The concept, manufacturing praxis with IBS, and a limitation during the design process regarding the pulse duration of THG mirrors are described. To increase the precision of the thicknesses of the manufactured films a high resolution, in-situ transmission measuring system is used and further improved. Addionally the dispersion data of the utilized hafnium is constantly investigated and process routines concerning the stability of the refractive index are introduced. During the project in which these mirrors are produced it became clear that the pulse duration is a factor which cannot be neglacted during the design synthesis. Especially for very short pulses the efficiency to expact is lowered bei multiple orders of magnitude. The current highest achieved efficiency is 1.8\%.