Laser Tomographic systems, such as ATLAS, will rely on natural guide stars (NGS) to sense low order perturbation.
This low order perturbation contains low order turbulence and Telescope Windshake, which strength
lead to NGS wave front sensor (WFS) frame rate of several hundred Hertz. Therefore, the ability of the NGS
WFS to deliver precise low order measurements in low signal to noise conditions will drive the limit magnitude
of the NGS, hence the sky coverage. We have investigated the use of a focal plane sensor for this purpose, and
consider it as the most efficient sensor in this context. We propose LIFT (LInearized Focal-plane Technique),
and compare it to classical sensors, such as Quad Cell WFS, Pyramid WFS and Shack-Hartmann WFS. We
derive an analytic model of the noise propagation law, which we validate on End-to-End diffractive simulations,
based on realistic phase screens.