A beam-expander optical system, comprising: a convex mirror having a first mirror substrate made of metal and having a convex substrate surface and a first reflective multilayer coating formed on the convex substrate surface; a concave mirror having a second mirror substrate made of the metal and having a concave substrate surface and a second reflective multilayer coating formed on the concave substrate surface, with the convex and concave mirrors being arranged in an off-axis, afocal configuration having greater than unity magnification; characterized in that the first and second reflective multilayer coatings (24, 34) each includes alternating layers of HfO 2 and SiO 2 that define at least first and second sections (S1 and S2), wherein the first section is closest to the convex or concave substrate surface and is configured to optimally reflect a first wavelength of light and substantially transmit mid-wavelength IR light, wherein the second section resides atop the first section and is configured to optimally reflect a second wavelength of light that is shorter than the first wavelength of light and wherein the second section substantially transmits the mid-wavelength IR light; the first and second reflective multilayer coatings (24, 34) each include a third section atop the second section and configured to optimally reflect a third wavelength of light that is shorter than the second wavelength of light and substantially transmit the mid-wavelength IR light; the first wavelength of light is an infrared wavelength, the second wavelength of light is a visible wavelength and the third wavelength of light is an ultraviolet wavelength; and the beam-expander optical system has a high-efficiency transmittance T BE > 95% for the first and second wavelengths of light. a convex mirror having a first mirror substrate made of metal and having a convex substrate surface and a first reflective multilayer coating formed on the convex substrate surface; a concave mirror having a second mirror substrate made of the metal and having a concave substrate surface and a second reflective multilayer coating formed on the concave substrate surface, with the convex and concave mirrors being arranged in an off-axis, afocal configuration having greater than unity magnification; characterized in that the first and second reflective multilayer coatings (24, 34) each includes alternating layers of HfO 2 and SiO 2 that define at least first and second sections (S1 and S2), wherein the first section is closest to the convex or concave substrate surface and is configured to optimally reflect a first wavelength of light and substantially transmit mid-wavelength IR light, wherein the second section resides atop the first section and is configured to optimally reflect a second wavelength of light that is shorter than the first wavelength of light and wherein the second section substantially transmits the mid-wavelength IR light; the first and second reflective multilayer coatings (24, 34) each include a third section atop the second section and configured to optimally reflect a third wavelength of light that is shorter than the second wavelength of light and substantially transmit the mid-wavelength IR light; the first wavelength of light is an infrared wavelength, the second wavelength of light is a visible wavelength and the third wavelength of light is an ultraviolet wavelength; and the beam-expander optical system has a high-efficiency transmittance T BE > 95% for the first and second wavelengths of light.