An H-band Vector Vortex Coronagraph for the Subaru Coronagraphic Extreme Adaptive Optics System

Abstract

The vector vortex is a coronagraphic imaging mode of the recently commissioned Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) platform on the 8 m Subaru Telescope. This multi-purpose high-contrast visible and near-infrared (R-to K-band) instrument is not only intended to serve as a VLT-class "planet-imager" instrument in the northern hemisphere, but also to operate as a technology demonstration testbed ahead of the ELTs-era, with a particular emphasis on small inner-working angle (IWA) coronagraphic capabilities. The given priority to small-IWA imaging led to the early design choice to incorporate focal-plane phase-mask coronagraphs. In this context, a test H-band vector vortex liquid crystal polymer waveplate was provided to SCExAO, to allow a one-to-one comparison of different small-IWA techniques on the same telescope instrument, before considering further steps. Here we present a detailed overview of the vector vortex coronagraph, from its installation and performances on the SCExAO optical bench, to the on-sky results in the extreme AO regime, as of late 2016/early 2017. To this purpose, we also provide a few recent on-sky imaging examples, notably high-contrast ADI detection of the planetary-mass companion κ Andromedae b, with a signal-to-noise ratio above 100 reached in less than 10 mn exposure time.

Atmospheric Characterization and Further Orbital Modeling of κ Andromeda b

“…75, the planet-tostar contrasts for the SCExAO/CHARIS broadband data in Currie et al (2018) are about a factor of 2-5 better than those reported here for SCExAO/HiCIAO at H band due to the CHARIS data's better PSF quality and utilization of ADI+SDI for PSF subtraction. Similarly, the SCExAO/HiCIAO H-band contrasts in Kühn et al (2018), which were taken on a different date: 2016 November 12 UT, are typically a factor of 2 deeper, likely due to usage of the vector vortex coronagraph.…”

SLM-based digital adaptive coronagraphy: current status and capabilities

“…Second, the SLM-induced phase shift is highly chromatic, as it relies on a scalar phase delay instead of a geometric phase shift like, e.g., the vector vortex 24 or vAPP 25 coronagraphs. One can estimate the related theoretical chromatic coronagraphic null depth leakage from Riaud et al 2003 26 as about 2·10 -3 for 10% bandwidth at H-band: this is a rather serious limiting effect, albeit still negligible by roughly an order of magnitude as compared to routinely achieved on-sky null depths with more achromatic coronagraphs, 6 where wavefront control errors are still dominating the leakage budget. However, as ground-based AO performance and loworder NCPAs (especially tip/tilt jitter control) correction progress in future instruments, one can expect this limitation to detrimentally kick in and set a hard contrast limit, and even more immediately so for space-applications or broadband implementations (as typically required for an integral field spectrograph unit).…”

“…This level of null depth is still very modest, but we note that it is still very rarely achieved on-sky, even with latest extreme AO instruments. 6 Further custom optimization steps to mitigate the parasitic reflection at the glass-LC interface would involve operating with a thicker high-retardance SLM panel, where the relative change of refractive index of the LC material is smaller, hence possibly opening the door to designing some kind of A/R coating for this optical interface. Even more so, we note that the manufacturer quotes a phase flicker standard deviation of ~ 0.2 rad for our PLUTO-014 panel, which theoretically could limit the null depth to ~ 1.2·10 -2 , 26 while higher retardance panels typically exhibiting phase flicker specifications nearly ten times lower.…”

“…This is in spite of the recent commissioning of several secondgeneration so-called "planet-imager" instruments, including the Gemini Planet Imager (GPI), 3 the Spectro-Polarimetric High contrast Exoplanet Research (SPHERE) instrument, 4 and the Subaru Extreme-Adaptive Optics (SCExAO) facility. 5,6 These near-infrared (NIR) HCI instruments incorporate the latest Adaptive Optics (AO) systems to correct for the atmospheric turbulences, to reach the extreme AO regime with Strehl ratio in excess of 0.8. Furthermore, these facilities make use of the latest techniques of wavefront sensing (WFS) to correct for non-common path aberrations (NCPAs) present in the beam train between the AO WFS and the science focal-plane array (FPA), 7,8 and integrate coronagraphs to mask, or diffract away, the bulk of the on-axis stellar point-spread function (PSF).…”

High-contrast imaging of tight resolved binaries with two vector vortex coronagraphs in cascade with the Palomar SDC instrument

“…However, in practice this limitation needs to be evaluated, as the contrast penalty for, e.g., a support spider being not masked in the Lyot pupil-plane, is usually quite modest as compared to wavefront control aspects. 33 Another solution compatible with field rotation and ADI -and which could also address the case of higher multiple systems (triples, etc.) -would be to rely on some kind of adaptive focal-plane coronagraphs, which would null several stars in the field-of-view and "follow" their rotation while observing.…”