Test and analysis of low-light characterisation of the ICCD Hartmann-Shack wavefront sensor

Hartmann-Shack wavefront sensors (Hartmann WFS) are widely used for optical wavefront measurement in adaptive optics. They are especially useful in the conjunction with laser guide stars that provide a quasi-point-source reference with continuous and pulse operating patterns. The principle of the sensor is that an array of lenslets is used to divide the coming wavefront into subaperatures and form images at their focal planes where the detector is placed. If the wavefront is plane, each lenslet forms an image of the source at its focus. If the wavefront is disturbed, each lenslet receives a tilted wavefront and forms an image out of axis in its focal plane. The measure of the image position gives the angle of arrival of the wave for each lenslet. The Hartmann WFS generally consists of a lenslet array, a detector which is a CCD or an intensified CCD, and an image processing system. For both natural and guide stars, a very low-light photo-counting level characterisation of the sensor is needed. And the knowledge on the low-light performance of the sensor for the both operating patterns is useful for the AO system design.