The importance of tip-tilt correction
Because it is optimized for the detection of companions at low angular separations, a high-performance coronagraph such as the PIAA is pushed to become highly sensitive to low-order aberrations, and especially pointing errors. Indeed, for angular separations close to 1 λ/D, the signal created by a small (<< λ/D) pointing error can perfectly mimic the one of a high-contrast companion that would be located at a larger separation. Since the coronagraph is designed to transmit the signal of such companions, it will transmit the pointing error signal. The SCExAO system therefore includes a robust and accurate wavefront sensor to measure the tip-tilt as well as defocus, so-called Coronagraphic Low-Order Wavefront Sensor (CLOWFS).
CLOWFS uses the light reflected by the focal plane mask located after the apodizer (c.f. Figure). However, instead of reflecting all the on-axis stellar light, the central part of the mask is opaque and only a reflective annulus around this central part sends light to the CLOWFS camera, which acquires a defocused image of this annulus. Discarding the central part of the light rejects a lot of flux that contains very little signal. In a manner that is reminiscent of Schlieren photography or the Foucault knife-edge, this central dark zone makes tiny changes in the wavefront produce macroscopic changes in the CLOWFS image. For wavefront aberrations smaller than 1 radian, the CLOWFS image is a linear function of the wavefront aberration modes. In the lab, CLOWFS has demonstrated very good performance, successfully helping to stabilize the tip-tilt with residuals approaching 10-3 λ/D.
The Lyot-based CLOWFS
For phase mask based coronagraphs like the PIAA, PIAACMC, 4QPM, 8OPM and Vortex versions, the light is not reflected but transmitted by the focal plane mask and hence the original CLOWFS concept outlined above does not apply. Instead we have adapted our CLOWFS so that light diffracted by the focal plane mask is reflected at the Lyot-stop towards the LOWFS camera. The beam is refocused onto the camera which is slightly defocused in a manner similar to the original concept.
The Lyot-based LOWFS concept. An image of a reflective Lyot-stop mask (chrome plated).
A reference image of the PSF with the 4-quadrant phase mask in the focal plane.
The Lyot-based LOWFS has been tested on the first 3 modes (tip/tilt and focus) in the laboratory. Here is a video of the lowfs loop closing on the coronagraphic PSF generated by the vortex coronagraph. The video shows a constant tip/tilt modulation being applied in a circle while simultaneously the turbulence applied is varied (to simulate varying degrees of seeing). Keep an eye on the two images to the bottom right the left of which shows the image the LOWFS sees and the right the internal science camera. Also note the tip/tilt residual graph improvement when the loop is closed and tuned.