[Paco, Tega, Anchal]

Today, we started work on AS4 SOS by checking the OSEM and cable. Swapping the connection preserved the failure (no counts) so we swapped the long OSEM for a short one that we knew was working instead, and this solved the issue. We proceeded to swap in a "yellow label", long OSEM in place and then noticed the top plate had issues with the OSEM threads. We took out the bolt and inspected its thread, and even borrowed the screw from PR2 plate but saw the same effect. Even using a silver plated setscrew such as the SD OSEM one resulted in trouble... Then, we decided to not keep trying weird things, and took our sweet time to remove the UL, UR OSEMs, top earthquake stops, and top plate carefully in-situ. Then, we continued the surgery by installing a new top plate which we borrowed from the clean room (the only difference is the OSEM aperture barrels are teflon (?) rather than stainless. The operation was a success, and we moved on to OSEM installation.

After reaching a good place with the OSEM installation, where most sensors were at 50% brightness level and we were happy with the damping action (!!), we fixed all EQ stops and proceeded to push the SOS to its nominal placement. Then upong releasing the EQ stops, we found out that the sensor readings were shifted.