The basic outline of a pupil plane WaveFront Sensor is reviewed taking into account that the source to be sensed could be different from an unresolved source, i.e. it is extended, and that it could deploy also in a 3D fashion, enough to exceed the field's depth of the observing telescope. Under these conditions it is pointed out that the features of the reference are not invariant for different position on the pupil and it is shown that the INGOT WFS is the equivalent of the Pyramid for a Laser Guide Star. Under these conditions one can imagine to use a Dark WFS approach to improve the SNR of such a WFS, or to use a corrected upward beam in order to achieve a better use of the LGS photons with respect to an ideal Shack-Hartmann WFS.
@article{arxiv.2012.07560,
title = {Pupil plane wavefront sensing for extended and 3D sources},
author = {Roberto Ragazzoni and Valentina Viotto and Elisa Portaluri and Maria Bergomi and Davide Greggio and Simone Di Filippo and Kalyan Radhakrishnan and Gabriele Umbriaco and Marco Dima and Demetrio Magrin and Jacopo Farinato and Luca Marafatto and Carmelo Arcidiacono and Federico Biondi},
journal= {arXiv preprint arXiv:2012.07560},
year = {2020}
}