Preparing the big CGH

The original CGH mounting assembly needed some extra measures, such as the springs shown below, to safely cope with the extra weight of the big CGH.

The XYZ stages & tip/tilt actuators also had to be tested & characterised to ensure that they could reliably manipulate a dummy mass representing the CGH, both when the test is done in the vertical & at the operational angle of 37 degrees.

Following successful testing of the mount assembly, the CGH was bonded to its titanium base & allowed to cure for a few days before being attached to the actuator system.

 

A gauge rod was also added to the setup to allow measurement of the height of the CGH as this will be needed when setting its axial position for the test.

It was found that the Faro probe could be supported below the end of the gauge rod & the CGH slowly lowered until the rod makes contact with & deflects the probe - an axial resolution of 5 microns is easily achieved in this way.

Time for some more CGH art!

Spectacular rainbow rings from illuminating the CGH with an array of LEDs.

The year that was...

Yesterday it was a year since the SAC was brought down from the tracker!  Execution of the SALT IQ fix has taken longer than we'd hoped it would, but we've also learnt far more than we'd ever have imagined.

We stand now at step 11: the testing & alignment of M3 - the final phase of the original SAC repair plan.  After this, the SAC will be returned to the payload & the alignment re-checked before we begin on-sky testing.

The major milestones from the past year include:

* Removing the SAC from the telescope *

* Installing the new interface collars *

* Getting to know all our new toys *

* Testing & aligning M5 *

* Revamping the M4 mount *

* Washing M5 *

* Aligning M4 to M5 *

* Testing M2 *

* Washing M3 *

Overall, there have been many more hightlights than can possibly be listed & the lowlights were relatively few & far between during this most extraordinary year!

This blog will go on hold for the next few weeks while the author attends to a major fixup on the family front - we'll catch up the clean room news asap...

Setting up the alignment telescope

With M3 clean, our next task is to prepare for the CGH tests...  The first test will use the big (210 mm diameter) CGH to check the surface figure of M3, then the small (148 mm diameter) CGH will be used to align M3 to the rest of the mirrors.  More about that later - for now the first step is to get the alignment telescope set up on the optical axis of the corrector.

A camera on the back of the alignment telescope provided this rather unusual view of the alignment telescope (in its mount) reflecting off the aluminised spot on the M2 null lens!

Some fiddling to get the alignment telescope centred up on the laser from the interferometer down below...

Here we have Ockert & Martin sporting their uber-stylish laser safety goggles - great smouldering look, without the fried eyeball... nice ;)

Making M3 Byoootiful!

Our main reason for electing to re-wash M3 was an extremely thin line of squiz we found at the outer edge of the coating.  Some of this material dissolved during yesterday's wash & caused mild streaking at the very edge of the mirror.  Although far outside M3's clear aperture, we decided it would be best to remove this if at all possible.

A small area was successfully tested with a cotton swab before proceeding to clean the whole edge with sodium lauryl sulphate. 

The offending substance was carefully removed & then the edge of the mirror was drag-wiped.

 

Yesterday's cleaning process was then repeated - the radial drag-wiping, rinsing with distilled water & finally blow drying.  Afterwards, the lights were switched off & the mirror inspected with a flashlight...

The result was Superb! :)

After the inspection & photo-shoot, M3's cover was replaced & the mirror lifted to allow it to be dried thoroughly.

At that stage it could be lowered back into its box...

& the team could breathe a huge sigh of relief :)  Great job guys!!

Washing M3

We're done with M2 now & before we get started on the CGH tests to check the figure of M3 & the overall alignment of the system, it's time to wash M3... 

First the mirror - still in the cell that bolts on to the rest of the SAC cage - had to be lifted out of its storage box.

It could then be lowered onto the washing trolley & wheeled into the wash bay next to the clean room.

Safely in the washing area, the mirror cover was removed...  Not quite as bad as M5 was, but still pretty disgusting!

It's even more gruesome up close - Eeeuw :(

Since the SAC's usually at an angle of 37 degrees, the down-hill side's particularly bad as years worth of dust & muck's collected in that sector.

The mirror cell had to be water-proofed before the washing could start - black plastic bags & loads of duct tape did the trick!

Tight-fitting plastic skirts were then stretched over the mirror to help channel the water away.

De-ionised water was used to rinse all the loose dust & dirt down the central hole...

Within a few minutes the mirror was already looking great!

Confident that all abrasive material had been flushed off the mirror, it was repeatedly sprayed with a 10 grams/litre solution of sodium lauryl sulphate.

The mirror was soaked like this for several minutes...

With Jonathan continually spraying the mirror, Hitesh could start the delicate process of drag-wiping the surface with wads of cotton wool soaked in sodium lauryl sulphate.

The mirror was then thoroughly rinsed with distilled water.

The clean surface sheds water beautifully!

Everyone watched eagerly while Jonathan blasted away the remaining drops of water.

Time then for a careful inspection of the surface...  It looks Fantastic, but we're not entirely happy with a couple of areas so we'll repeat the washing process tomorrow...

Done with M2 - bring on M3!

First day back on the hill in a while & treated to a wonderful aerial encounter :)  One of the local black eagles seemed to decide she'd had enough of the pesky ravens & gave chase...  Much squawking from the clearly disconcerted raven while the eagle loomed ominously large on its radar screen!  No real casualties though, other than some raven pride.

Meanwhile, the clean room was cooling down so that we could repeat the M2 null test at lower temperature.  After some fiddling to get everything tweaked up again we obtained superb interferograms with a RMS of 0.04 waves - the testing of M2's now complete so we can move on to M3 :)

When not working on the new MASS DIMM system that will monitor atmospheric turbulence up here, SALT Operator Freddie's found yet another instrument to keep busy with :)

We plan to wash M3 later this week so watch this space!