An essential new piece of gear for the CGH tests is the coat hanger... This is a thin piece of steel that fits in between M2 & M5, carrying a small reticle that will be used to mark the optical axis of the SAC.
Similar to the old paddle mirror used to align M5, the coat hanger has a pair of 100 thread-per-inch jacking bolts to allow the reticle to be set up perfectly level on the back of M2. Initial coarse adjustment was done with the interferometer looking down on the coat hanger that was placed on a high-quality flat surface - the back of a piece of the ill-fated primary mirror segment!
The reticle is an extremely flat aluminised disk with a transparent crosshair on the surface. We'll spare everyone the gory details of the fiddly multi-step process used to get it to coincide with the optical axis of the corrector - suffice to say that it's there now!
New invar buttons were glued in place on the back of M2 so that the coat hanger can be positioned repeatably whenever needed. Here we can see the reticle near the edge of the hole in M5 as the coat hanger had been moved out of the way to allow access to the pinhole.
The long-term plan is to produce an invar version of the coat hanger which would be used to represent the optical axis whenever necessary.