Manufacturing Large Mirrors – The Challenge

The production of such mirrors presents a unique set of challenges for optical manufacturers. Firstly, there is accuracy of surface form – whether it is spherical, aspheric, parabolic, or free form. Tolerance requirements of the order of 0.2 fringe of HeNe light are not uncommon. The off-axis form is specified in systems that cannot tolerate the central obscuration. These off-axis mirrors are substantially more challenging in terms of mirror fabrication, testing, and system assembly.

As large mirrors are often used for multispectral applications, they must perform to a high level across a wide range of wavelengths. This means that the mirrors must have minimum roughness, especially when they are used in VIS wavelength, under 40Å RMS, to prevent light from scattering. Additionally, as detectors are increasing in resolution, there has been growth in the demand for mirrors with increasingly accurate surfaces.