Last week we mounted the H2 iLIGO PSL table on a new leg system for aLIGO, discussed here.

Our design philosophy was to minimize RMS motion in the aLIGO band by raising the table sway resonances from iLIGO’s 20 Hz to about 100 Hz, and the vertical bounce on the legs from the 40-60 Hz region to about 200 Hz. We did not want to raise the resonances much higher, because they would overlap with the periscope resonance. We chose not to float the table because the system we have experience with only controls 3 degrees of freedom and it increases low frequency motion in the 1 Hz region and below.

Figures 1 and 2 show displacement spectra and RMS from accelerometers mounted on the H1 (iLIGO) table (red), and the H2 aLIGO table (blue) for comparable locations near the periscope position, taken at the same time, and a spectrum from the floor near the H2 table, taken at a slightly different time (black). Clean rooms were off for these spectra. The lowest of the sway resonances of the new table is at about 95 Hz, not quite 100 Hz, but acceptable, and the bounce mode is at about 200 Hz. 

The RMS motion of the new H2 aLIGO table above 10 Hz is about 20% greater than the rms for the black floor trace, while the RMS motion of the iLIGO H1 table is 730% greater. 

These results were comparable with those from the first installation, made at LLO, discussed here.

Robert Schofield, Rick Savage, Michael Rodruck, Mark Dodson