With the tests of M5's surface figure completed, the next mirror on our list is M2. We have the null lens that the manufacturers used to check M2 & a plan as to how to go about the test has been formulated. So now we need to establish whether we can meet the alignment tolerances in order to make a meaningful measurement...
The back of the lens is flat & the front surface is concave so the plan is to use the interferometer to get fringes off the back to align it in tip & tilt. Decentre looked like posing more of a challenge, but the shape of the lens comes to our rescue: you get circular fringes from the front surface thrown in for free!
Getting the lens squared up with the interferometer in tip & tilt kills the straight fringes (off the flat back of the lens) & then one can centre the circular fringes (from the curved front surface) with respect to the reference beam by translating the lens in X & Y - too easy :)
The remaining requirement is to get the axial spacing from the interferometer to the null lens, for which you need to know exactly where the point source focus of the interferometer is. A cunning trick for doing that is to place "a ball bearing on a stick" in the laser beam (close to where it emerges from the diverger) & adjust the focus until the circular fringes disappear. The centre of the ball is then at the point source focus & its position can be measured with the Faro Arm.
Midst all this fun stuff, there's still time for some funky photos :)