The May 14 post described how to get the CGH aligned - the next step was to put M4 back in before tipping & tilting the CGH & decentring the interferometer to get the best fringes possible.
The actuator readings indicated where the CGH ended up after optimising the image quality and with the scheme below we could work out where M3 needed to be moved to. The decentre's set by adjusting M3's tie-rods that attach to the mirror at 1, 5 & 9 o'clock while tilt's taken care of by placing shims (in the form of feeler-gauges) under the mirror's spherical bearings.
After each M3 adjustment, we again had to tweak the CGH & interferometer to get the best image quality & then decide what the next M3 move needed to be. This seemingly endless iterative process was interspersed with regular re-alignments of the CGH (as described previously) & handfulls of rapidly greying hair were torn out along the way... But eventually things did converge so - believe it or not, the SAC is now ALIGNED!!
Now we need to tip the whole contraption over & make sure it all still works at angle, then we'll also check it over a range of temperatures...