Exciting news from the SCExAO instrument

Sébastien Vievard, Julien Lozi, Subaru telescope, August 16th 2018

The Subaru Coronographic Extreme Adaptive Optics (SCExAO) instrument is a versatile high contrast instrument installed on the Subaru telescope in Hawaii (See Fig. 1 below). SCExAO benefits from a first stage of wavefront correction with the facility adaptive optics AO188 and splits the 600-2400 nm spectrum towards a variety of modules, in visible and near-infrared, optimized for a large range of science cases. Thanks to a pyramid wavefront sensor (PyWFS), SCExAO can perform a second stage of wavefront correction allowing to obtain a high Strehl ratio (>80% in median seeing in H-band) critical for high contrast imaging. One of SCExAO’s current science mission is to feed the integral field spectrograph CHARIS in J-, H-, or K-band for exoplanet detection and characterization. In the visible, SCExAO feeds the VAMPIRES module that performs differential polarization imaging sometimes coupled with non-redundant masking for diffraction limited scattered-light imaging of circumstellar environments.

Fig. 1: SCExAO instrument on the Subaru telescope Nasmyth InfraRed platform.

Contrary to other instruments, SCExAO has a unique design and operation allowing for continuous upgrades with the latest hardware software and algorithms, while still performing cutting-edge science on-sky.

One of the latest in date is the upgrade of the internal NIR cameras with two new C-RED2 (Fig. 2, left). They replaced the Lyot-stop low-order WFS camera and the focal plane WFS camera. With 400 Hz (600 Hz with the new firmware), 30 electrons of noise and a non-destructive read mode, these cameras will allow us to perform wavefront correction on fainter stars, and run faster on bright stars. A sub-window mode can also be used to increase the frame rate to a few kHz.
SCExAO will soon perform the injection of the post-coronographic planet light into a single-mode fiber coupled to the InfraRed Doppler high-resolution spectrograph (IRD). We would then obtain a high-resolution spectrum of the exoplanet light, free from modal noise, allowing to measure the atmosphere’s composition. During an observing run in the end of June, the light of Alpha Herculis (mag -3 in H-band) was successfully injected from SCExAO to IRD despite bad seeing conditions, promising a bright future with the new fiber injection optimization developments to come.
The most important recent upgrade of SCExAO is the integration of MEC, a new generation camera using a 20 000-pixel MKIDs (Microwave Kinetic Inductance Detector) detector. This noiseless photo-counting camera allows measurements of the energy -therefore the wavelength- of every photon with a resolution of about 10. This camera and the wavelength information will be useful for example for high-speed focal plane wavefront sensing to correct chromatic effects in the speckle field.
SCExAO has also the ability to test various coronographic designs and pupil masks thanks to a pupil wheel, apodizing lens wheels, a focal plane wheel and a Lyot stop wheel. Among the dozen of coronographic options, a Phase-Induced Amplitude Apodization Complex Mask Coronograph (PIAACMC, Fig. 2, middle) was tested for the first time in a high-constrast imager, showing some promising results. In the pupil wheel, a shaped pupil mask creating four high-contrast square zones was characterized on-sky. Another promising high-contrast device is the vector Apodizing Phase Plate (vAPP, Fig. 2 right). It uses a liquid crystal pattern to create any type of phase aberration like grating, Zernike aberration and diffraction patterns.
In addition to these upgrades, modules like FIRST, a visible single-mode fiber injection interferometer module, GLINT, an IR Ultrafast Laser Inscription integrated pupil remapper and nulling interferometer, as well as RHEA, a visible 3x3 single-mode fiber injection integral field spectrograph, demonstrated good results during the May and June 2018 engineering runs and will be upgraded soon for even more efficiency.

Fig. 2: Left: CRED-2 camera; Middle: PIAAMC under microscope; Right: Focal plane image after vAPP, creating symmetric dark zones around copies of the PSF, as well as defocused images for phase diversity measurements.

We would like to thank all our collaborators for their work and commitment to make SCExAO a competitive and efficient instrument. We also thank the Japan Society for the Promotion of Science (Grant-in-Aid for Research #23340051, #26220704 & #23103002), the Astrobiology Center of the National Institutes of Natural Sciences in Japan, the Mt Cuba foundation and the directors contingency fund at Subaru for their financial support. The SCExAO team wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

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