Variation around a pyramid theme: optical recombination and optimal use of photons

We propose a new type of wave-front sensor (WFS) derived from the pyramid WFS (PWFS). This new WFS, called the flattened pyramid-WFS (FPWFS), has a reduced pyramid angle in order to optically overlap the four pupil images into an unique intensity. This map is then used to derive the phase information. In this Letter, this new WFS is compared to three existing WFSs, namely the PWFS, the modulated PWFS (MPWFS), and the Zernike WFS (ZWFS) following tests about sensitivity, linearity range, and low-photon-flux behavior. The FPWFS turns out to be more linear than a modulated pyramid for the high-spatial order aberrations, but it provides an improved sensitivity compared to the non-modulated pyramid. The noise propagation may even be as low as the ZWFS for some given radial orders. Furthermore, the pixel arrangement being more efficient than for the PWFS, the FPWFS seems particularly well suited for high-contrast applications.