Aligning M5 with M2

The first step in this procedure is to set the alignment telescope up so that it's aligned with the optical axis of M2. Reflections from a mirror resting on the back of M2 & from the alignment target in the hole in M2 need to land on the graticule of the alignment telescope. This is achieved by carefully (& tediously!) adjusting the cones on the alignment telescope's mount & the tip-tilt stage on the fold mirror (that allows the telescope to look downwards).

One then has to view both the pinhole & the image of the pinhole as those two points give you M5's optical axis. That was made easier by etching a crosshair onto a piece of glass & attaching it to the top of the pinhole carrier.

The crosshair marks the location of the pinhole, which can then be viewed with the alignment telescope, as can the image of the pinhole (aka "the spot"). The pinhole carrier position is tweaked to get the best image quality for the spot & these two points then land roughly on the alignment telescope's graticule.

At this stage, the only degrees of freedom left belong to M5 & so the mirror itself needs to be moved to get all these points precisely centred on the graticule. M5 movements are effected by placing shims under the spherical bearings where the tie rods attach to the mirror.

That's the story in principle & all the fixturing's in place - now we just have to DO all this to the required tolerances... A task made tougher by the fact that Simon's led IT to quite the comeback on the pool table this past week!

The Anti-Gravity Quest

The Texans are back - this time to copy the anti-gravity drive on our tracker & to see what other useful ideas we have down here...

They seemed impressed with SALT's CO2-snow mirror cleaning system, but Adelaide's curry vetkoek took the cultural exchange to a new level!

Frosty Friday

Woke up to heavy frost all over this morning!

Eben brought his new baby in to show us how it's coming along - see the 12 July entry for how it all began. The handle's made of buffalo horn.

The remaining polishing will be done by hand...

It's been a good week up here - the M5 test's complete & we're confident that we'll be able to align the null lens for the M2 test when the time comes. So now, with the temperature rocketing up to almost 3 degrees already, we're heading home for the weekend :)

Playing with the M2 null lens

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 :)

Suffering for our art

Having tested M5 at 17 C & 23 C, the clean room was violently cooled throughout the day. The temperature eventually bottomed out around 6 C (%*?#$@&!!), at which point we repeated the wavefront measurements.

This somewhat bizarre situation evoked long-lost memories of being an astronomer up here - freezing to death while observing late at night in a fridge-like dome...

In fact, throughout the past few weeks, Darragh's spent 10's of hours up the ladder in the dark, doing battle with the alignment telescope - it really is just like old times ;)

With the alignment telescope out of the way, one can look directly at the pinhole carrier or the image of the pinhole via the fold mirror.

Most frustratingly, it seems impossible to get the 2 to line up!

I guess now that the M5 test's out of the way, we were due for a new puzzle...

More new bits!

Sutherland really cleans up beautifully after a good storm... From the SW corner of the plateau you can see 13 of the 14 telescopes up here. Only tiny SLODAR manages to hide behind MoNet & SALT lurks unimpressively in the background...

After struggling endlessly to get the alignment telescope to work - a clever new mount design was proposed. Manufacturing was completed today & Darragh spent the evening getting acquainted with the new gear.

Here's the new assembly on top of the super-structure platform, it looks down via a fold mirror.

Part of the plan for aligning M5 to M2 is to view an alignment target, positioned just below the hole in M5, with the alignment telescope up on the platform. For now, this target's held in place by the tube from the M4/M5 test that used to hold the pinhole.

Later on in the process we'll need the pinhole for the M4/M5 test & so the target won't be able to sit where it is now. An elaborate new target carrier that will be placed between M2 & M5 & locate within the hole in M2 was produced on the CNC machine in Cape Town.

The aluminium vanes in the delicate flexure system are only 0.5 mm thick!

Twilight on the plateau - another great night for the astronomers!

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 :)