The challenges of a large-scale mirror design for the in-vessel collection system of ITER divertor Thomson scattering (DTS) are under consideration. These are the mirrors located out-of-line vision of fusion plasma (so called 'second mirrors') with a high-reflective Ag-based coating. The paper outlines: approaches providing optical surface shape and angular position stability; Ag coating applicability for ITER in-vessel environment, including resistance to accidental steam ingress. The proposed solutions may also appear to be of some use in other ITER diagnostics and in diagnostics of other fusion devices.