It seems that what we see on the ISS second loop photodiodes is real intensity noise in transmission of the IMC. Indeed, as visible in the attached plot, the signal on all ISS PDs has the same shape and is very well coherent with other monitors of the IMC transmitted and reflected powers.

So, as suggested in my previous entry, the RIN at the IMC output is very large, 1e-5 at 10 Hz and 1e-6 at few hundreds Hz.

To investigate the origin of this broadband excess of noise, I had a look at some angular channels related to the IMC.

As visible in the third attached plot, the IMC transmission shows some coherence with the IMC WFS signals and with the periscope accelerometer. In particular:

• WFS_B_YAW shows the same structure at 250-280 Hz as the IMC transmission
• WFS_B_PITCH has the same structure visible at 700 Hz

There is some broadband coherence also, so maybe most of the noise floor is due to angular motion of the beam, converted to intensity noise by the IMC.