A method of determining a reference calibration setting for an adaptive optics system comprising a detecting device for detecting light from an object; and at least one controllable wavefront modifying device arranged such that light from the object passes via the wavefront modifying device to the detecting device. The method comprises: arranging a light-source between the object and the wavefront modifying device to provide a reference light beam to the detecting device; for each of a plurality of orthogonal wavefront modes: controlling the wavefront modifying device to vary a magnitude of the orthogonal wavefront mode; acquiring a series of readings of the detecting device, each reading corresponding to one of the magnitude; determining a quality metric value for each reading, resulting in a series of quality metric values; and determining a reference parameter set for the wavefront modifying device corresponding to an optimum quality metric value.