There are multiple methods by which the arm loss can be measured, including, but not limited to:
We found that the second method is extremely sensitive to errors in the ITM transmissivity. The first method was not an option for a while because the AOM (which serves as a fast shutter to cut the light to the cavity and thereby allow measurement of the cavity ringdown) was not installed. Johannes and Shubham have re-installed this so we may want to consider this method.
Most of the recent efforts have relied on the 3rd method, which itself is susceptible to many problems. As Yutaro found, there is something weird going on with ASDC which makes it perhaps not so reliable a sensor for this measurement (unfortunately, no one remembered to follow up on this during the vent, something we may come to regret...). He performed some checks and found that for the Y arm, POY is a suitable alternative sensor. However, the whitening gain was at 0dB for the measurements that Johannes recently performed (Yutaro does not mention what whitening gain he used, but presumably it was not 0). As a result, the standard deviation during the 10s averaging was such that the locked and misaligned readings had their 'fuzz' overlapping significantly. The situation is worse for POX DC - today, Eric checked that the POX DC and POY DC channels are indeed reporting what they claim, but we found little to no change in the POX DC level while misaligning the ITM - even after cranking the whitening gain up to 40!
Eric then suggested deriving ASDC from the AS110 photodiode, where there is more light. This increased the SNR significantly - in a 10s averaging window, the fuzz is now about 10 ADC counts out of ~1500 (~<1%) as opposed to ~2counts out of 30 previously. We also set the gains of POX DC, POY DC and ASDC to 1 (they were 0.001,0.001 and 0.5 respectively, for reasons unknown).
I ran a quick measurement of the X arm loss with the new ASDC configuration, and got a number of 80 +/- 10 ppm (7 datapoints), which is wildly different from the ~250ppm number I got from last night's measurement with 70 datapoints. I was simultaneously recording the POX DC value, which yielded 40 +/- 10 ppm.
We also discovered another possible problem today - the spot on the AS camera has been looking rather square (clearly not round) since, I presume, closing up and realigning everything. By looking at the beam near the viewport on the AS table for various configurations of the ITM, we were able to confirm that whatever is causing this distortion is in the vacuum. By misaligning the ITM, we are able to recover a nice round spot on the AS camera. But after running the dither align script, we revert to this weirdly distorted state. While closing up, no checks were done to see how well centered we are on the OMs, and moreover, the DRMI has been locked since the vent I believe. It is not clear how much of an impact this will have on locking the IFO (we will know more after tonight). There is also the possibility of using the PZT mounted OMs to mitigate this problem, which would be ideal.
Long story short -
GV Edit 8 Oct 2016: Going through some old elogs, I came across this useful reference for loss measurement. It doesn't talk about the reflection method (Method 3 in the list at the top of this elog), but suggests that cavity ringdown with the Trans PD yields the most precise numbers, and also allows for measuring TITM