All done with M5!

On our way up to the clean room this morning we got to see one of the locals - a magnificent Black Harrier - out looking for breakfast...

This week we've repeated the M5 alignment process a few times, refining the procedure & checking Everything, over & over again. Satisfied with the spot positions & the quality of the wavefront, it was time to lock the tie-rods so that the mirror can no longer translate in X & Y. All the tests were then repeated to confirm that the locking process had not disturbed the alignment.

Next we needed to measure the positions of M2 & M5 with the Faro, before tilting the SAC over to its operational angle of 37 degrees. We removed the M5 pinhole stage assembly & the M4 mounting blocks to make things a liiittle easier for the Faro jockey.

Faro'ing at angle's a whole new game - Darragh resembled a weary boxer each time he plonked down to review the data after making a 1000-point measurement of the 14 invar buttons (each done 3x, not to mention the outer diameters of M2 & M5 which are even trickier).

We left for Cape Town straight after completing the measurements, but a quick look at the data showed that M2 & M5 both moved less than 80 microns in the Y-direction & they flew in formation :)

New vines & a new CGH

Another week, another season in the Hex River Valley! Spring has sprung & now summer's well on its way - the vines are already looking spectacular :)

Hopes of installing the new M4 mount later this week were dashed by the discovery of a subtle slip-up by the manufacturers... Luckily this can be overcome by modifying smaller parts of the assembly so James & Craig returned to Cape Town to work on that.

In the meantime, an eagerly anticipated parcel arrived from Siberia: our huge, new CGH! It's another work of art, made more beautiful by the fact that (unlike its baby brother) it was born with a cross-hair to mark its centre...

By far the most elegant pieces of "hardware" in this entire project!

Shimming & twiddling!

Some violent SAC interventions today! To get M5's optical axis aligned with that of M2, we had to break the epoxy locking the M5 tie rods - 1 size 10 spanner to hold the rod & another to torque the nut at each end & break the glue...

The tie rods (which attach at 1, 5 & 9 o'clock) allow adjustment of M5 in decentre. The 9 o'clock rod moves M5 in Y only, while the 1 & 5 o'clock rods move M5 in X only - provided you rotate them equal amounts in opposite directions.

There is a spherical bearing where each of the tie rods attach to M5 & this is where shims get inserted to adjust the tip & tilt of the mirror.

Combinations of shims & tie rod rotations enabled us to get the 2 points defining M5's optical axis to coincide & then these had to be made to line up with the centre of the alignment target that represents the optical axis of M2. The next 2 images show the positions of the pinhole itself & the pinhole image (both marked with a red cross-hair) with respect to the centre of the M2 target (indicated by the blue circle).

It then became an iterative process to optimise the image quality by twiddling the tie rods (using the wavefront camera's live aberration breakdown for feedback) & moving the pinhole image ever closer to the target by shimming accordingly.

Inserting & removing the shims involves lifting M5 with our soon to be patented "shimmin' tool" & slipping the required feeler-gauge in or out from under the particular spherical bearing.

By the end of a very long day we had the 3 spots lined up & M5 delivering an excellent wavefront - with that, M5 aligned with respect to M2! A fantastic way to end an epic 10 day run - off to Cape Town tomorrow for a much needed long weekend :)

Tune in next week when we hope to reinstall M4 with its improved mounting scheme!

Improving repeatability

Aligning M5 to M2 requires us to alternate between the wavefront camera & the alignment telescope. We found that the fold mirror used with the alignment telescope didn't always go back in a repeatable enough way so Eben came to our rescue once again.

The new scheme has the mirror attaching to the alignment telescope mount, instead of the stage assembly that the wavefront camera bolts on to - a much more reliable solution.

Chasing spots & cross-hairs

This weekend we spent a lot of time getting the alignment telescope set up on the super-structure platform. A CCD camera on the back of the alignment telescope allows us to use Francois' nifty software to determine centroids and cross-hair positions for the various points we're interested in.

The paddle mirror on the back of M2 was used to get the tip & tilt of the alignment telescope right...

& the alignment target in the hole in M2 takes care of the decentre.

With the alignment telescope ready, it was time to replace the M5 pinhole carrier & try to find the 2 points that define the optical axis of M5 (the cross-hair on top of the pinhole carrier that represents the pinhole & the image of the pinhole formed by M5).

Here's the pinhole cross-hair...

& here's the spot - the pinhole image formed by M5.

These are all the points we'll need for aligning M5 to M2 - watch this space!

Going forward again - at last...

Since our return to the clean room on Tuesday, the SALT Faro's been subjected to a number of tests before being allowed to take up its post on the aluminium ring of the NRS Simulator.

We remain satisfied with the Faro's performance so we've now established the master coordinate system on the back of M2, measured the 14 invar buttons on M2 & M5 (1000 points at a time, several times each), checked the concentricity of the alignment target within its holder (another 10000 odd points), accurately positioned the target within the hole in M2, replaced the M5 pinhole assembly & put back the super-structure of the NRS Simulator.

Without wishing to jinx anything, we should now be able to install the alignment telescope & proceed with the alignment of M5 with respect to M2. See the 31 July blog entry if, like us, you need some reminding as to how this is all supposed to work...

Recalibrated & looking Gooood!

The SALT Faro made it back from Europe last week & finally got through SA customs on Friday, along with its fresh calibration certificate...

A series of Single Point Accuracy Tests (aka SPATs) carried out in Cape Town on Sunday confirmed that the Arm's now performing within its 20 micron specification, a HUGE relief to all concerned! The plot below shows a comparison between typical SPAT results obtained with the Faro we'd borrowed while ours was being fixed (the thicket of points to the left with average values of ~45 microns) & the latest measurements made with the recalibrated SALT Faro to the right.

This really is fantastic news :)