The coupling from the BSC-ISI Stage 1 Z drive to the T240 RZ signal reported a few weeks ago has been lately our first guess as for what's limiting the RZ (Yaw) isolation performance.

This week we modified our real time model to do subtraction tests. The modification has been installed on ITMY.

The first plot attached shows:

- the coupling measurement in green

- a simple fit to be used in the subtraction block, in black

- the expected residual coupling in red

The second plot shows the residual coupling measured after installing the correction. The subtraction results look pretty good. Unfortunately, I did not observe substantial improvement in the optical lever Yaw motion. To be further investigated...