I've spent a little more time working on the HAM1 ground to HEPI feedforward and did another test. I can get pretty good improvement on the HEPI L4Cs, but it doesn't make much of an improvement on the interferometer. DARM doesn't see any improvement, but CHARD P sees a bit of an improvement over 5-10hz, see attached plot. Top is the table HEPI Z L4Cs, middle is the tabletop L4Cs,  bottom is CHARD P. For each, red is with the FF on, blue is with the FF off. The HEPI and table top L4Cs see some reinjection 1-2hz, but this doesn't show up in CHARD. CHARD P does see a not quite factor 2 improvement at 8hz.

I haven't had a chance to see how this interacts with the HAM1 TT to ASC feedforward, but that, I think, is generally a bit higher in frequency.

I've talked to Jenne and Sheila, I'm going to leave it on overnight to get some good low frequency measurements.

If there is concern this is somehow causing problems (it really shouldn't!) it can be turned off by setting the gain to 0, in a terminal :

caput H1:HPI-HAM1_3DL4C_FF_Z_GAIN 0

The new gain would then need to be accepted in SDF. But, this really shouldn't hurt anything.