Adaptive Optics for Extremely Large Telescopes 4 – Conference Proceedings

The concepts and considerations that could leads to the development of a novel kind of classes of wavefront sensors inwhich the information is retrieved by the absence of photons rather than from their presence are shown. In quantumoptics the concept of ”sensing” an object without actually having photons interacting with it is already known. Thepotentiality in terms of sensitivity has been already pointed out. Under the conditions that background counts are smallenough (something that evolved with time with respect to a couple of decades ago, as today close to zero RON detectorsare available, although sky background would continue to impose some limit to this approach) detecting ”no photons”can be made with an SNR larger than detecting a ”given amount of photons” in a number of practical situations. Whilewe review the case of the coronograph coupled with a conventional WFS -a case already proposed in the literature- weexplore variations on the theme of the Smartt wavefront sensor and in particular we explore the case of a double channelWFS where two images of the pupil are exploited and the flat wavefront is perceived as no photons on the pupils. Thepresence of a signal on subapertures on one pupil or the other indicates the sign of the wavefront perturbation. Thesystem can be tuned in its sensitivity and -potentially- adjusted in order to have the two detectors working with close tozero flux on them while in closed loop operations. As the ”signal” is coming from the condition of no photons Idescribed these as a ”Dark” WFS.