An optical apparatus comprising a first mirror (12), a second mirror (14), and at least one support (30) for holding the second mirror in substantially a predetermined position relative to said first mirror (12), wherein said at least one support (30) comprises at least one actuator (32) arranged to adjust the position of the second mirror (14), and said optical apparatus further comprises at least one light source (40a-40-c) rigidly fixed to the first mirror (12) in a predetermined orientation for providing a beam of light (42a-42c) directed at said second mirror (14), at least one corresponding alignment sensor (46a-46c) for detecting the beam of light (44a-44c) reflected from said second mirror (14), and arranged to provide an output signal indicative of the position of the incident reflected beam, and a controller (50) arranged to receive said output signal, and to thereby control said actuator (32) to adjust the position of the second mirror (14).