Summary:

There was some suspicion that the fast shutter was firing prematurely during CARM transition at 2W, breaking the lock, but it doesn't seem likely.

Details:

With a single bounce beam with 5W into IMC, H1:SYS-MOTION_C_SHUTTER_G_TRIGGER_VOLTS outputs about 0.02 volts (after subtracting dark offset of about 2mV). 

With this input, the IR power coming to HAM6 should be roughly 5W*0.03(PRM)*0.25(BS)*0.2 0.37(surrogate SRM) = 14mW.

[Voltage]/[HAM6 power] of that signal is 20mV/14mW = 1.4 Volt per Watt on HAM6.

By changing threshold (H1:SYS-MOTION_C_SHUTTER_G_THRESHOLD) I confirmed that the shutter triggers when the trigger voltage crosses the threshold, which is usually set at 2V.

So the shutter should trigger when HAM6 power is 2/1.4=1.4W, which sounds OK to me.

I don't know where exactly the H1:SYS-MOTION_C_SHUTTER_G_TRIGGER_VOLTS signal is monitored, and there's something odd about how the threshold voltage receiver has a gain of 0.2 while the trigger voltage receiver has a gain of 1 in D1102312, but anyway just from observing how the signals behave at which power, things don't look crazy.

If people still think that the shutter is triggered prematurely, we need to hook up a fast ADC channel to the trigger voltage. That voltage comes from the summation output of the AS_C interface board and recorded in EPICS only, AS_C_SUM is a test point, so we cannot do any serious trigger analysis except in real time for the moment.