An apparatus for optical imaging of a low radiance object is disclosed. It comprises a fixed polychromatic imaging device 104, the optical path between the polychromatic imaging device and the object being defined by a first section extending from the polychromatic imaging device to a fixed point 111, and a second section extending from the fixed point to the object 107. A fixed high-sensitivity imaging device 101 has an optical path between itself and the object defined by a third section extending from the high-sensitivity imaging device to the fixed point, and the second section. A focus controller is arranged to control the focus of the high-sensitivity imaging device based on the focus of the polychromatic imaging device, which may be tracked by an encoder, and the respective lengths of the first and third sections. The apparatus may also comprise an optical system 109 arranged to vary the length of the second section.
