J. Warner, K. Venkateswara

This morning we tried X and Z sensor to HEPI. The results on Z are very positive. The first plot of the first pdf shows the ground Z motion / Stage 1 T240Z motion without and with Z sensor correction. It shows an improvement of ~10 at the microseism, with a factor ~2 reinjection between 20-50 mHz. The next plot of the first pdf shows the Pitch motion without and with Z sensor correction, which again shows an improvement of ~10 at the microseism.

With X, things are a bit different. The first plot of the second pdf shows the ground X motion / Stage 1 T240X motion without and with X sensor correction. It shows an improvement of ~2-5 between  a large range 20 mHz to 2 Hz, with maybe a factor ~2 reinjection at 10 mHz. This was similar to the increase in performance when we tried sensor correction to Stage 1. The next plot of the second pdf shows the Pitch and Yaw motion without and with X sensor correction, which looks the same above 50 mhz, but shows a factor ~5 increase below that frequency. We saw a similar increase when doing the sensor correction to X as described in 14570. Jim and I suspect that the tilt and yaw decoupling from X at Stage 1 (or HEPI) might not be good enough so we will repeat that measurement later.

Bottomline: Z sensor correction to HEPI works great. It works better than Z sensor correction to Stage 1. X sensor correction to HEPI/Stage 1 produces similar results - It improves performance at Stage 1 X but increases Pitch and Yaw motion at low frequencies, probably due to tilt and yaw coupling with X drive/sensing. Better tilt decoupling may help.

Edit: I had a calibration error in the plots, which has now been corrected.