  High contrast coronagraphic imaging of the immediate surrounding of stars
requires exquisite control of low-order wavefront aberrations, such as tip-tilt
(pointing) and focus. We propose an accurate, efficient and easy to implement
technique to measure such aberrations in coronagraphs which use a focal plane
mask to block starlight. The Coronagraphic Low Order Wavefront Sensor (CLOWFS)
produces a defocused image of a reflective focal plane ring to measure low
order aberrations. Even for small levels of wavefront aberration, the proposed
scheme produces large intensity signals which can be easily measured, and
therefore does not require highly accurate calibration of either the detector
or optical elements. The CLOWFS achieves nearly optimal sensitivity and is
immune from non-common path errors. This technique is especially well suited
for high performance low inner working angle (IWA) coronagraphs. On
phase-induced amplitude apodization (PIAA) type coronagraphs, it can
unambiguously recover aberrations which originate from either side of the beam
shaping introduced by the PIAA optics. We show that the proposed CLOWFS can
measure sub-milliarcsecond telescope pointing errors several orders of
magnitude faster than would be possible in the coronagraphic science focal
plane alone, and can also accurately calibrate residual coronagraphic leaks due
to residual low order aberrations. We have demonstrated 1e-3 lambda/D pointing
stability in a laboratory demonstration of the CLOWFS on a PIAA type
coronagraph.
