An optical system (120) for detecting optical aberrations of light from an object (101), the optical system comprising: a reference light-source (102) providing collimated reference light； an optical element (212) configured to focus at least one collimated light beam incident on the optical element (212) to a plurality of focal points in a conjugate object plane (214), the optical element (212) being arranged in an optical path between the reference light-source (102) and the object (101) for transmitting a plurality of reference light beams towards the object (101)； and a wavefront sensor (112) configured to detect a property indicative of an optical aberration of light incident on the wavefront sensor； wherein the optical element (212) is further arranged to transmit a plurality of reflected guide star light beams resulting from reflection of the reference light beams at the object (101) towards the wavefront sensor (112).
