Saturday, December 5, 2009

The puzzle of the M4-M5 axial spacing

Our main objective for the past few days has been to sort out the axial spacing for the M4-M5 pair.  This apparently humble goal's turned out to be quite a challenge as we've found it impossible to get all the Z-distances - from the pinhole to the back of M2 to the back of M5 to the vertex of M5 to the vertex of M4 to the back of M4 to the wavefront camera - to add up to what we'd expect from the SAC metrology report & the Zemax models!

The contraption above is a mirror showing modifications made to the wavefront camera to allow us to use the Faro to find its height above M2.  Having measured all the relevant separations, & checked repeatedly that the Faro's indeed behaving itself, we were forced to conclude that the problem must lie with the numbers in the metrology report.

Thus we needed to measure the vertex distances of M4 & M5 - the separations between the backs of the mirrors & the centres of their optical surfaces.


Fortunately M4's vertex lies at the centre of an unused 3 mm patch on the surface of the mirror & so it can be touched with the Faro...

Everything was set up to be as ergonomic as possible - the photographer even refrained from snapping during the scary part!  All went well & the interesting result is a 133 micron discrepancy between the stated & measured values for the M4 vertex distance.

M5's a trickier prospect as its vertex lies within the central hole!  The surface of the mirror had to be touched in few places & those measurements matched to the surface sag at the measured radii in order to establish the vertex distance. 

The result in this case was a discrepancy of 188 microns!

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