Jennie W, Sheila

I noticed yesterday that the power reported as being reflected from the OMC is more than the power reported entering HAM6 (see this alog #87629). This does not affect any controls, but it does affect anyone who is using this channel to do comissioning tests.

Sheila checked this alog for the calibration of OMC-REFL_A PD, alog #65082. It looks like there should be a -20dB switch on in the filter bank when the gain is set to 20dB:

FM7/8 should be the inverse of H1:OMC-REFL_A_DC_GAINSETTING (e.g. enable "-20dB" if the gain is 20dB)

Checking the screen we found the gain setting was set to 0 on the front-end side but the '-20dB filter bank is still turned on. We have turned this filter bank off (see image with relevant parts circled), but the front-end H1:OMC-REFL_A_LF_OUT value still does not match the Beckhoff value, H1:OMC-REFL_A_DC_POWER.

Looking back at our last long lock (when the -20dB filter was still on) you can see that the OMC-REFL_A_LF_OUT16 and OMC-REFL_A_DC_POWER are off by a factor of 100 even though they are both supposed to be in mW. Therefore our switch off of the -20dB filter will not correct this problem and the rest of the calibration of this PD in the front-end filters needs more investigation.