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The Subaru Seminar is usually held in Room 104 of the Hilo Base Facility, adjacent to the main lobby. Everyone is welcome to attend. If you are interested in giving a seminar, please contact Subaru seminar organizers (Tae-Soo Pyo, Sherry Yeh, Nagayoshi Ohashi) by email : (please change"_at" to @).

September 17, Thursday, 1:00 pm in 104

" Low-order Wavefront Control and Calibration for Phase Mask Coronagraphs "

Garima Singh

(Observatoire de Paris/Subaru Telescope)

The direct detection of young and warm extrasolar giant planets in the habitable zone of nearby cool stars is one of the major goals of current ground-based high contrast imaging instruments. To characterize such exoplanets by spectroscopy of their atmospheres requires isolating the planet light from the brighter stellar light, which is challenging due to high contrast at small angular separation. Using high performance small inner working angle (IWA) coronagraphs, it is possible to detect faint companions in the proximity of the stellar source. However, the uncontrolled pointing errors and other low-order wavefront aberrations degrade the rejection capability of these coronagraphs by leaking starlight around the coronagraphic focal plane mask. How well these aberrations upstream of various coronagraphs can be controlled is the focus of my research. To prevent coronagraphic leaks at small IWA, I worked on a concept where within a coronagraph, the low-order wavefront aberrations are sensed at the Lyot plane. The starlight diffracted by the focal plane mask is reflected by the Lyot stop towards a detector, which reliably estimates low-order aberrations present in the wavefront. I called this new sensor a Lyot-based low-order wavefront sensor (LLOWFS), which is now operational on Subaru coronagraphic extreme adaptive optics instrument (SCExAO) at the Subaru Telescope (8.2 meter). On SCExAO, I have validated the closed-loop performance of the LLOWFS for 15 Zernike modes with the Phase-Induced Amplitude Apodization (PIAA) coronagraph and for 35 Zernike modes with the Vector Vortex Coronagraph (VVC) and the four quadrant/eight octant phase masks. Closed-loop pointing residual between 10-3 l/D and 10-4 l/D is obtained in H-band at 170 Hz for slow-varying errors (frequencies < 0.5 Hz) for all the coronagraphs. On the on-sky post-adaptive optics wavefront residuals, correction of 10 Zernike modes are demonstrated with the VVC and the PIAA coronagraph. Under good seeing and for bright target (mH < 2), the low-order control provides a closed-loop pointing residuals of 10-4 l/D whereas under moderate seeing and for targets brighter than mH < 2.5, the low-order control routinely provide a closed-loop residual of 10-3 l/D. In this talk, after introducing the LLOWFS principle in the context of high contrast imaging, I will describe its implementation on the SCExAO instrument followed by the results obtained in the laboratory and on-sky under the extreme-AO (ExAO) and non ExAO regime. As a closing remark, I will explain briefly how the LLOWFS-like technology can benefit the planned ground/space-based ExAO instruments.

Seminars are also held at JAC, CFHT, and IfA.

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