Atmospheric Self-coherent camera (SCC) Technique (FAST), which enables the SCC to operate down to millisecond timescales even when only a few photons are detected per speckle. Here we present preliminary experimental results of FAST on the Santa Cruz Extreme AO Laboratory (SEAL) testbed. In particular, we illustrate the benefit second stage AO-based focal plane wavefront control, demonstrating FAST closed-loop compensation of evolving residual atmospheric turbulence on millisecond-timescales.
Residual wavefront errors in optical elements limit the performance of coronagraphs. To improve their efficiency, different types of devices have been proposed to correct or calibrate these errors. In this paper, we study one of these techniques proposed by Baudoz et al. 2006 and called Self-Coherent Camera (SCC). The principle of this instrument is based on the lack of coherence between the stellar light and the planet that is searched for. After recalling the principle of the SCC, we simulate its performance under realistic conditions and compare it with the performance of differential imaging.
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