Backscattering at ISCT1 has been a problem in the past ( https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=35538) and we have installed new beam dumps (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=35636). To answer the question of whether backscattering from ISCT1 will be a problem at higher sensitivities (assuming the current beam dumping), we placed a rented large-amplitude shaker on the leg cross beams.

An estimate from the data shown in the plot suggests that, assuming linearity, the background motion of the table would produce noise at the level of 1.1e-19 m/sqrt(Hz) at 11.4 Hz (a coupling factor of 1.25e-11 m test mass motion per meter of table motion).

We double checked that the features associated with shaking in the figure were produced by scattering from the POP path by closing the beam diverter. The features did go away when we closed the beam diverter.

Philippe Nguyen, Sheila Dwyer, Robert Schofield