A mirror device for providing an image of a remote object, suitable for use in a multi-spectral collimator, comprises a primary concave mirror whose surface is an off-axis portion of a paraboloid having a rotational symetry about an axis, and a secondary concave mirror whose surface is an off-axis portion of a hyperboloid, one focus of which coincides with the focus of the paraboloid. The mirrors have sizes of the same order of magnitude. The primary mirror is so located that a light beam received by it and having a mean ray parallel to the axis of the paraboloid is unobscured by the secondary mirror. The radii of curvatures of the two mirrors at locations along their respective axes do not differ by more than 10%.