Andy, Josh, TJ, Laura, 

When inspecting periods of poor DQ in H1 today we noticed some clear scattering that has multiple arch structures (see fig 1). When we ran scatter frequency predictions, ETMY and ETMY R0 were the two optics whose length motion most closely resembled the fringes (see figs 2,3 and link). If you gave each of them 10, 20, 30 bounces, they would line up perfectly. We observed something just like this in LLO before, see 29114. There, folks on site were able to dramatically reduce it by minimizing the scatter arches from a drive signal while introducing an offset in the reaction mass alignment, as described on slide 12 of Bryan Barr's fellow's project here. Perhaps this procedure could also be helpful for reducing this particular scatter mechanism at LHO.