There is provided an optical imaging device (18) for splitting an initial image into at least two images with different optical characteristics. The device comprises a dichroic mirror (32) to create first and second optical pathways respectively incident on first and second mirrors (41, 41′) carried on a centrally pivoted rotatable arm, characterised in that the first and second reflective means are moveable along the arm (42) whilst held in fixed relationship to each other, thereby to adjust separation of the first and second optical pathways. A third mirror (46) in fixed relationship to the beam splitter (32) is positioned adjacent where the first and second optical pathways intersect, or just before the intersection of the first and second optical pathways, or just after the point of intersection.