The cross hair on the SAC reference mirror had to be replaced with a thicker one that will be visible through the alignment telescope when looking down on it from the CCAS tower.
Then the reference mirror had to be attached to the back of M4...
Once in position, it could be aligned with respect to the alignment telescope, that had earlier been trained on the coat hanger which marks the SAC's optical axis.
To help prevent stray light issues, the shiny mounting bolts that lock M4 in place were painted black.
& the new M5 aperture stop was installed to cut off any light from outside the clear aperture of the mirror.