Methods and apparatus of slot time synchronization for coexistence of 5 MHz, 10 MHz, and 20 MHz systems in TV white space are provided. The anode terminal includes a first and a second anode terminals, and circuit current flows from the first anode terminal toward the second anode terminal through the porous sintered body. The invention is suitable for astigmatism, fourfold wavefront-deformations due to lens heating, compaction, and the like.