Now that the CGH is in place, it has to be positioned to within 25 microns in decentre, about 70 microns in despace & less than 10 arcseconds in tip & tilt... As always, a cunning plan is required & fortunately we hardly ever leave home without one ;)
First of all, the M4 "steering wheel" - the bit that can be removed & replaced repeatably - has to be removed to allow the alignment telescope on the platform to see the CGH & the coat hanger below.
The alignment telescope looks down via a fold-mirror - which too is repeatbly replaceable as it needs to get out of the way whenever the interferometer's in use.
By adjusting the focus of the alignment telescope, it's possible to find the centroids of the 4 features we need to get the CGH aligned to the coat hanger (which marks the optical axis of the SAC) in both decentre & tilt.
First we focus on the coat hanger reticle & find the coordinates for the centre of the cross, which was meticulously set up previously to coincide with the optical axis of the SAC.
Next, we centroid the image of the alignment telescope's auto-reflection target as it bounces off the coat hanger reticle.
The coat hanger (which also locates repeatbly) is then moved out of the way to allow the alignment telescope to access the CGH. The auto-reflection target image from the CGH is centroided & those coordinates compared to the coat hanger's. Adjustments are made with the tip & tilt actuators on the CGH until the coordinates match.
We then focus on the cross etched on the surface of the CGH & compare the coordinates of its centroid to those of the coat hanger cross. The necessary adjustments are made with the X & Y actuators on the CGH mount.
It all sounds simple enough in principle, but by now we ought to be used to the fact that procedures like this end up being iterative & thus take Time & Loads of patience!