An apparatus for optical imaging of a low radiance object is disclosed. The apparatus comprises a fixed polychromatic imaging device, the optical path between the polychromatic imaging device and the low radiance object being defined by a first section extending from the polychromatic imaging device to a fixed point, and a second section extending from the fixed point to the low radiance object. The apparatus also comprises a fixed high-sensitivity imaging device, the optical path between the high-sensitivity imaging device and the low radiance object being defined by a third section extending from the high-sensitivity imaging device to the fixed point, and the second section; and a focus controller arranged to control the focus of the high-sensitivity imaging device based on the focus of the polychromatic imaging device and the respective lengths of the first and third sections.