Having previously calibrated the IMC mirror alignment offsets (see aLOG 9870), on the evening of Thursday 6th Feb I proceeded to misalign the IMC in all 4 D.O.F.s in turn and measure the RIN spectra in transmission using MC2 trans QPD. This is the same measurement that was performed for the PSL commissioning mode beam jitter measurement as reported in aLOG 8190, the only differences being the more precise calibration of the applied offsets this time, and of course the PSL being in science mode this time.

The first attached plot shows the newly measured jitter spectra on top of the spectra measured in October with the PSL in commissioning mode. Here it is clear to see an improvement of greater than 1 order of magnitude between about 1Hz and 30Hz or so. This roughly agrees with what we'd expect given the measurements on the PSL table reported for L1 in LIGO-T1300368.

The second attached plot shows the unscaled RIN spectra for the IMC aligned, and misaligned in the 4 orthogonal D.O.F.s. Since the aligned spectrum looks very similar to the IMC misaligned spectra, it's reasonable to assume that even in the aligned case the RIN out of the IMC is dominated by the effects of beam jitter. In the ~f^(-0.5) part of the spectrum above about 5Hz the MC mirror motion is not expected to contribute to the jitter, so we can assume everything here is due to coupling from input beam jitter through RMS or DC misalignment of the IMC into RIN in transmission of the IMC.

From the difference in magnitude of the aligned and misaligned spectra we can make some estimates of the RMS alignment offset in each D.O.F. Since the jitter to RIN coupling is linear with alignment offset, the RMS offset in each D.O.F. can be estimated as follows: RMS offset = aligned RIN / (misaligned RIN - aligned RIN) * misaligned offset. For the D.O.F.s studied this comes out as:

I'll continue with some more analysis of the data, and write it up in LIGO-T1300378.