Pending a T measurement on the second 9-m Y1S mirror, we will be installing these in the place of the 6-m ones that are in the cavity right now. The reason is twofold:
The input waists for the cavity given these mirrors (the average, for spherical MM, and the two orthogonal waists, for cylindrical) are:
Since we will have to realign the cavity after replacing the mirrors anyway, we are going to slide everything down the table a pinch so that we can make room for the final MMT (the table is just too crowded for a proper IOO solution right now). After this, we will work out both a cheap (read: quick) modematch to the the average, and a full cylindrical modematch. The former will allow us to continue working on the experiment while we wait for parts for the latter to arrive.