Dan, Fil

Today we swapped R23 on the AS_C transimpedance board (D1001974) to increase the gain on the single-ended sum output that is used for the triggering of the HAM6 shutter functions (the fast shutter, and OMC PZT shutter).  The old resistors were 424 ohms, the new ones are 26.7k.  The increase of 63x compensates for the reduced transmission of the OM1 mirror (was 5%, now is 800ppm - half of which goes to AS_C).  The new resistor value allows us to keep the same threshold setting for the shutter logic: 1W into the chamber results in 400uW on AS_C, with a 1k transimpedance resistor and 80% quantum efficiency the threshold should be (400e-6 * 0.8 * 1000 * 26.7e3/4.99e3) = 1.7 volts.  The maximum threshold we can set via Beckhoff is 2V, so this is a good fit.

As Rich pointed out the last time we made this swap, it would be better to account for changes in the gain at the input to the shutter controller, so that the max output of the AS_C sum channel (10V) matches the full range of the PD (10mA).  Unfortunately the Beckhoff readbacks of the PD input to the shutter controller are picked off before the input gain stage, so if we adjusted the gain on that board, the shutter threshold setting in EPICS would not longer agree with the PD input channel.  This feels like a bigger source of confusion than the signal loss we suffer from changing the gain on the output from the transimpedance box, so we've stuck with the old kludge for now.

Also since the timing of the Beckhoff readbacks violates causality in strange ways, it would be good to have a 16k RCG readback of this single-ended sum output from AS_C.  There's a spare channel in the PD interface box on ISCT6.

I have updated the E-traveler for this board (S1301506).

With this change the HAM6 shutter should be functional with the new OM1, we will test it tomorrow to be sure.