Calibration of residual aberrations in exoplanet imagers with large numbers of degrees of freedom

Pourcelot, Raphaël; Vigan, A.; Dohlen, Kjetil; Rouzé, Bastien; Sauvage, Jean-François; Morsy, M. El; Lopez, Maxime Abellan; N’Diaye, Mamadou; Caillat, A.; Choquet, Élodie; Otten, G. P. P. L.; Abbinanti, Alain; Balard, Philippe; Carbillet, Marcel; Blanchard, P.; Hulin, Jérémy; Robert, Émilie

doi:10.1051/0004-6361/202040157

Cited by 11 publications

(9 citation statements)

References 63 publications

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“…Some of the key advantages of ZELDA are its flexibility, its simple implementation, and the fact that it can easily measure a wide range of spatial frequencies (e.g., Pourcelot et al 2021). The presence of this sensor inside SPHERE was not originally planned and is the result of several happy coincidences (N'Diaye et al 2014(N'Diaye et al , 2016b.…”

Section: Conclusion and Discussionmentioning

confidence: 99%

Calibration of quasi-static aberrations in exoplanet direct-imaging instruments with a Zernike phase-mask sensor

Vigan

Dohlen

N’Diaye

et al. 2022

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Coronagraphic imaging of exoplanets and circumstellar environments using ground-based instruments on large telescopes is intrinsically limited by speckles induced by uncorrected aberrations. These aberrations originate from the imperfect correction of the atmosphere by an extreme adaptive optics (ExAO) system; from static optical defects; or from small opto-mechanical variations due to changes in temperature, pressure, or gravity vector. More than the speckles themselves, the performance of high-contrast imagers is ultimately limited by their temporal stability, since most post-processing techniques rely on difference of images acquired at different points in time. Identifying the origin of the aberrations and the timescales involved is therefore crucial to understanding the fundamental limits of dedicated high-contrast instruments. In previous works we demonstrated the use of a Zernike wavefront sensor called ZELDA for sensing non-common path aberrations (NCPA) in the VLT/SPHERE instrument. We now use ZELDA to investigate the stability of the instrumental aberrations using five long sequences of measurements obtained at high cadence on the internal calibration source. Our study reveals two regimes of decorrelation of the NCPA. The first, with a characteristic timescale of a few seconds and an amplitude of a few nanometers, is induced by a fast internal turbulence within the enclosure. The second is a slow quasi-linear decorrelation on the order of a few 10 −3 nm rms/s that acts on timescales from minutes to hours. We use coronagraphic image reconstruction to demonstrate that these two NCPA contributions have a measurable impact on differences of images, and that the fast internal turbulence is a dominating term over to the slow linear decorrelation. We also use dedicated sequences where the derotator and atmospheric dispersion compensators emulate a real observation to demonstrate the importance of performing observations symmetric around the meridian, which minimizes speckle decorrelation, and therefore maximizes the sensitivity to point sources in difference of images.

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Section: Conclusion and Discussionmentioning

confidence: 99%

Calibration of quasi-static aberrations in exoplanet direct-imaging instruments with a Zernike phase-mask sensor

Vigan

Dohlen

N’Diaye

et al. 2022

A&A

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“…MITHiC is a HCI testbed located in the optics laboratory at the Laboratoire d'Astrophysique de Marseille (France). It was developed in 2010 and has been used to develop and test various methods for HCI, such as COFFEE (Paul et al 2013), the ZELDA wavefront sensor (N'Diaye et al 2013;Pourcelot et al 2021), and the Roddier and Roddier coronagraph N'Diaye, M. et al 2010;N´Diaye, M. et al 2012). The testbed has already been described in detail in Pourcelot et al (2021), so we only mention the key elements in this section.…”

Section: Optical Setupmentioning

confidence: 99%

“…The spatial light modulator (SLM), which acts as a deformable mirror (DM) in the pupil plane (274 pixels across the pupil diameter), allows a phase correction to be applied in closed-loop to flatten the wavefront and the wavefront to be modulated for particular needs, such as creating the satellite spots or introducing known aberrations. The WFS on MITHiC is a Zernike wavefront sensor called ZELDA (Pourcelot et al 2021;N'Diaye et al 2013). Due to the aberrations in the MITHiC setup, if no corrections are applied on the SLM, the optical aberrations of the MITHIC bench are quantified to ∼33 nm root mean square (rms), and can be decreased to 20 nm rms or better with a correction based on a ZELDA measurement.…”

Section: Optical Setupmentioning

confidence: 99%

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Validation of strategies for coupling exoplanet PSFs into single-mode fibres for high-dispersion coronagraphy

Morsy

Vigan

Lopez

et al. 2022

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On large ground-based telescopes, the combination of extreme adaptive optics (ExAO) and coronagraphy with high-dispersion spectroscopy (HDS), sometimes referred to as high-dispersion coronagraphy (HDC), is starting to emerge as a powerful technique for the direct characterisation of giant exoplanets. The high spectral resolution not only brings a major gain in terms of accessible spectral features, but also enables a better separation of the stellar and planetary signals. Ongoing projects such as Keck/KPIC, Subaru/REACH, and VLT/HiRISE base their observing strategy on the use of a few science fibres, one of which is dedicated to sampling the planet's signal, while the others sample the residual starlight in the speckle field. The main challenge in this approach is to blindly centre the planet's point spread function (PSF) accurately on the science fibre, with an accuracy of less than 0.1 λ/D to maximise the coupling efficiency. In the context of the HiRISE project, three possible centring strategies are foreseen, either based on retro-injecting calibration fibres to localise the position of the science fibre or based on a dedicated centring fibre. We implemented these three approaches, and we compared their centring accuracy using an upgraded setup of the MITHiC high-contrast imaging testbed, which is similar to the setup that will be adopted in HiRISE. Our results demonstrate that reaching a specification accuracy of 0.1 λ/D is extremely challenging regardless of the chosen centring strategy. It requires a high level of accuracy at every step of the centring procedure, which can be reached with very stable instruments. We studied the contributors to the centring error in the case of MITHiC and we propose a quantification for some of the most impacting terms.

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“…The phase reconstruction using our analytical method based on phase conjugation can be used as a wavefront error estimator in closed-loop for wavefront stabilization (Pourcelot et al 2021).…”

Section: Closed-loop Sensing and Controlmentioning

confidence: 99%

Low-order wavefront control using a Zernike sensor through Lyot coronagraphs for exoplanet imaging

Pourcelot

N’Diaye

Por

et al. 2022

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Context. The combination of large segmented space telescopes, coronagraphy, and wavefront control methods is a promising solution for producing a dark hole (DH) region in the coronagraphic image of an observed star in order to study planetary companions. The thermal and mechanical evolution of such a high-contrast instrumental setup leads to wavefront drifts that degrade the DH contrast during the observing time, thus limiting the ability to retrieve planetary signals. Aims. Lyot-style coronagraphs are starlight-suppression systems that remove the central part of the image for an unresolved observed star, that is, the point spread function, with an opaque focal plane mask (FPM). When implemented with a flat mirror containing an etched pinhole, the mask rejects part of the starlight through the pinhole which can be used to retrieve information about low-order aberrations. Methods. We propose an active control scheme using a Zernike wavefront sensor (ZWFS) to analyze the light rejected by the FPM, control low-order aberrations, and stabilize the DH contrast. We first present the concept formalism and then describe how we characterized the sensor behavior in simulations and in the laboratory. We performed experimental tests to validate a wavefront control loop using a ZWFS on the HiCAT testbed. Results. By controlling the first 11 Zernike modes, we show a decrease in the standard deviation of the wavefront error by a factor of up to 9 between open- and closed-loop operations using the ZWFS. In the presence of wavefront perturbations, we show the ability of this control loop to stabilize a DH contrast around 7 × 10−8 with a standard deviation of 7 × 10−9. Conclusions. Active control with a ZWFS proves to be a promising solution in Lyot coronagraphs with an FPM-filtered beam for controlling and stabilizing low-order wavefront aberrations and DH contrast for exoplanet imaging with future space missions.

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Calibration of residual aberrations in exoplanet imagers with large numbers of degrees of freedom

Cited by 11 publications

References 63 publications

Calibration of quasi-static aberrations in exoplanet direct-imaging instruments with a Zernike phase-mask sensor

Calibration of quasi-static aberrations in exoplanet direct-imaging instruments with a Zernike phase-mask sensor

Validation of strategies for coupling exoplanet PSFs into single-mode fibres for high-dispersion coronagraphy

Low-order wavefront control using a Zernike sensor through Lyot coronagraphs for exoplanet imaging

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