Because of the LM deformation, heaters on the rear side of the large mirrors reduces the temperature difference between front and rear side. The temperature difference is obtained from existing thermocouples (formerly read by the UEI), a custom made 4-channel heater controller reads the temperature and controls the heating pads. There's one single device for all four large mirrors (called HS4K
). As the data conversion of the temperature sensors is very slow, it's enough to read the values not faster than twice per second.