(Alexa, Sheila)

For the noise budget, we had tried measuring the PDH shot noise by misaligning ITMX and reading out the amplitude spectrum from IMON of the demod. This amplitude spectrum was clearly not pure shot noise at low frequency; there seemed to be some scattering and fringe wrapping (see green line in plot). We went to investigate this by putting a HR 532nm mirror at the bottom of the periscope and repeating the measurement; the noise at low frequency subsequently decreased significantly (see red line in plot -- unfortunately I should have taken more frequency spans). There is stil some acoustic noise at around 100Hz but it's less than in the first measurement (and in the overall PDH noise, the fiber noise is dominant in this region anyway). We removed the HR mirror and tried misaligned ITMX to various different positions and saw that the amplitude spectrum changed around. Clearly, in this measurement we are picking up scattering in the cavity which comes about from misaligning ITMX. We have now convinced oursevles that we do not have any major scattering on the ISCTEX table. 

I have attached the data with the HR mirror in place.