Without getting into such extremely boring topics as signal‐to‐noise ratio, detectivity, or noise‐equivalent power, let’s just say that wavefront sensors must have reasonably bright light in order to work. Shack‐Hartmann and pyramid sensors need a point source of light to work best. But what if the thing that we are looking at is not bright enough? Remember, we must use some of the light for the wavefront sensor and then have enough light left over for the image to form, which is the reason for doing the whole thing in the first place.
Because a wavefront sensor must have a bright beacon or source of light to work, artificial laser guide stars are used. The guide star must be close to the science object because of anisoplanatism. If the guide star is too low, the wavefront sensor only measures the small cone of light between the telescope and the guide star. The cone effect is avoided by placing a sodium laser guide star at a very high altitude. Beam tilt cannot be measured with a laser guide star because we don’t really have control over exactly where the guide star will form.
Online access to SPIE eBooks is limited to subscribing institutions.