Anamaria, Varun, Georgia
Today we tried to project the noise from some of the OMC ASC loops to DARM, using the same method as here. We only managed to get to the POS_X and POS_Y loops today, but we project that the POS_X loop is within a factor of a few of measured DARM, and the POS_Y loop we project to be *above* DARM, which is pretty questionable. We'll need to think about this measurement some more.
To do these measurements we first added an offset to the loop such that the noise is visibile in DARM. See the purple (quiet) and green (offset) traces in the first 2 attached screenshots. For the POS_X loop we started to see noise peeking above DARM at an offset of ~3, while for the POS_Y loop the noise was barely visible at an offset of 6. Any more offset and the OMC suspension (T2 & T3) started to saturate. We then run an excitation in the loops, and look at the RMS noise in DARM in the excitation band as a function of loop offset. This is the pink/brown/green/orange traces in the attached screenshots.
We measure the RMS area under DARM in the band of the excitation using DTT, and then fit a linear term (large offsets) and flat term (small offsets) using Anamaria's matlab script. This allows us to project the noise in DARM for 0 offset and 0 excitation.
We first tried a much small excitation (Amplitude=0.15) in the POS_X loop, but it yeilded similar but noisier results. The results of POS_Y are strange - there is ever-so-slightly less noise coupling with a small offset in the loop (green trace) compared to 0 offset (gold trace). We've seen this in the past, but it seems reduced now, maybe there is some less junk light on the QPD?
Other things we considered:
- As we put offsets in the OMC ASC the DARM calibration suffers (this is what caused jumps in the BSN range). While POS_X offset = 6, kappa_c = 0.796. Nominally Kappa_c = 1.02. Whiule POS_Y offset = 6, kappa_c = 0.953. We think that at the frequencies we're injecting changes in kappa_c don't matter. While we were running these injections we also kept an eye on H1:CAL-CFTD_DELTAL_EXTERNAL_DQ and saw the same response as in CAL_DELTAL.
- Our OMC ASC is using the QPDs, rather than the dither. Perhaps for some reason this measurement doesn't work when using the QPDs?
- We might in the future try to redo these measurements with the OMC ASC on the dither. See third attachment screenshot of the awggui used to make these injections. For POS_Y the amplitude was 1.5.
