On the NE pillar of the SRM, spotted the witness optic looking pinker than it should and maybe even seeing the pull tab. Decided to ignore my growling stomach and call my bet. Sure enough, while the front surface FC was removed, the back side was still in place. See the attached photo after I flipped the Optic in the holder on the bench. I looked through the ham5 alogs and see no indication of anyone setting up the CC witness optic although I may have missed it or that was not logged. And, someone may have the scheme of pulling the backside FC and plan to remove and flipped the optic at the last minute but I'm not so sure. If correct, this means we had this batch of FC under vacuum for about 20 days. I was going to replace everything with new but realized that was unnecessary. The optic is ready to have its FC pulled to exposed the clean surface.
Do we normally leave FC on components under vacuum? This material seems noxious and not vacuum friendly. This might explain why we have been seeing an unsual corner pump down not breaking the 1e-6 Torr barrier.
I'm not familiar with the procedure for these guys. On HAM6 earlier this week, I pulled FC off the 1" optic attached to the OMC. I was not aware if I had to worry about FC on the back side of the optic--Hugh's email has me wondering/worrying. Wonder if this needs to be checked for HAM6 before pumpdown.
How many of this type of contamination control optics are installed per chamber?
Corey--I'd say at most we have an average of one 1" witness optic in each chamber, +- one. Yes, the witness optic has FC on both sides, I'd guess one would be sufficient but it has been both sides in my limited experience.
Regarding Chandra's question o first contact (fc) left behind affecting recent pumping issues I would say probably not, see details in approval doc for outgassing https://dcc.ligo.org/LIGO-T0900438.
Regarding how many per chamber, it is 1x 1” optic or less and yes please not only should one look for fc on both sides but also use top gun while removing.