M5's outer edge is a Mess - but that's OK!

When we first got to look carefully at M5 we noticed that the outer 5-10 mm is rather ratty, particularly that the coating's irregular out near the edge. This isn't too surprising given how steep that part of the mirror is & since it's so close to the edge, we didn't think it would matter too much.

You can also see the effect when you line a reflection of a straight object up against the edge of the mirror...

Given our ongoing vignetting problems, we turned our attention to this area. Ages were spent peering at reflections along the edge & marvelling at how bad it all looks, but we needed a way to measure how far this mess intrudes into our field of view. To this end, a laser spirit level was mounted on a camera tripod, which in turn was attached to the NRS simulator so that the laser could be directed onto M5.

With some careful arrangement (& a Lot of masking tape), an appropriate reflection of the laser could be projected onto a sheet of paper fixed to the inside of the birdcage.

The bright red slash shows an exaggerated profile of M5 & pushing a strip of paper in above that part of the mirror blocks the reflection, making it easy to see how far in the bad ring extends. This turns out to be exactly the same region we believed we were losing to the vignetting problem...

At this stage we delved into the gory details of the SAC data pack (written exclusively in French, but of courrrse) & found that M5's in fact deliberately oversized to avoid having to contend with the edge: AARRRRHH!! But at least that does explain all our puzzles: why the Zemax & CAD models (which use the clear aperture diameter) were right & yet we weren't getting the whole mirror, why the "chromosphere" appeared after opening up the holes in the pinhole stages, why the outer diameters quoted in the data pack & measured with the Faro Arm agree etc etc.

So with renewed faith in our gear, at last it was back to the M5 wavefront test - happily only measuring the part we care about. The RMS = 0.046 waves - what a great mirror afterall! We apologise for all the unkind things we said about you, M5...

We've also been having fun with the alignment telescope set up on its new mount on the optical table, figuring out how that works in anticipation of the other optical tests coming up.