CO2 snow & a midnight raid on the gym

Extremely cool to get to see the primary mirror cleaning system in action today! A huge arm with a nozzle attached swings over the array, blasting CO2 snow over the segments.

I also relished the chance to play with a fisheye lens again, what a treat :) Note the plume of CO2 just to the left of the centre of the array.

Back in the clean room, Eben & James had finished installing the framework for attaching the dial gauges & LVDTs to the SAC.

We also spent time testing a new setup for measuring the reflectivity of M5. This involved shining a red laser down through the hole in the super-structure platform & getting the return to land on a photodiode mounted next to it. The effect of the laser hitting M4 along the way is shown below - the translucent gold-coloured Zerodur that the mirrors are made of lights up beautifully!

Further effort went into adjusting the CGH fringes obtained with the interferometer...

The incredible sensitivity of the interferometer's essential to us, but being capable of measuring to fractions of a wavelength of light also has its drawbacks! The system easily registers the disturbance caused by a car driving up towards the plateau, let alone the effects that pumps, fans, people & wind have on the building.

We decided that more weight on the NRS simulator might help damp out some of the jittering so a posse rushed down to the gym & made off with all the weights. The poor little bakkie barely made it back up the hill again...

Other than the exhausting amounts of pool being played, this was the most exercise we've had in a month. It added a whole new dimension to Clean Room Olympics, but alas did little to stabilize the fringes.

That's it for now - we're heading back to Cape Town tomorrow for a much needed recharge, tune in again when we return on 1 June!

First CGH fringes!

Last night a package containing a new camera for the interferometer arrived so we fired that up this morning. Its larger field of view & range of gain settings made it easier to tweak the interferometer.

After Much fiddling with all the degrees of freedom for both the CGH & the interferometer, as well as adding a neutral density filter to the reference beam, we obtained our first fringes from the CGH.

With that, we proclaim the CGH test commissioned! :)

Happy 1 month anniversary :)

Today it's a month since this adventure began! Hard to believe, in both ways... Being so busy, the time's really flown - but on the other hand, normal life seems such a distant memory. We've now spent 26/31 days at or travelling to/from SALT, an average working day's ~13 hours & we typically sleep less than 5 hours a night.

James spent part of the morning on the couch with an empty champagne bottle... Not as much fun as it sounds - he was making up a new cone-shaped baffle to help cut down stray light & make it easier to find the CGH return spot.

We also inserted another small baffle (known to the SALT Ops team as Blackbeard's Patch) between M2 & M3. The rubber sleeve for the lower part of the SAC had to be lifted all the way up to do this, offering a very cool photo-op!

Peering through the cage at the CGH being lit up by my illumination assistant, Dazzle.

Bored with white light, out came a red headlamp & a green laser :) Apologies to the red-green colour blind!

After our indulgent photographic interlude, we got back to business chasing the CGH return spot. With stray light eliminated by the various baffles & a webcam set up to give us live feedback while moving the stages around, we could identify the spot & tweak it to perfection - Fantastic!!

Re-organising the clean room

We decided to tackle clean room ergonomics this morning... The cabling had gotten somewhat out of control & this prevented us from tipping & tilting the SAC through its full operational envelope.

The spirit of recycling prevailed when all the duct tape holding our world together was removed.

The cables were all disconnected & piled onto the desk which was wheeled over to the other side of the room.

The spaghetti wranglers then did their thing & life just seems simpler now...

Feeling smug about our redecorating effort, we turned our attention back to hunting down the elusive return spot from the CGH. By attaching the camera responsible for the blog photos (shameless plug for my precious Nikon D200!) to the NRS super-structure, we could get a better sense of what moving the various CGH stages does to all the confusing spots & smudges. Although they make no particular sense yet, they do look rather nifty!

Setting up the CGH

The CGH on its mounting assembly was inserted into the hole in M3 today. It can be seen through the lower section of the SAC framework (aka the birdcage) when the rubber sleeve surrounding the SAC is lifted up.

Neat to finally get to see M3 properly as well!

The SAC Reference Mirror's a small flat mirror that bolts onto the back of M4 & is used to align things with the optical axis of the SAC. We attached that in order to adjust the tip & tilt of the interferometer so that we'll be able to pick up a beam returning off the CGH. A rather cute little parasol had to be employed to prevent the reference mirror sending an extra beam straight back up that would swamp the CGH return.

The angry volcano's calling for a SACrifice...

To be able to reach M3 & the CGH, the Faro Arm sits on a lower perch than its usual one & wears an extended probe on the end.

More absolutely wicked optics :)

The current priority is to try out the Computer Generated Hologram (CGH) test that will eventually be used to align M3 with respect to the other mirrors & hence to check the optical alignment of the whole SAC system.

At first glance, the CGH looks like a thick CD without the hole in the middle, but it's quite a bit fancier than that... It's specifically designed to generate all the spherical aberration that SALT's primary mirror array would ordinarily impose on the incoming wavefronts - exactly the aberration that the SAC's required to clean up. By putting that aberrated light through the SAC & evaluating what comes out, we can test whether the 4 SAC mirrors are working as they should.

The CGH has been bonded to a base that will attach to the stage assembly & a set of safety hooks was installed around the edges for additional peace of mind.

The CGH consists of 23590 concentric chrome rings of varying thickness & separation deposited on a 15 cm wide flat piece of Sitall (the same low-thermal-expansion material used for SALT's primary mirror segments). It behaves like a diffraction grating & is thoroughly spectacular when illuminated & viewed on-axis!

As for the pinhole in the M4/M5 test, the CGH stage assembly lives in a tube that will be inserted into the hole in M3 to locate the CGH at the appropriate axial position.

Wires, wires - everywhere! It's all got to be controlled somehow...

Craig had to test some of his gear so the interferometer, NRS simulator super-structure & SAC cover had to be removed, leaving the Faro stranded without its usual hitching post.

Not an extractor fan for the SAC - this plate will serve as a drilling jig for improving the way the M4 mirror cell is secured to the 3 vanes of the mounting structure.

Janus works his magic

Swooping in for just 2 days, Janus kept himself busy getting the mount for the Computer Generated Hologram (CGH) together & sorting out the control systems for for all the stages & actuators.

He also spent time setting up the LVDTs - high precision linear displacement measuring devices that will be used instead of some of the dial gauges.

Things didn't look anywhere near this complicated a couple of hours ago?!

With all the Smurf Suits already occupied, Craig's left no choice but to go with the Shrek Suit option...

Hitesh to the rescue!

An autopsy on the Shack-Hartmann wavefront camera indicated death by power-surge :( Fortunately we have another identical camera (without a lenslet array attached) so Hitesh opened both up, swapped out the damaged card & we're back in business :)

Other progress for the day was the installation of the new cover for the top of the SAC which allows a test mirror to be set up for use with the interferometer.

With that in place, the NRS super-structure could be put back up & the interferometer re-attached so that Francois can play while the rest of us disappear for the weekend...

2 kinds of wavefront cameras

Darragh earned his learner's licence for driving the Faro for the first time this morning as we measured the top of M4, the plane of the back of M2 & the surface of the NRS simulator ring that the Faro & the SAC are attached to.

With the Shack-Hartmann wavefront camera misbehaving this evening, the old Hartmann wavefront camera was dug out. Very neat to be able to see the hexagonal lenslet array in the former & a similar array of holes in the latter (left & right respectively in the images below).

Tomorrow we're off to Cape Town again for a couple of days. It's not clear that we'll be able to function without playing a game of pool after every meal...

Firing up the Faro

The positions of all the invar buttons & reference points for various planes to be measured with the Faro Arm were marked on pieces of funky yellow duct tape.

The Faro (aka Langarm) is equipped to take care of itself if left to contend with gravity. This is all very well, but also means that handling the arm can at times seem more like a snake-wrangling exercise! Especially tricky when trying to avoid the mirror surfaces while at the same time negotiating around the NRS simulator super-structure - very well done James!

The Faro + its associated software is an incredibly powerful tool. At worst, it measures the position of a point to better than one fifth the thickness of a human hair!

With the primary mirror cover out of the way, the guys could start re-installing segments today - in this case, a beautifully clean specimen!

Mirror segment on its way back up to the primary via the hatch in the loading bay...

The white-board in the control room got busy as Darragh started jotting down the current set of to-do lists for individual aspects of the IQ plan...