I made a calibration of the IM4trans and MC2trans SUM channels into uW.
The conversion from W incident on the PDs to counts registered on IMC-IM4_TRANS_SUM_OUTMON is as follows:
[counts/W] = responsivity [0.16 A/W] x transimpedance gain [1000V/A] x differential to single input gain [2V/V] x whitening gain [36dB=63.1 V/V] x ADC gain [1.6384x10^3 cts/V].
This is actually the same for both MC2trans QPD and IM4trans QPDs: 2.087x10^9 [counts/W]
I used this calibration factor to calculate the power on IM4trans and MC2trans currently:
IM4trans counts ~= 57890 counts, which converts to 1.75mW. Comparing this with the expected power on IM4trans:
8.73W into IMC * 0.85 IO throughput * 2400ppm IM4 transmission * 0.1 transmission of BS between IM4 and IM4trans QPD = 1.78mW
MC2trans counts ~= 2.17x10^4 counts, which converts to 655uW. Comparing this with the expected power on MC2trans:
8.73W into IMC * (IMC gain = IMC Finesse/pi = 166 ) * 5.1x10^-6 MC2 transmission * 0.1 transmission of BS between MC2 and MC2trans QPD = 740uW.
Both these estimates for the power incident on each PD agree pretty well 
so I'll go ahead and add a new filter called "cts2uW" in the SUM_OUTMON paths to give the outputs in uW incident on the PDs.
The fact that the SUM output is used to normalize PITCH and YAW signals meant that adding the filter in front of just the SUM channel increased the gains on PITCH and YAW significantly, affecting the IMC ASC loops. I have now put the cts2uW filters in the input filter banks for each segment of both QPDs. It works fine now, so the MC2trans SUM and IM4trans SUM are calibrated in uW. If anyone changes the whitening gains on these PDs, please be sure to adjust the calibration factor accordingly. I may look into automatically factoring the whitening gain into the calibration to avoid this issue in future.
Just to be clear, the calibration from counts to uW for both IM4trans and MC2trans was 0.032 uW per count. The number 2.087x10^9 [counts/W] was in error: this number should have been 3.3080x10^7 [counts/W]. Although the number in the alog post was in error, the correct number was used in the filters which were applied.