J. Kissel, J. Rollins

Summary: I've zeroed out the RX to Z element of the IPSALIGN matrix for H1 HPI ITMX, because it's SDF system was showing it containing a small, non-zero value -- different front the expected value of zero. This coefficient was small to begin with, and never proven to be effective. See details below.

-----------------

Jamie pointed me to the last SEI SDF system that was reporting differences in its table: H1 HPI ITMX. The single difference was in H1:HPI-ITMX_IPSALIGN_6_5, which is the RX to Z element of the IPSALIGN matrix, used to decouple RX tilt from Z drive. It had a current value of -0.0013 and a SDF table value of 0.000.

Hugh's aLOG on the derivation of this specific coefficient (see LHO aLOG 16118) is wishy-washy [edited for brevity]: 
"[W]ith the [RX to Z] correction factor of +0.0013 installed, [the] HEPI Z to ISI tilt was [...] looking pretty consistently wrong [...] so I switched the sign and [...] it looks as equally bad.
[...]
This leads me to the conclusion that [...] the plant condition/measurement setup is [inconsistent] at these [small] coupling levels. [...] The next step is to remove the coupling factor and remeasure this Z to RX and see if it is similar.
[...]
The likely course will be that we don't care about the coupling at this level and leave them unpopulated for now."

But I found no further follow-up aLOG of whether Hugh assessed the performance without the element installed. A dataviewer trend reveals the value had been -0.0013 *since* Jan 16th 2015 (when he wrote the above quoted log). However, the SDF system for HPI ITMX -- which according to the date of the log that Hugh posted when he turned all of the HEPI's SDF systems ON (16217) -- didn't exist until Jan 22nd, so I don't understand why the table suggests it should be 0.0. Maybe Hugh was leaving the coefficient installed, but different from the SDF table to remind himself that this was still to be investigated? Dunno.

Anyways -- I've zeroed the element, as the SDF system wanted, and there are no longer any diffs. If the coupling is as small as the measurement / calculation suggests, then it should have negligible impact on the SEI system's performance.