On-Sky Demonstration of Low-Order Wavefront Sensing and Control with Focal Plane Phase Mask Coronagraphs

Abstract

The ability to characterize exoplanets by spectroscopy of their atmospheres requires direct imaging techniques to isolate planet signal from the bright stellar glare. One of the limitations with the direct detection of exoplanets, either with ground-or space-based coronagraphs, is pointing errors and other low-order wavefront aberrations. The coronagraphic detection sensitivity at the diffraction limit therefore depends on how well low-order aberrations upstream of the focal plane mask are corrected. To prevent starlight leakage at the inner working angle of a phase mask coronagraph, we have introduced a Lyot-based low-order wavefront sensor (LLOWFS), which senses aberrations using the rejected starlight diffracted at the Lyot plane. In this paper, we present the implementation, testing and results of LLOWFS on the Subaru Coronagraphic Extreme Adaptive Optics system (SCExAO) at the Subaru Telescope.We have controlled thirty-five Zernike modes of a H-band vector vortex coronagraph in the laboratory and ten Zernike modes on sky with an integrator control law. We demonstrated a closed-loop pointing residual of 0.02 mas in the laboratory and 0.15 mas on sky for data sampled using the minimal 2-second exposure time of the science camera. We have also integrated the LLOWFS in the visible high-order control loop of SCExAO, which in closed-loop operation has validated the correction of the non-common path pointing errors between the infrared science channel and the visible wavefront sensing channel with pointing residual of 0.23 mas on sky.

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

“…As previously introduced, this specific Lyot stop arrangement is uniquely available on SCExAO, and enables Lyot-plane loworder wavefront sensing (LLOWFS, see Singh et al 2015) in situ, using the coronagraphically rejected starlight. Practically, the reflected starlight is focused on an InGaAs CMOS camera that serves as a low order wavefront sensor.…”

Lyot-plane phase masks for improved high-contrast imaging with a vortex coronagraph

“…Although unwanted speckles appear in the dark region, the VC and LPMs offer contrast improvement in the presence of wavefront errors potentially achieved on current and next-generation high-contrast imaging instruments (ω < λ/100). These aberrations levels require post-coronagraphic, low-order wavefront sensing solutions, such as those proposed in Codona & Kenworthy (2013), Singh et al (2014Singh et al ( , 2015, or Huby et al (2015).…”

Laser Guide Star for Large Segmented-aperture Space Telescopes. I. Implications for Terrestrial Exoplanet Detection and Observatory Stability

“…Coronagraph designs, such as WFIRST-Coronagraph Instrument (CGI) and the HabEx VVC use rejected starlight from a reflective focal plane mask at the center of the field to feed a ZWFS. Other designs use reflected light from the Lyot stop (Singh et al 2015;Mendillo et al 2015). For an LGS with precision station keeping (Sec.…”