  Full sky coverage Adaptive Optics on Extremely Large Telescopes requires the
adoption of several Laser Guide Stars as references.With such large apertures,
the apparent elongation of the beacons is absolutely significant.With few
exceptions,WaveFront Sensors designed for Natural Guide Stars are adapted and
used in suboptimal mode in this context. We analyse and describe the
geometrical properties of a class of WaveFront Sensors that are specifically
designed to deal with Laser Guide Stars propagated from a location in the
immediate vicinity of the telescope aperture. We describe in three dimensions
the loci where the light of the Laser Guide Stars would focus in the focal
volume located behind the focal plane (where astronomical objects are
reimaged). We also describe the properties of several types of optomechanical
devices that, through refraction and reflections, act as perturbers for this
new class of pupil plane sensors, which we call ingot WaveFront Sensor. We give
the recipes both for the most reasonable complex version of these WaveFront
Sensors, with 6 pupils, and for the simplest one, with only 3 pupils. Both of
them are referred to the ELT case. Elements to have a qualitative idea of how
the sensitivity of such a new class of sensors compared to conventional ones
are outlined. We present a new class of WaveFront Sensors, by carrying out the
extension to the case of elongated sources at finite distance of the pyramid
WaveFront Sensor and pointing out which advantages of the pyramid are retained
and how it can be adopted to optimize the sensing.
