Minimization of non-common path aberrations at the Palomar telescope using a self-coherent camera

Abstract

Context. The two main advantages of exoplanet imaging are the discovery of objects in the outer part of stellar systems -constraining the models of planet formation -, and its ability to spectrally characterize the planets -giving information on their atmosphere. It is, however, challenging, because exoplanets are up to 10 10 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, a new class of 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 10 8 in space-like conditions at angular separations down to 2 λ/D. Aims. In this paper, we demonstrate the SCC on the sky for the first time. Methods. 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. Results. On internal source, we demonstrated that the SCC improves the coronagraphic detection limit by a factor between 4 and 20 between 1.5 and 5 λ/D. Using this SCC calibration, the on-sky contrast is improved by a factor of 5 between 2 and 4 λ/D. These results prove the ability of the SCC to be implemented in an existing instrument. Conclusions. 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.

On-sky verification of Fast and Furious focal-plane wavefront sensing: Moving forward toward controlling the island effect at Subaru/SCExAO

“…The Fast and Furious (F&F; Keller et al 2012;Korkiakoski et al 2014) algorithm is an extension of the sequential phase diversity technique originally introduced by Gonsalves (2002). In conventional phase diversity techniques (Gonsalves 1982;Paxman et al 1992), the degeneracy in estimating even phase modes is solved by recording two images, one in focus and another strongly out of focus. This forces the user to either split the light into two imaging channels or alternately record in-and out-of-focus images.…”

“…Here, is a regularization parameter for the pixels where a goes to zero that would otherwise amplify the noise. This solution only solves for |v|, which is a well-known sign ambiguity (Gonsalves 1982;Paxman et al 1992). The sign of v is solved by introducing an additional image that has a known phase diversity Φ d .…”

“…Ideally, these aberrations are detected by wavefront sensors close to, or in the science focal plane and subsequently corrected by the deformable mirror (DM). Many variants of such wavefront sensors have been developed, and some of these have been successfully demonstrated on-sky (Martinache et al 2014(Martinache et al , 2016Singh et al 2015;Bottom et al 2017;Wilby et al 2017;Bos et al 2019;Galicher et al 2019;Vigan et al 2019).…”

An Empirical Mass–Radius Relation for Cool Giant Planets

“…This can be further improved to 10-20 nm with a Zernike phasemask sensor (Vigan et al 2019), which corresponds to better than λ/100 per mode for K and L band. Furthermore, NCPA can be accounted for by focal-plane wavefront sensing techniques (Galicher et al 2019), which shows promising prospects of reducing wavefront error to lower than 10 nm given ample photons (Bos et al 2019). We therefore use λ/100 (or 0.063 rad) per mode as a reasonable baseline RMS wavefront error for subsequent calculations for exposure time.…”

Increasing the raw contrast of VLT/SPHERE with the dark hole technique

“…A few sensors have already been implemented and validated on optical test beds fed by an artificial residual turbulence (Singh et al 2019;Potier et al 2019;Herscovici-Schiller et al 2019). Some of these sensors were tested in the past to calibrate the quasi-static aberrations on calibration sources: the self-coherent camera (SCC; Galicher et al 2019) and the electric field conjugation (EFC; Matthews et al 2017) were tested at the Palomar Observatory, the coronagraphic phase diversity (COFFEE; Paul et al 2014) and the Zernike sensor for extremely low-level differential aberration (ZELDA; N'Diaye et al 2016a) were studied on SPHERE, while the speckle nulling technique was implemented at Palomar and Keck (Bottom et al 2016). However, only the speckle nulling technique with SCEXAO/Subaru (Martinache et al 2014) and ZELDA with SPHERE (Vigan et al 2019) were tested directly using on-sky measurements.…”