Radio-Frequency (RF) plasma glow discharge is an attractive candidate for in-situ mirror
cleaning of ITER optical diagnostics. In particular, the capacitively coupled radio frequency (CCRF) discharge type is selected as a baseline design solution for first mirror unit (FMU1) with RF discharge mirror cleaning function. A number of questions to CCRF discharge mirror cleaning have been already addressed. Many of them resulted in additional R&D tasks that are ongoing now; however, new open questions are appearing in the mirror cleaning project in line with engineering work activity on the FMU design development and FMU integration in diagnostic ports.