M. Todd, C. Cahillane, K. Kawabe

In the last quarter of 2025, I was running a bunch of injections to the various ASC DOFs of the IMC, and looking at the response in the ISS --- this was all in an effort to explain the various features in the witnessed RIN from the out of loop diodes of the ISS array, or in other words create a noise budget for the ISS. In short, I believe we are IMC ASC limited below 5Hz while acheiving the shot noise limit from 20-1000Hz.

There are some peaks in the witnessed RIN that are not in the in-loop ASD, suggesting it is some noise being imposed by the loop itself. Further measurements will try and explore the source of some of these, particularly the ones around 18 and 24 Hz, with a suspicion of the IMs motion.

I wanted this alog to serve as a reference for where all the data lives for these measurements, as well as the code and noise budget plot. Note, the ISS_OUTER and INNER rin channels are saved with quick_psd, but the gpstimes are recorded in the table below.

For the projections, a transfer function is measured during the excitation of the measured DOF --- looking at the TF from the best witness channel to the ISS OUTER, and then that transfer function is applied to the witness during the quiet time when there is no excitation.

ISS Noise Budget O4c