Lyot-based low order wavefront sensor for phase-mask coronagraphs: Principle, simulations and laboratory experiments

High performance coronagraphic imaging of faint structures around bright stars at small angular separations requires fine control of tip, tilt, and other low order aberrations. When such errors occur upstream of a coronagraph they result in starlight leakage, which reduces the dynamic range of the instrument. This issue has been previously addressed for occulting coronagraphs by sensing the starlight before or at the coronagraphic focal plane. One such solution, the coronagraphic low order wave-front sensor (CLOWFS), uses a partially reflective focal plane mask to measure pointing errors for Lyot-type coronagraphs. To deal with pointing errors in low inner working angle phase mask coronagraphs which do not have a reflective focal plane mask, we have adapted the CLOWFS technique. This new concept relies on starlight diffracted by the focal plane phase mask being reflected by the Lyot stop towards a sensor which reliably measures low order aberrations such as tip and tilt. This reflective Lyot-based wavefront sensor is a linear reconstructor which provides high sensitivity tip-tilt error measurements with phase mask coronagraphs. Simulations show that the measurement accuracy of pointing errors with realistic post adaptive optics residuals are ≈10^-2λ/D per mode at λ = 1:6 μm for a four quadrant phase mask. In addition, we demonstrate the open loop measurement pointing accuracy of 10^-2λ/D at 638 nm for a four quadrant phase mask in the laboratory.