1. An optical coupler comprising: an afocal prism assembly configured to receive light from an optical instrument, the light from the optical instrument including visible light and near-infrared light having different optical path lengths to respective visible and near-infrared focal plane locations, the prism assembly being configured to cause the visible light and near-infrared light to traverse separate optical paths to correct for path length difference such that visible and near infrared images can be formed on substantially the same image plane.