IQ *crushes* IT!

We're finally back to testing M5 - with the SAC vertical, as well as tipped & tilted to the extreme attitudes of its operational envelope. It also needs to be tested at different temperatures, which means killing time while we wait for the clean room to heat up, & then cool down again. Yaaawn... What to do?!

With a willing victim on hand, the IQ team could suspend the daily battles in their endless pool war & unleash their fury on another! & who better to be on the receiving end than the head of IT??

Afterall, it's a position that's uniquely equipped him to cope with abuse & frustration ;)

It was all rather too gory to describe! The motion blur on the cue ball conveys some of the violence of the encounters - R.I.P. IT!

Finally - some decent snow!

Last night it started snowing - quite heavily at times, so by morning the place looked absolutely wonderful :)

Poked around taking photos before making our way up to SALT - thanks Charl for the 4x4 shuttle service!

The sun fought its way through by late morning & most of the snow had disappeared by dinner time, but we're sure to get a few more doses of this before winter's over...

Comparing microscope objectives

We spent time testing a variety of microscope objectives to find the one best suited for use in conjunction with the wavefront camera. The collection included two reflecting objectives that look a bit like miniature telescopes as they each have a small secondary mirror suspended on a spider above the primary.

Measurements were made with the objectives attached to the wavefront camera & placed close to an illuminated pinhole on an XYZ stage assembly (the tube from the M4/M5 wavefront test).

The 15x magnification reflecting objective has a lot of spherical aberration & the 74x magnification one is too powerful...

A refracting objective with 30x magnification turned out to be ideal & so we'll proceed with that one.

Happy with that conclusion, we put everything back together on the platform, connected up the direct camera & found our pinhole image again. While trying to identify a shadow extending across the comatic image, Darragh traced his finger along a cable. What happened next amounts to our highlight of the week: his finger disappeared into the image!

In between all that excitement, Ockert installed his mount for the alignment telescope on the super-structure platform & tried it out...

& now for something completely different...

For a change of scenery this afternoon, we indulged ourselves with a visit to Eben's workshop to see how he goes about forging Damascus steel for his incredible knives.

This surely represents The most extreme departure from our clean room & the delicate world of optical alignment & testing. Showers of sparks don't tend to burn holes into our clothing while we do what we do...

Titanium's the way to go if you prefer white sparks!

The flash & smoke as Eben started welding a handle onto the billet produced a shot that I'll claim as my best attempt at astrophotography to date.

Into the >1200 degree Celsius forge it goes!

This ferocious contraption's a stamping machine that takes all the pain out of hammering - for the human anyway.

The temperature of the steel's around 800 C at this stage.

Cooling down to about 600 C on the anvil after a vigorous encounter with a wire brush, soon to be put back into the forge.

After a couple of rounds of this, a grinder's used to remove the dull outer layer...

& then a dip in a bath of ferric chloride etches the surface & begins to reveal the patterns in the steel.

After some polishing, the piece's ready to start being shaped into a blade.

There's an endless variety of complex, beautiful patterns to be created.

Just one of Eben's many amazing works of art :)

Interferometer vs Wavefront Camera

We returned from Cape Town yesterday to find a very smart new workbench in the clean room. This is actually turning into quite a classy place...

In fact - now's a good time to reflect on what this room looked like about 12 weeks ago & compare it to the current scene...

Keen to check that the wavefront camera & interferometer give the same results, Darragh & Francois set about comparing their measurements of a test mirror. The laser on the interferometer serves as the light source & the wavefront & interferometer cameras are easily interchanged. When using the wavefront camera, one has to block off the reference arm of the interferometer.

Once everything's appropriately aligned, the interferometer produces fringes. These are usually recorded by the camera attached to the interferometer, but here they're seen projected on the wall.

When you match up the number of fringes you see that the two instruments are indeed measuring the same thing, albeit with a subtle sign change - the 2 interferograms are vertically inverted with respect to each other.

Sutherland continues to churn out spectacular sunsets!

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.

The M5 mystery

Despite significantly enlarging the holes in the pinhole stages, we still couldn't get the M5 wavefront test to work without apparently vignetting the outer 25 mm of the mirror... Most bizarre, since the Zemax & CAD models are fine, the M5 test hardware was appropriately manufactured & the measured physical size of the mirror is correct.

So what's going on?? & why do we now (after attacking the pinhole stage assembly with the plasma cutter) see a weird halo/"chromosphere" around the edge of out-of-focus images of the mirror?

Extraordinary levels of ingenuity went into investigating this frustrating puzzle, including every possible way of viewing the system: with eyeballs, webcams, projections, photographs, the wavefront camera, the alignment telescope...

Still no helpful clues - although we did get a rather nifty "globular cluster" image when we peered through the alignment telescope at what was probably the dust on the mirror being illuminated by the pinhole!

At times the clean room took on a sinister look as various forms of lighting were employed.

When the going really got tough, some of us retreated to the relative safety of the optical bench where we unwrapped the N2 null lens & played around with getting fringes off its flat back surface. We need to establish how accurately the interferometer will allow us to align the null lens in tip & tilt for the M2 test that will happen at a later stage.