Gravitational wave detectors such as Virgo and LIGO use long-baseline Michelson interferometers with high finesse Fabry-Perrot cavity in the arms. The symmetry of these cavities is essential to prevent the interferometer from sensitivity to laser fluctuations. For this purpose the difference between the transmissions of the two input mirrors has to be minimized. Advanced LIGO, the upgrade of LIGO, plans a transmission matching between the two input mirrors as high as 99%. A small deviation in the process fabrication from run to run might induce transmission mismatch larger than 1%. Consequently, the two input mirrors have to be coated during the same coating run. That requires ability to deposit the reflective coating, based on a stack of titanium doped tantala (Ti:Ta2O5) layers and silica layers, uniformly over a 800 mm diameter aperture. This paper presents the study to improve the thickness uniformity of a reflective coating and the preliminary results achieved on two Ø350mm substrates coated in the run.© (2011) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.