Abstract: A start of scan circuit for an FTIR spectrometer includes a mode counter, counting through various states of operation of the moving mirror in the FTIR interferometer, and a positioning counter circuit which counts laser pulses from a positioning laser also directed through the interferometer. By appropriately loading and counting up or down the counter in various states, as determined by the mode counter, the circuit can flexibly control the start of scan of the FTIR so that that start of scan can be selectively varied by switches and/or under software control.